Asteroid and comet impacts can trigger widespread havoc, killing off life on a global scale. Now, one new study reveals that the molten wreckage of these explosions can entomb the remains of life that once dwelt in the blast zones and preserve them for millions of years, while another study hints that these impacts could even create novel habitats where life can flourish.
These findings suggest that impact craters on alien worlds might be good places to look for past and present signs of life, researchers say.
The blazing heat generated by cosmic impacts can heat, melt and even vaporize tons of soil and rock, some of which forms glass as it cools. Impact geologist Peter Schultz at Brown University in Providence, Rhode Island, has explored impact glasses in Argentina for more than 20 years. An area roughly the size of Texas in eastern Argentina (south of Buenos Aires) is littered with impact glass created by at least seven different impacts that occurred between 6,000 years ago and 9.2 million years ago. [5 Bold Claims of Alien Life]
“As we collected these glasses, we could see what appeared to be leaf-like materials trapped inside,” Schultz said.
He and his colleagues detailed their findings online April 15 in the journal Geology.
Plant material was seen in each impact. In two impacts in particular — one from 3 million years ago and the other from 9 million years ago — Schultz and his colleagues discovered centimeter-size leaf fragments, including structures such as papillae, tiny bumps that line leaf surfaces. Bundles of vein-like structures found in several samples are very similar to modern pampas grass, a species common to that region of Argentina.
“Impact glass may actually trap and preserve remnants of past life,” Schultz said.
The fragile plant matter in these glass samples was exquisitely preserved down to the cellular level. Moreover, the glasses at times also preserved organic compounds as well, including remnants of chlorophyll and related pigments.
To understand how this plant material could have survived the scorching conditions that created the impact glass, Schultz and his colleagues attempted to replicate those conditions in the lab. They mixed pulverized impact glass with fragments of pampas grass leaves, heated the mixtures at different temperatures for various amounts of time, and then quickly cooled them.
The experiments revealed that plant material was preserved when the samples were quickly heated to more than 2,730 degrees Fahrenheit (1,500 degrees Celsius). The water in the exterior layers of the leaves apparently protect the inner layers in a way similar to deep frying, in which the food on the outside dries quickly while the inside cooks more slowly.
This glass could yield insights on environmental conditions at the time of these impacts, shedding light on the climate and life of ancient Earth. In addition, if the wreckage of impacts can preserve signs of life on Earth, it may well do so on distant planets such as Mars. Coincidentally, the soil conditions in Argentina that helped preserve these plant samples are not unlike those found on Mars.
“Marsis covered by dust deposits more than 2 kilometers (1.2 miles) thick in some areas,” Schultz said. “In Argentina, similar deposits of loess [windblown sediment] are 200 to 300 meters (650 to 975 feet) thick.”
Impacts on such dust deposits not only have a chance of melting matter in a way that can preserve signs of Martian life that may have lived billions of years ago, but the dust deposits can also serve as a soft cushion to capture such entombed life.
“The strategy would be to find the right type of impact glass that would most likely have trapped materials inside,” Schultz said. [The Search for Life on Mars (A Photo Timeline)]
Schultz cautioned this work does not mean one should expect to find signs of plants on Mars. Rather, scientists might want to look for relics of microbes in Martian impact glass.
“The next step is to understand the limits of preservation, to understand the conditions of trapping better, and to establish criteria for looking for similar materials on Mars,” Schultz said. “I’m very hopeful we can answer these questions with enough time.”
Schultz cautioned that the findings only apply to impact glass that stayed on the planet it was created, not to rocks blasted into space, as might be the case with meteorites originating from Mars.
“These impact glasses are typically pretty fragile and would break up, whether once launched at high speeds into space … or once they hit the surface at high speeds,” he said. “So, there is little bearing of these findings on life being delivered to the Earth, as yet.”
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In another study, researchers discovered the first known trace fossils of microbes unearthed from within an impact crater. These findings suggest that cosmic impacts could generate novel habitats for life in their blast zones.
Haley Sapers, an astrobiologist at the Canadian Astrobiology Training Program at McGill University in Montreal and at Western University in London, Canada, examined nearly 120 samples of impact glass from Nördlinger Ries, a 15-mile-wide (24-kilometer-wide) crater located in Bavaria, Germany. The energy required to create a crater like Ries is estimated to equal the power generated by 1.8 million atomic bombs, enough to melt many cubic miles of rock at this location about 14.6 million years ago.
“Near the center of the crater, there is a little city called Nördlingen, a double-walled medieval city that is about one kilometer (0.6 miles) in diameter, about the size of the impactor that created the crater,” Sapers said.
Sapers and her colleagues discovered unusual tubular features just one to three microns wide in these glasses, or roughly one-hundredth to three-hundredths the average diameter of a human hair. These formed after the glasses did.
Some of these tubes were straight, while others were curvy, wavy or spiraled. The investigators noted conventional mineral-forming processes could not readily explain the shapes and distribution of these tubes. Instead, they suggest these were formed by microbes etching the glass with organic acids.
“I believe they were etching the rock to extract elements they needed for their metabolism, such as iron,” Sapers said. “They were also creating habitats that could be quite protective that other microbes might have lived in.”
Sapers and her colleagues detailed their findings online April 10 in the journal Geology. Sapers emphasized these findings are from Earth microbes that colonized rocks after the impact.
“These aren’t bugs from space — they didn’t come from the meteor,” Sapers said.
The researchers suggest cosmic impacts on Earth may have created areas friendly to the origin of life.
“It’s interesting that 4.2 billion to 3.8 billion years ago, the early Earth experienced a period known as the Late Heavy Bombardment where there were a lot of impacts, including large impacts, and this period also overlaps with the evidence of the earliest life on Earth,” Sapers said. “One might ask why life arose during such an inhospitable part of Earth’s history. Maybe impact cratering had a role in the origin of life.” [Fly Over Earth's Best-Preserved Crater (Video)]
Impacts on a water-rich planet like Earth or even Mars can generate hydrothermal activity — that is, underwater areas boiling with heat. Seafloor hot springs known as hydrothermal vents more than a mile beneath the ocean’s surface can be home to thriving ecosystems on Earth, including giant tube worms 6 feet (2 meters) tall. The impact that created the Ries crater may have generated hydrothermal activity lasting as long as 10,000 years, giving microbes time enough to colonize the area.
Impacts could provide an otherwise cold, dead planet with heat and energy useful for life, Sapers said.
“Impact events occur not only on Earth, but on pretty much every other rocky and icy object in the solar system,” she said. “As far as we know, they’re the only ubiquitous geological process in the solar system.”
By studying impacts on Earth, scientists might get a better view of how life elsewhere might originate and survive.
“When thinking about future missions to Mars, this could suggest that an impact crater with mineral deposits associated with hydrothermal activity could be a very exciting astrobiology target,” Sapers said.
An international team of astronomers has discovered an exoplanet in the star Gliese 832′s “habitable zone” — the just-right range of distances that could allow liquid water to exist on a world’s surface. The planet, known as Gliese 832c, lies just 16 light-years from Earth. (For perspective, the Milky Way galaxy is about 100,000 light-years wide; the closest star to Earth, Proxima Centauri, is 4.2 light-years away.)
Gliese 832c is a “super-Earth” at least five times as massive as our planet, and it zips around its host star every 36 days. But that host star is a red dwarf that’s much dimmer and cooler than our sun, so Gliese 832c receives about as much stellar energy as Earth does, despite orbiting much closer to its parent, researchers said. [10 Exoplanets That Could Host Alien Life]
Indeed, Gliese 832c is one of the three most Earth-like exoplanets yet discovered according to a commonly used metric, said Abel Mendez Torres, director of the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo.
“The Earth Similarity Index (ESI) of Gliese 832c (ESI = 0.81) is comparable to Gliese 667Cc (ESI = 0.84) and Kepler-62e (ESI = 0.83),” Mendez wrote in a blog post today (June 25). (A perfect “Earth twin” would have an ESI of 1.)
“This makes Gliese 832c one of the top three most Earth-like planets according to the ESI (i.e., with respect to Earth’s stellar flux and mass) and the closest one to Earth of all three — a prime object for follow-up observations,” he added.
A team led by Robert Wittenmyer, of the University of New South Wales in Australia, discovered Gliese 832c by noticing the tiny wobbles the planet’s gravity induces in the motion of its host star.
They spotted these wobbles in data gathered by three separate instruments — the University College London Echelle Spectrograph on the Anglo-Australian Telescope in Australia, the Carnegie Planet Finder Spectrograph on the Magellan II telescope in Chile and the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph, which is part of the European Southern Observatory’s 11.8-foot (3.6 meters) telescope at the La Silla Observatory in Chile.
Gliese 832c is the second planet to be discovered around the star Gliese 832. The other one, Gliese 832b, was found in 2009; it’s a gas giant that circles much farther out, taking about nine years to complete one orbit.
“So far, the two planets of Gliese 832 are a scaled-down version of our own solar system, with an inner, potentially Earth-like planet and an outer, Jupiter-like giant planet,” Mendez wrote.
However, it’s unclear at the moment just how much Gliese 832c resembles Earth. Indeed, its discoverers think the newfound world may be more similar to scorching-hot Venus, with a thick atmosphere that has led to a runaway greenhouse effect.
“Given the large mass of the planet, it seems likely that it would possess a massive atmosphere, which may well render the planet inhospitable,” Wittenmyer and his team wrote in their paper, which has been accepted for publication in The Astrophysical Journal. “Indeed, it is perhaps more likely that GJ [Gliese] 832c is a ‘super-Venus,’ featuring significant greenhouse forcing.”
Huge Earth-like planets that have both continents and oceans may be better at harboring extraterrestrial life than those that are water-only worlds. A new study gives hope for the possibility that many super-Earth planets orbiting distant stars have exposed continents rather than just water-covered surfaces.
Super-Earths likely have more stable climates as compared to water worlds, and therefore larger habitable zones where alien life could thrive. In the new study, researchers used the Earth as a starting point for modeling how super-Earths might store their water on the surface and deep underground within the mantle. The work is detailed in a paper titled “Water Cycling Between Ocean and Mantle: Super-Earths Need Not Be Waterworlds” that was published in the January issue of The Astrophysical Journal.
Researchers typically expect super-Earths to exist as water worlds because their strong surface gravity creates relatively flattened surface geography and deep oceans. But the new study found that super-Earths with active tectonics can have exposed continents if their water is less than 0.2 percent of the total planetary mass. [The Strangest Alien Planets (Gallery)]
“A planet could be 10 times wetter than Earth and still have exposed continents,” said Nicolas Cowan, a planetary scientist at Northwestern University and co-author on the new paper. ”That’s important for what the planet looks like and how it ages.”
Cowan and Dorian Abbot, a climate scientist at the University of Chicago, built the model in the study. The model uses Earth as a starting point in defining how a planet’s water distribution could end up balanced in a steady state between the surface oceans and the mantle, which allows the researchers to calculate whether a super-Earth is likely to be a water world or not.
The movement of tectonic plates on Earth transfers water continuously between the surface oceans and the mantle. Ocean water enters the mantle as part of deep-sea rocks when one tectonic plate slides under another and goes down into the mantle.
“Earth is the only known planet with plate tectonics, a deep water cycle, etc., so it’s a good place to start,” Cowan said. “On the other hand, if it turns out that Earth’s deep water cycle is in nowhere near a steady-state, then our conclusions are way off the mark. “
Water in the mantle can re-enter the ocean when volcanic activity splits the planet’s crust at mid-ocean ridges. The loss of the crust causes a drop in pressure that leads the underlying mantle rock to melt and lose volatiles such as water. (An additional twist is that super-Earths with their stronger gravity could have greater seafloor pressure that suppresses the mantle’s loss of water, so that more of the planet’s overall water remains in the mantle.)
There are other uncertainties that could make a big difference in the model’s accuracy in predicting a super-Earth’s likelihood of having dry continents. One unknown is the amount of water hidden deep within Earth’s own mantle; Cowan and Abbot cite estimates of one to two oceans’ worth of water. Another factor is whether or not super-Earths have tectonic processes. If the researchers’ assumptions about either factor are wrong, that would change their model’s calculation of the “water world boundary,” which represents the mathematical model’s dividing point between water-worlds and worlds with dry continents.
Cowan and Abbot tried to compensate for the unknowns by drawing conservative conclusions with the results from their mathematical model. But even those conclusions suggest that super-Earths need not be water worlds.
“If some of our input parameters are wildly off, then the actual water world boundary might differ by an order of magnitude,” Cowan said. ”No matter how you cut it, though, the water world boundary is unlikely to be as damning as previously thought.”
The debate over super-Earths will continue until space missions begin collecting hard data on how much water exists on such planets. A space telescope with an interior coronagraph or exterior starshade could block the blinding light of distant stars to get a peek at orbiting planets. But no active space telescopes can currently do the necessary work of mapping the surface of super-Earths.
At the very least, you’d need a space telescope with a mirror a few meters wide, coupled to a starshade tens of thousands of kilometers away,” Cowan explained. “NASA is mooting this idea, but it is not the next priority.”
One space telescope that could fit the bill would be NASA’s Wide-Field Infrared Survey Telescope (WFIRST) — a planned 2.4-meter telescope with an instrument for imaging exoplanets. The $1.6 billion mission remains up in the air until NASA can squeeze it into the budget, but Cowan expects that WFIRST could get off the ground by the mid-2020s or 2030s. If so, that would bring researchers one step closer to understanding whether super-Earths truly work like our own world.
A NASA mission arriving in the Pluto system next year could help scientists figure out if the dwarf planet’s largest moon Charon might have once harbored a subsurface ocean of liquid water.
Researchers think it’s possible that the icy surface of Charon — Pluto’s largest moon — is cracked, which could, in turn, mean that its interior was once warm enough to support an ocean, NASA officials said. Two frigid moons with cracks, Saturn’s Enceladus and Jupiter’s Europa, have underground oceans beneath their icy shells. It’s possible that Charon resembled those two moons sometime in the past.
Charon probably cannot support a liquid ocean today. However, friction created by tidal forces earlier in the solar system’s history could have warmed Charon’s interior. NASA’s New Horizons mission, scheduled to reach Pluto and its moons in 2015, could help scientists learn if Charon was cracked and even wet in its early days.
“Our model predicts different fracture patterns on the surface of Charon depending on the thickness of its surface ice, the structure of the moon’s interior and how easily it deforms, and how its orbit evolved,” Alyssa Rhoden of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said in a statement. “By comparing the actual New Horizons observations of Charon to the various predictions, we can see what fits best and discover if Charon could have had a subsurface ocean in its past, driven by high eccentricity.”
At the moment, Charon is extremely cold like Pluto — the dwarf planet’s surface temperatures are expected to be around minus 380 degrees Fahrenheit (minus 229 degrees Celsius).
Scientists think that the tidal forces that warmed the interior of the moon could have been created if Charon had a very eccentric orbit sometime in the past. The forces would have caused telltale cracks in the moon’s surface.
Charon probably formed after a huge impact blasted pieces of Pluto off into space. Those pieces orbited Pluto, eventually coalescing into the moons, NASA officials said. At the moment, scientists have discovered four other moons orbiting the dwarf planet.
“Initially, there would have been strong tides on both worlds as gravity between Pluto and Charon caused their surfaces to bulge toward each other, generating friction in their interiors,” NASA officials said in a statement. “This friction would have also caused the tides to slightly lag behind their orbital positions. The lag would act like a brake on Pluto, causing its rotation to slow while transferring that rotational energy to Charon, making it speed up and move farther away from Pluto.”
The moon’s orbit around Pluto is very circular now, making it unlikely that Charon is still harboring a liquid ocean, Rhoden said. If Charon does have fractures — surface features that are relatively easy to get, according to Rhoden — it would help scientists learn a little more about the early history of the moon and Pluto’s interactions with it.
“Since it’s so easy to get fractures, if we get to Charon and there are none, it puts a very strong constraint on how high the eccentricity could have been and how warm the interior ever could have been,” Rhoden said. “This research gives us a head start on the New Horizons arrival — what should we look for and what can we learn from it. We’re going to Pluto and Pluto is fascinating, but Charon is also going to be fascinating.”
In 1975, physicist Kip Thorne and astronomer Anna Zytkow proposed the existence of odd objects that are hybrids between red supergiants and neutron stars — the collapsed, superdense remnants of supernova explosions.
These so-called Thorne-Zytkow objects (TZOs) likely form when a red supergiant gobbles up a nearby neutron star, which sinks down into the giant’s core, researchers said. TZOs look like ordinary red supergiants, like the famed star Betelgeuse in the constellation Orion, but differ in their chemical fingerprints, the theory goes.
“Studying these objects is exciting because it represents a completely new model of how stellar interiors can work,” study leader Emily Levesque, of the University of Colorado Boulder, said in a statement.
“In these interiors we also have a new way of producing heavy elements in our universe,” she added. “You’ve heard that everything is made of ‘star stuff’ — inside these stars we might now have a new way to make some of it.”
And now Levesque and her team say they have probably found the first TZO — a star called HV 2112 in the Small Magellanic Cloud, a dwarf galaxy that lies about 200,000 light-years away.
The researchers used the 6.5-meter Magellan Clay telescope in Chile to study the light emitted by HV 2112. They found the starlight to be highly enriched in rubidium, lithium and molybdenum, just as theory predicts for TZOs. (Normal red supergiants produce these elements as well, but not in such abundance, scientists said.)
The new data, while suggestive, do not represent a slam-dunk discovery for TZOs quite yet, researchers said.
“We could, of course, be wrong,” co-author Philip Massey, of Lowell Observatory in Flagstaff, Arizona, said in a statement.
“There are some minor inconsistencies between some of the details of what we found and what theory predicts,” he added. “But the theoretical predictions are quite old, and there have been a lot of improvements in the theory since then. Hopefully our discovery will spur additional work on the theoretical side now.”
The find means a lot to Zytkow, who is a co-author of the new study.
“I am extremely happy that observational confirmation of our theoretical prediction has started to emerge,” said Zytkow, who is based at the University of Cambridge in England. “Since Kip Thorne and I proposed our models of stars with neutron cores, people were not able to disprove our work. If theory is sound, experimental confirmation shows up sooner or later. So it was a matter of identification of a promising group of stars, getting telescope time and proceeding with the project.”
The newfound exoplanet candidate Kapteyn b, which lies a mere 13 light-years away, is about 11.5 billion years old, scientists say. That makes it 2.5 times older than Earth, and just 2 billion years or so younger than the universe itself, which burst into existence with the Big Bang 13.8 billion years ago.
“It does make you wonder what kind of life could have evolved on those planets over such a long time,” study lead author Guillem Anglada-Escude, of Queen Mary University of London, said in a statement. [10 Exoplanets That Could Host Alien Life]
Anglada-Escude was referring to Kapteyn b and its newly discovered sister world, Kapteyn c, which both orbit a nearby red dwarf known as Kapteyn’s Star. But only Kapteyn b, a “super-Earth” about five times as massive as our own planet, is thought to be potentially habitable; the larger Kapteyn c is likely too cold, researchers said.
The astronomers spotted both alien planets by noting the tiny wobbles their gravitational tugs induced in the motion of Kapteyn’s Star. These tugs caused shifts in the star’s light, which were first detected using the HARPS spectrometer at the European Southern Observatory’s La Silla Observatory in Chile. Further observations by two other spectrometers — HIRES at the Keck Observatory in Hawaii and the PFS instrument at Chile’s Magellan II Telescope — backed up the finds.
The team didn’t expect to find a possibly habitable world around Kapteyn’s Star, which is one-third as massive as the sun but so close to Earth that it’s visible in amateur telescopes, in the southern constellation of Pictor.
“We were surprised to find planets orbiting Kapteyn’s Star,” Anglada-Escude said. “Previous data showed some moderate excess of variability, so we were looking for very short-period planets when the new signals showed up loud and clear.”
Kapteyn b lies in the star’s habitable zone, the range of distances that could support liquid water — and thus, perhaps, life as we know it — on a world’s surface. The exoplanet completes one orbit every 48 days. The colder Kapteyn c is much farther out, circling the star once every 121 days.
Adding to the intrigue is the strange history of the Kapteyn system. The star originally belonged to a dwarf galaxy that our own Milky Way eventually absorbed and disrupted, researchers said, throwing Kapteyn and its planets into their speedy, elliptical orbit in the galactic “halo” — the region surrouding the Milky Way’s familiar spiral-armed disk.
The remnant of this gobbled-up dwarf galaxy is likely Omega Centauri, a globular cluster about 16,000 light-years away that contains many thousands of stars that are around 11.5 billion years old, researchers said.
“The presence and long-term survival of a planetary system seems a remarkable feat given the peculiar origin and kinematic history of Kapteyn’s star,” the researchers write in the new study, which will be published in the Monthly Notices of the Royal Astronomical Society. “The detection of super-Earth mass planets around halo stars provides important insights into planet-formation processes in the early days of the Milky Way.”
The new discovery is an exciting one that could inform the search for alien life throughout the galaxy, outside researchers said.
It suggests that many potentially habitable worlds will be found in the next years around nearby stars by ground-based and space-based observatories such as ESA’s PLATO mission,” said Richard Nelson of Queen Mary University of London, who was not a part of the study team. “Until we have detected a larger number of them, the properties and possible habitability of the near-most planetary systems will remain mysterious.”
Dubbed a “mega-Earth,” the exoplanet Kepler-10c weighs 17 times as much as Earth and it circles a sunlike star in the constellation Draco. The mega-Earth is rocky and also bigger than “super-Earths,” which are a class of planets that are slightly bigger than Earth.
Theorists weren’t actually sure that a world like the newfound exoplanet could exist. Scientists thought that planets of Kepler-10c’s size would be gaseous, collecting hydrogen as they grew and turning into Jupiter-like worlds. However, researchers have now found that the newly discovered planet is rocky, Christine Pulliam, a spokeswoman with the Harvard-Smithsonian Center for Astrophysics, wrote in a statement announcing the find. [The Strangest Alien Planets Ever Found (Gallery)]
“This is the Godzilla of Earths!” the CfA’s Dimitar Sasselov, director of the Harvard Origins of Life Initiative, said of Kepler-10c in a statement. “But unlike the movie monster, Kepler-10c has positive implications for life.”
The discovery of Kepler-10c was presented today here at the 224th American Astronomical Society meeting.
The mega-Earth orbits its parent star once every 45 days. Kepler-10c is probably too close to its star to be hospitable to life, and it isn’t the only orbiting the yellow star. Kepler-10 also plays host to a “lava world” called Kepler-10b that is three times the mass of Earth and speeds around its star in a 20-hour orbit.
NASA’s Kepler space telescope first spotted Kepler-10c, however, the exoplanet-hunting tool is not able to tell whether an alien world it finds is gaseous or rocky. The new planet’s size initially signaled that it fell into the “mini-Neptune” category, meaning it would have a thick envelope of gas covering the planet.
CfA astronomer Xavier Dumusque and his team used the HARPS-North instrument on the Telescopio Nazionale Galileo in the Canary Islands to measure Kepler-10c’s mass. They found that the planet is, in fact, rocky and not a mini-Neptune.
“Kepler-10c didn’t lose its atmosphere over time. It’s massive enough to have held onto one if it ever had it,” Dumusque said in a statement. “It must have formed the way we see it now.”
Scientists think the Kepler-10c system is actually quite old, forming less than 3 billion years after the Big Bang. The system’s early formation suggests that, although the materials were scarce, there were enough heavy elements like silicon and iron to form rocky worlds relatively early on in the history of the universe, according to the CfA.
“Finding Kepler-10c tells us that rocky planets could form much earlier than we thought,” Sasselov said in a statement. “And if you can make rocks, you can make life.”
The new finding bolsters the idea that old stars could host rocky Earths, giving astronomers a wider range of stars that may support Earth-like alien worlds to study, according to the CfA. Instead of ruling out old stars when searching for Earth-like planets, they might actually be worth a second look.
It’s also possible that exoplanet hunters will find more mega-Earths as they continue searching the universe. CfA astronomer Lars A. Buchhave “found a correlation between the period of a planet (how long it takes to orbit its star) and the size at which a planet transitions from rocky to gaseous,” meaning that scientists could find more Kepler-10c-like planets as they look to longer period orbits, according to the CfA.
Astronomers have so far confirmed the existence of more than 1,000 planets beyond our solar system with the aid of NASA’s Kepler spacecraft and other telescopes. They are investigating thousands more candidate worlds to see if they, too, are exoplanets, or extrasolar planets.
About 75 percent of all known exoplanets discovered by the Kepler space observatory are less than four times Earth’s diameter. However, despite their sheer abundance, the compositions of such planets are largely unknown.
“We don’t have any planets in our system between one to four times the size of Earth, so we’d really like to find out what they are,” said lead study author Lars Buchhave, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
Rocky or not: What’s inside that alien planet?
The available evidence suggests these worlds range in composition from small, high-density rocky planets with thin envelopes of gas like Earth, Mars, Venus and Mercury to bigger, low-density planets consisting of rocky cores with thick envelopes of hydrogen and helium gas. However, to deduce what the compositions of these exoplanets might be, astronomers need to know their densities, which entails knowing both their sizes and masses, and while scientists can now readily observe what size an exoplanet is, “determining the mass of a planet is very difficult,” Buchhave said.
To learn more about this unknown range of exoplanets, scientists tried looking at their stars. They focused on metallicities — that is, the abundances of elements heavier than hydrogen and helium — of more than 400 stars hosting 600 candidate exoplanets.
The investigators discovered these remaining exoplanets could be placed into three groups based on how metallic their stars were. The greater the average metallicity of a star, the larger its planets usually were.
The investigators noted the metallicity of a star was a hint of how much solid material initially existed in the protoplanetary disks of gas and dust surrounding them. The greater the metallicity of a star, “the more rocky cores of planets form quickly and have time to accrete hydrogen-helium envelopes and become big planets,” Buchhave told Space.com.
How planets are made
The researchers suggest these three groups reflect three different kinds of planetary composition. Exoplanets less than 1.7 times Earth’s diameter are predominantly rocky planets. Worlds between 1.7 and 3.9 as wide as Earth are so-called gas dwarf planets with rocky cores and envelopes of hydrogen and helium gas. Planets larger than 3.9 times Earth’s diameter are gas giants like Jupiter and Saturn or ice giants like Uranus and Neptune.
The scientists also predicted that planets farther from their star could be large rocky planets without thick atmospheres of hydrogen and helium. Their analysis not only confirmed this prediction, but also suggested that gigantic planets could form with relatively thin atmospheres. “These would be massive, massive rocks,” Buchhave said.
In the future, Buchhave noted the Transiting Exoplanet Survey Satellite (TESS), a space telescope planned for NASA’s Explorer program, “could discover a large number of exoplanets orbiting bright stars, making them amenable to subsequent mass measurement and thus helping to find out what they are made of.”
Buchhave and his colleague detailed their findings in the May 29 issue of the journal Nature.
The Hubble Space Telescope is famous for finding black holes. It can pick out thousands of galaxies in a patch of sky the size of a thumbprint. The most powerful space telescope ever built, the Hubble provided evidence that the universe is slowing down in its infinite rush into whatever lies beyond.
But Hubble’s cosmic firepower was recently put to a new purpose: searching for a billowing cloud of water vapor on Jupiter’s moon Europa. The plumes are a sign that extraterrestrial life could be lurking within our own solar system. Before we head way out there, we need to know a little about the eruptions happening at home.
On Earth, the plumes are a hallmark of energy in motion. Here, active geology often takes the form of pyroclastic eruptions. Pyro is Greek for “fire,” while “clastic” derives from “broken.” Pyroclastic eruptions feature solid rock, semi-solid fragments and hot gases expelled from the mantle through areas of weakness in the crust.
Pyroclastic eruptions create the plumes of ash and smoke we typically associate with volcanos. Even when volcanos are underwater, as many are, they send up steaming columns of lava fragments, bits of rock and heated gas. These underwater plumes of hot material rise hundreds of meters. The heated underwater plumes that make it to the surface of the ocean can be seen from space.
While these displays are impressive, not all that explodes from the Earth’s crust is pyroclastic. Geysers are long columns of water. Their bases lie close enough to the mantle to be heated by its 1,000° C (1,832° F) temperatures. The heated water expands and rises, forcing its way to the surface. Once the water and steam reach the surface, the pressure falls, as does the plume of vapor, after inertia shoots it briefly into space.
In all of these formations, heated gases escape from the interior and reach the surface. There, they rapidly expand and cool, dissipating the fierce energies that drove them to erupt. In this way, volcanism reflects the build-up of pressure within a planet or other large body on which it is known to occur.
In March of 2006, geysers were discovered spewing waterfromthe surface of Enceladus, one of Saturn’s icy moons. Thus began a race to explain how a moon with surface temperatures of -330° Fahrenheit (-201° Celsius)could have active geology, and to discover if those geysers could signal a warm core for Enceladus and other icy moons.
Plumes: Cold and Far Away
Cold cryoclastic plumes may play a key role in finding superhabitable zones in areas around gas giants-places where tidal forces create enough heat to sustain life. Nonetheless, they differ from traditional volcanism in many respects. One respect is temperature.
“Much of what we see taking place or see evidence of having taken place looks like lava and sort of normal volcanism on Earth except it involves warm water,” said Bruce Marsh, a geophysicist at the Department of Earth and Planetary Sciences, Johns Hopkins University.
Warm, in this case, is relative. Cassini, a NASA satellite, noticed that the area around these vents is substantiallywarmerthan the surrounding ice: as high as -120o Fahrenheit (190 Kelvin).Where pyroclastic explosions emit fragments of broken fire, cryoclastic eruptions are bursts of icy material that are hundreds of degrees warmer than the surrounding surface, but still well below freezing. [See photos taken by the Cassini spacecraft]
Another point of difference between Earth’s plumes and icy world plumes is contents. Enceladus’ plumes spew water vapor and dust, which rapidly disperse into surrounding space. Then, there’s the point of origin. There are no volcanoes on Enceladus. Instead, these plumes originate from vents in the southern polar terrain, also known as “tiger stripes.” Now, at least two of the ingredients necessary for life are present on Enceladus: water and warmth.
Unfortunately, the existence of relatively warm vents and escaping water are insufficient to prove that active heating is occurring deep within the moon, or that an ocean lies within its ice sheet. Local radioactivity and flexing forces from Saturn may be creating underground liquid reservoirs in the regions around the vents. That would raise temperature in just that region, melting ice that could then pool into the cracks. If this were true, water vapor plumes on Enceladus might originate relatively near the surface, instead of from a life-breeding ocean.
The discovery of the plume on Enceladus was sufficient to prove that icy moons undergoing tidal forces are geologically active. It was a good sign that we should look for others.
As it happens, there is another icy moon, namely Europa, circling a closer planet, that has all of ingredients for life as well. It also has an extremely thick ice sheet, a saltwater signature and possibly an enormous ocean with twice as much water as all of the oceans on Earth combined. Organic material has been found on the surface. The surface itself is known to undergo continual remodeling, bringing organics down and any subsurface life-signatures up. Many scientists believed that if plumes could be found on Europa, that would be a portent that we should head there to look for further signs of life.
Until recently, though, none had been seen. It turns out that finding a geyser 500,000 miles away is tricky business. How the Hubble happened to be looking right at Europa the moment a plume exploded is more than pure coincidence.
Pretty Elusive Eruptions
“Plumes on Europa could be tough to catch in the act,” said Lynnae Quick, a planetary scientist and postdoctoral fellow at NASA’s Goddard Institute of Space Studies. “If observing at visible wavelengths, there have to be enough particles (in the plume) in order for it to be seen. Icy satellites (like Europa and Enceladus) have very bright, reflective surfaces. The light reflected off of these surfaces can obscure plumes from being seen. For this reason, you want to look at the limb.”
Looking at the limb of a planetary body requires a perpendicular point of view. It’s not a natural viewpoint for humans. We tend to look straight an object of interest, focusing on the center of it.
Imagine examining a tree. The natural thing to do is to look directly at the trunk. The limbs of a tree are everywhere, scattered in all directions at the periphery of your visual field. The tiniest tree limbs are nearly impossible to see without standing close to the tree and craning your neck. In the case of Europa, standing close isn’t an option. That’s one reason a really big telescope is needed.
Considering the tree analogy, another problem becomes obvious: tree limbs are only visible against a different-colored background, like a blue sky or white clouds. Europa’s surface is made of water. The plumes are too.
“On Europa, plumes are very hard to see because you are putting ice crystals on top of an icy surface,” said Louise Prockter, a superviser at Johns Hopkins University’s Applied Physics Laboratory’s planetary exploration group.
Looking at the limb of Europa, where the edge of the moon meets the dark space of space is like dropping a black curtain in the background. That’s a better contrast than the white ice. However, there’s another problem.
“The problem with [water] plumes is that they are very tenuous,” said Prockter, “We’ve seen plumes off the limb of Io (Jupiter’s innermost Galilean moon) because they tend to be pretty fierce and pretty long lasting. They caused massive changes on the surface on the time scale that Galileo was there.”
On smaller Enceladus, plumes remain suspended for a time. By contrast, the expulsion of water vapor into the near-vacuum surrounding Europa is short-lived. Europa’s more substantial gravity yanks the frozen water spouts back toward the surface. Ice lands on ice, leaving barely a trace. So the only chance to catch a plume on Europa is to look at the disc and the limb, and keep looking until one happens right in front of us.
The Hubble Space Telescope is currently the only tool powerful enough to crane its neck at just the right angle to catch a transient puff of the water vapor 400 million miles away. Even so, Hubble hunted for a plume on Europa for years without success. It was aimed at Europa in October 1999, November 2012 and December 2012 before finally catching one using its spectrograph.
Caught on Camera in invisible light
A spectrograph is an instrument that separates light into color components by wavelength. Spectrographs are invaluable instruments in astronomy. Objects in space that can’t be seen with visible light can still be detected by the radiation around them. Hubble’s Space Telescope Imaging Spectrograph (STIS) allows it to detect black holes. In Europa’s case, the STIS picked up the plume’s vapor cloud by its ultraviolet light, which is just beyond what’s visible to the eye.
In the ultraviolet spectrum, the plume of water vapor seen by Hubble Space Telescope in December 2012 extended more than 125 miles (200 kilometers) into space.
“I’m really excited about these observations because they seem to suggest that some areas of Europa’s crustare being intensely heated to create the vapor,” said Jason Goodman, assistant professor of physics at Wheaton college, who has published research on hydrothermal plumes on Europa.
“We can’t say for sure yet whether this is a cyclical process or a chance event, or whether the vapor is coming from warm but solid ice, partially-melted ice, or from the ocean, but it’s definitely a sign that Europa has some exciting internal activity going on right now.”
Putting On the Gloves
Even with the Hubble Space Telescope hard at work, we are uncertain how water gets just below the surface of Europa. We have many models about how, and quite a few notions about when. For example, at the farthest point, or apocenter, Europa experiences tensions that may cause plume production at weak spots in the ice. [See amazing photos from the Hubble Space Telescope]
“At apocenter, the surface fractures in the south polar region experience tension. This tension might open the cracks and allow the water vapor to escape from a subsurface liquid reservoir,” said Lorenz Roth, first author on the Science paper that described the plume’s discovery. “Confirmation of the initial detection and the proposed connection to Europa’s orbital position is crucial now.”
Using Hubble to look for further plumes during the entire orbit will bring us closer to some answers. However, we won’t truly understand what’s going on inside Europa’s salty oceans, possibly warm for the last 4 billion years, until we take a much, much closer look.
“More Hubble data will help,” said Goodman. “But this vapor plume is right at the limit of what telescopes near Earth can see. To really get to the bottom of this story, we need to send a spacecraft to Europa.”
With plumes, water vapor bursts through the ice shell and arches away at amazing speed. The resulting ballistic arc of freezing droplets can be seen from half a million miles away. Better still, they can be sampled without having to land, drill, melt or dig.
“What this means is that we can now go and sample the subsurface by flying through a plume,” said Prockter. “There’s a very good chance that we can sample the material in five to 10 or 15 years.”
Now that we know when and where Europa is active, a mission can be launched with plumes specifically in mind.
“A spacecraft in a low enough orbit could fly through the plume,” said Quick.
A plume is a million pieces of Europa’s interior. These pieces might be from the depths of the sea or from just below the surface. Either way, to reach into the ice of a faraway moon, all we have to do is catch a plume.
“That’s really exciting. Europa comes to us,” said Prockter. “I can’t think of anywhere better for life than Europa.”
The most massive and luminous stars were long suspected to explode when they die, and astronomers now have the most direct evidence yet that these cosmic behemoths go out with a bang.
These findings shed light on the star explosions that provide the universe with the ingredients for planets and life, the researchers added.
With a mass more than 330,000 times that of Earth, the sun accounts for 99.86 percent of the solar system’s total mass. But as stars go, the sun is a lightweight. The largest and most luminous stars in the universe are Wolf-Rayet stars, which are more than 20 times as massive as the sun and at least five times as hot. Only a few hundred of these titan stars are known to astronomers. [Biggest Star Mysteries of All Time]
The intense heat of Wolf-Rayet stars forces their matter apart, making them extraordinarily windy stars. They usually lose the mass equivalent to that of the Earth each year, blowing winds at up to 5.6 million mph (9 million km/h).
How giant stars die
Astronomers long suspected that Wolf-Rayet stars violently self-destructed as supernovas, the most powerful stellar explosions in the universe. These outbursts are bright enough to momentarily outshine their entire galaxies, and enrich galaxies with heavy elements that eventually become the building blocks for planets and life.
However, the gigantic amounts of matter these stars blow out usually obscure them completely, so scientists weren’t sure how they form, live and die.
“Finding what kind of star exploded, after it already exploded, is, of course, a hard problem, since the explosion destroys much of the information,” said study author Avishay Gal-Yam, an astrophysicist at the Weizmann Institute of Science in Israel.
Some researchers even raised doubts as to whether Wolf-Rayet stars detonated as supernovas at all. “Some modelers predict that massive Wolf-Rayet stars will collapse into a black hole ‘quietly,’ without making a luminous supernova,” Gal-Yam told Space.com.
Now, for the first time, scientists have direct confirmation that a Wolf-Rayet star died in a supernova. They detail their findings in the May 22 issue of the journal Nature.
The researchers focused on a supernova named SN 2013cu, which exploded about 360 million light-years away from Earth in the Bootes constellation. This explosion was a Type IIb supernova, meaning it took place after the core of its star ran out of fuel, collapsing into an extraordinarily dense nugget in a fraction of a second and rebounding with a blast outward. What is left over after such supernovas is either a neutron star or a black hole.
A Wolf-Rayet smoking gun
By surveying the sky with the intermediate Palomar Transient Factory (iPTF), a project that charts the sky with a telescope mounted with a robotic observing system, the researchers discovered the supernova very soon after it happened.
“We now send high-quality supernova alerts to astronomers all around the globe in less than 40 minutes,” said study co-author Peter Nugent, a researcher at the University of California, Berkeley.
The scientists next rallied ground- and space-based telescopes across the world to observe the infant supernova approximately 5.7 hours and 15 hours after it detonated.
“Newly developed observational capabilities now enable us to study exploding stars in ways we could only dream of before,” Gal-Yam said. “We are moving towards real-time studies of supernovae.”
The explosion ionized surrounding molecules in an ultraviolet flash, giving them an electric charge. The ionized material that surrounded the star emits light that “tells us the elemental composition of the wind, and hence the surface composition of the star as it was just before it exploded,” Gal-Yam said. “That is a very powerful clue about the nature of the exploding star and how it evolved before it exploded, and this is the first time we managed to get this information.”
That opportunity lasts only for a day before the supernova blast wave sweeps the ionization away, Gal-Yam added.
This light suggested the precursor of the supernova was a nitrogen-rich Wolf-Rayet star. “This is the smoking gun,” Nugent said. “For the first time, we can directly point to an observation and say that this type of Wolf-Rayet star leads to this kind of Type IIb supernova.”
“When I identified the first example of a Type IIb supernova in 1987, I dreamed that someday we would have direct evidence of what kind of star exploded,” said study co-author Alex Filippenko, a researcher at the University of California, Berkeley. “It’s refreshing that we can now say that Wolf-Rayet stars are responsible, at least in some cases.”
Future studies could analyze more Wolf-Rayet stars, to see if these violent deaths are standard for them.
“If we can show that this is the norm for such massive stars, it would mean that new theories will have to be developed to explain how you can make a black hole and still throw out a lot of material and a lot of energy to make a luminous supernova,” Gal-Yam said.
Quaoar is a planetoid that lies beyond Pluto’s orbit in the solar system. Its discovery in 2002, as well as subsequent discoveries of other small worlds, led to a new classification and the redefinition of Pluto as a “dwarf planet.” Quaoar is probably massive enough to be considered a dwarf planet, but it has not been classified as such yet.
Quaoar lurks in the Kuiper Belt, a group of icy objects beyond Neptune. It is about 42 astronomical units, or Earth-sun distances, away. That’s about 4 billion miles (6 billion km) — a billion kilometers more distant than Neptune. It takes about 288 years for Quaoar to go once around the sun in a roughly circular orbit.
Mike Brown and Chadwick Trujillo of the California Institute of Technology found Quaoar using a 48-inch telescope at the Palomar Observatory in California. It showed up as an 18.5-magnitude object moving against the background stars in the constellation Ophiuchus. They suggested the name Quaoar [pronounced kwa-o-ar] after a creation god of the Native American Tongva tribe — the original inhabitants of the Los Angeles basin where Caltech is located. According to legend, Quaoar “came down from heaven; and, after reducing chaos to order, laid out the world on the back of seven giants. He then created the lower animals, and then mankind.”
Around the time of its discovery, the Hubble Space Telescope measured the object and found that its diameter was about 800 miles (1,300 kilometers) making it larger than the asteroid Ceres but smaller than Pluto. (Subsequent measurements have found it was slightly less than that, at about 1,092 km or 679 miles.)
“Quaoar is greater in volume than all known asteroids combined. Researchers suspect it’s made mostly of low-density ices mixed with rock, not unlike the makeup of a comet. If so, Quaoar’s mass is probably only one-third that of the asteroid belt,” NASA wrote at the time.
In 2007, Brown announced that Quaoar had a tiny moon. Named for the sky god Weywot, son of Quaoar, the moon is estimated to have only one two-thousandth the mass of its parent. Weywot is likely the result of a collision with Quaoar and another object, Brown wrote.
Red and icy
Quaoar is believed to be red and also coated in water ice, and that in the distant past it had an atmosphere with carbon monoxide, nitrogen and methane.
Some of these molecules bled away into space since the object’s gravity was so small, but methane stayed put. Radiation from the sun then gradually formed hydrocarbon chains out of the carbon and hydrogen atoms that make up methane. This process made the planet appear red in telescopes.
One research team in 2004 published research suggesting that Quaoar had or has volcanic activity because of the ammonia hydrate and water ice found on its surface. Radiation should have destroyed these particles in a few million years, which is a fraction of time compared to the solar system’s age.
“We conclude that Quaoar has been recently resurfaced, either by impact exposure of previously buried ices or by cryovolcanic outgassing, or by a combination of these processes,” the scientists wrote in their paper, which was published in Nature. More research will be needed to confirm this, they added. The study was led by David Jewitt of the University of Hawaii’s Institute for Astronomy.
Brown also led the teams that discovered several other dwarf worlds in the 2000s, such as Haumea, Eris and Sedna. These small worlds made astronomers realize that bigger objects lurked beyond Pluto than previously believed. In 2006, the International Astronomical Union met to determine how to define a planet, since some were wondering what to call these large new objects.
The IAU determined that for an object to be a “planet,” it must be big enough to be round, it must orbit the sun without orbiting another body and it must clear debris around its orbit. “Dwarf planet” meant that the object was round and orbited the sun, but it was too small to clear debris.
“Modern science provides much more information than the simple fact that objects orbiting the sun appear to move with respect to the background of fixed stars,” the IAU wrote in its justification for the change.
“For example, recent new discoveries have been made of objects in the outer regions of our Solar System that have sizes comparable with and larger than Pluto. Historically Pluto has been recognized as the ninth planet. Thus these discoveries have rightfully called into question whether or not the newly found Trans-Neptunian Objects should also be considered as new planets.”
There’s debate about whether Quaoar is actually round. It’s hard to spot because it’s far out in the solar system and it’s small, making it difficult to estimate its size. Brown says the planet would qualify as theoretically speaking, it has the mass required to be round. The IAU, however, only lists a few dwarf planets at this time: Ceres, Pluto, Eris, Makemake and Haumea.
Scientists spoke about the independent models at a news conference last month at a meeting of the American Physical Society in Savannah, Georgia. One simulation models some of the most energetic explosions in the universe in three dimensions for the first time, revealing why some produce black holes and oddly shaped clouds of material. You can watch an animation on how supernovas mark black holes on Space.com.
“We believe this could cause some of the asymmetries astronomers see when they observe supernova remnants,” Christian Ott of the California Institute of Technology said at the conference. Ott worked with a team of scientists led by Philipp Mösta, also of CalTech.
The other simulation provides an amazing detailed map of the growth and development of the universe since it burst into existence with the Big Bang 13.8 billion years ago.
“At the end, we got a simulated universe, and it’s something we can compare to observations,” Rupert Croft of Carnegie Melon said. Croft was part of a team led by Nishikanta Khandai of the Brookhaven National Laboratory in New York.
Magnetic fields in supernovas
Stars with masses ranging from 10 to 100 times more than the sun can end their lives in violent explosions known as supernovas. The expelled material creates beautiful objects known as supernova remnants (SNRs), combinations of stellar material and shock waves pushing outward through space. Most supernova remnants are fairly spherical, but some, such as SNR W49B, are oddly shaped.
Until now, models of these explosions have remained two-dimensional, producing results that can dramatically differ from observations. A new model that focuses on the interactions of the magnetic field with the supernova creates a less symmetrical result similar to several SNR that have been seen in the universe.
“We’re trying to understand how some of the most energetic explosions in the universe work,” Ott said. “For the first time, we’re able to simulate it in three dimensions.”
According to the new simulation, a rapidly spinning star with a collapsing strong magnetic field will find its field lines getting wound up around the spin axis. The wound-up field becomes unstable due to a process known as “kink instability,” in which the magnetic forces within the plasma become uneven, with the forces on the interior growing larger than those on the exterior.
Initially, inward-pressing gas dominates the star, but over time the magnetic field grows stronger. Regions of high magnetic pressure begin to press outward, ultimately causing the star to explode.
In previous two-dimensional models, jets of streaming material drive the explosion, pushing outward symmetrically along the axes. But the more detailed 3D models show the magnetic fields disrupting and fizzling the jets, creating a wide, double-lobed flow.
Such explosions may be more likely to leave behind a black hole rather than the dense neutron star scientists suspect is created by most core-collapse supernovae. In these more common, less-extreme cases, the magnetic field plays a minimal role in the stellar explosion.
The process itself isn’t unfamiliar, but the application to a supernova is.
“This is physics we’re familiar with,” Ott said. “It’s just the first time we’ve seen it in a supernova because we’ve been able to simulate it in three dimensions.”
The findings were published in the Astrophysical Journal Letters.
The complexity surrounding the evolution of the universe can make modeling the process a daunting task. But a new simulation resolves the growth of the universe down to the smallest galaxies, making it the most detailed of its kind.
The new model also supports the idea that the universe grew incredibly quickly shortly after its birth, expanding faster than the speed of light during a period of dramatic cosmic inflation.
“We’d like to simulate the whole universe, and we started with the Big Bang,” Croft said. [The Big Bang to Now in 10 Easy Steps]
It took about 45 million core hours on a supercomputer for the simulation to map out the basic evolution of the universe. Scientists can compare the millions of galaxies produced in the simulation to those that exist in the known universe, finding a statistical measurement of those objects and others. They can also study the universe at various stages in its growth.
Scientists ran the first full simulation of the evolution of the universe in the late 1970s, with several following in the years since. Each model improved as knowledge about the universe grew — original models didn’t include supermassive black holes or dark matter, for instance, because scientists didn’t know of their existence.
Evolving technology allowed for greater detail to be examined in each subsequent study. The first paper could only create super-clusters of galaxies. The latest model shows just how far the process has come.
“We’re actually able to resolve the tiniest galaxies that we can see,” Croft said, referring to dwarf galaxies, which contain less than 1 percent as many stars as the Milky Way.
The simulations are available online, and more than 2,000 scientists have already put them to use, downloading them for a variety of research projects.
By comparing the results of the simulation with observational evidence, scientists can get an understanding of how well the theories that went into the simulation function when forming a universe.
Croft emphasized the model’s role when it comes to understanding inflation. If inflation did not play a significant role in the evolution of the universe, the result should have been widely different from the universe observed today. Instead, the theoretical model matches up fairly well with reality.
“This shows us very directly that inflation seems to be correct,” Croft said.
When paired with recent findings on primordial gravity waves, the case for inflation seems very strong.
“We don’t have to investigate other ways of making our initial conditions,” Croft said. “This simulation seems to fit the conditions fairly well.”
The findings have been submitted for publication to the Monthly Notices of the Royal Astronomical Society and are available online as well.
Binary stars dampen each other’s solar radiation and stellar winds, thereby creating a more hospitable environment for life and increasing the habitable zone around such solar systems, according to research presented at the 223rd American Astronomical Society meeting in January.
“The two stars calm each other down in terms of activity,” said Paul Mason, an astrophysicist at the University of Texas at El Paso in an interview with Astrobiology Magazine.
Mason presented the results of a study, which used data collected by NASA’s Kepler spacecraft mission to discover potentially habitable exoplanets in our region of the Milky Way galaxy.
Stretching the habitable zone
Although more than a thousand planets have been found outside of the solar system, as well as a host of candidates waiting for follow-up observations, no moons have yet been confirmed. Scientists like Mason are performing theoretical calculations to determine which solar systems might be better for hosting potentially habitable moons.
Violent and active young stars spin rapidly, emitting radiation and stellar winds that could interfere with the habitability of planets and moons nearby. A close binary system of stars can help to dampen these effects, as the two stars synchronize their spins.
Binary stars exist in a range of configurations. Some are widely separated, so that a planet in orbit around one functions much like a planet around a single sun, while the companion is so distant that it appears as point-like as any other star. Others may be extremely close together, synching together to keep each other rapidly spinning for billions of years.
Mason’s research focuses on pairs of stars that orbit each other between 10 and 60 Earth-days, with a planet in orbit around both suns. These are known as circumbinary systems. The paired stars exert tidal forces on one another that cause a slowdown in spin, weakening the radiation and stellar wind of the pair faster than they would suffer as single stars. Fast-moving stellar winds can strip a moon or planet of its atmosphere, leaving it open to heavy radiation bombardment that can interfere with the development of life.
At the same time, the combined light from the duo pushes the edge of the region where water can exist, commonly termed the “habitable zone,” farther back than it would lie around a single star. Moving the entire zone a greater distance from its sun further reduces the negative effects from the stars. [The Strangest Alien Planets Ever (Gallery)]
“The habitable zone in a binary system is a little bit farther away, simply because you have the light from two stars rather than the light from one,” Mason said.
This distance is important because, if a planet orbits too close to its parent star, its moon can be stripped away completely.
“The closer a planet is to the star, the smaller its gravitational sphere of influence,” said David Kipping and astronomer at the Harvard-Smithsonian Center for Astrophysics in an interview with Astrobiology Magazine.
“Essentially, the star will rip off the moon if it gets too close,” he said.
Kipping, who was not involved in the research, searches for exomoons and is the principle investigator of The Hunt for Exomoons with Kepler project.
Pushing exomoons farther away also has ramifications for red dwarfs, the most populous stellar type in the galaxy. The habitable zone around these smaller, long-lived stars is so close to its parent star that stellar activity made many astronomers consider habitable planets around them unlikely to even exist, though recent researchhas increased the potential. In a binary system, the pushed-back habitable zone would decrease many of the negative effects that limit habitability around the plentiful stars.
According to Mason, if the sun had a companion star, the makeup of the solar system would change significantly. The water stripped from the atmosphere of Venus would likely still be present, making it potentially habitable. Earth itself could have been a very different environment.
“Earth would be a wetter planet if we were orbiting a binary star,” Mason said.
When it comes to potentially habitable exomoons, sun-like stars are always better.
“The ideal circumstance is solar twins,” Mason said.
Simply adding a sun-like star to the mix improves the chances for life.
“Solar-type stars with companions work really well,” Mason said. “They work better than our own solar-type star without a companion.”
Still, in a circumbinary system, it is not the type of star that matters nearly as much as how often they orbit one another. As long as the pair of stars dance around between once every 10 and 60 Earth-days, they increase the chances of the habitability of their moons and planets. (The exception is massive, giant stars, which burn through their fuel and die quickly, giving life little to no chance to evolve.)
Still, if at least one of the two stars in a binary system is sun-like, it provides a very wide habitable zone with plenty of room for water, a situation that Mason says he’s most excited about.
A wider habitable zone means a better chance of hosting planets capable of sustaining life, as well as exomoons that could support it.
“There’s plenty of room for several habitable planets,” Mason said. “These may be places where many worlds in system could be habitable.”
Unfortunately, systems with multiple planets make finding exomoons more challenging at the moment.
The hunt for exomoons
Astronomers hunt for distant moons essentially the same ways that they hunt for distant planets. They might watch for the planet and its moon to cross between their sun and the Earth. As a single planet crosses, it causes a dip in brightness; if a moon precedes or follows it, that dip is preceded or followed by a smaller dip as just the moon blocks the stellar light.
They can also watch a planet for a small wobble as the moon gravitationally tugs its host ever-so-slightly. A moon might also slightly change how quickly a planet orbits its sun.
In a multi-planet system, however, other planets can also cause the wobbles and velocity changes, making them “kind of a pain for looking for exomoons,” according to Kipping.
Kipping and his team have pared down the list of almost 5,000 planetary candidates detected by NASA’s Kepler spacecraft to approximately 250 bodies considered the best targets for hosting an exomoon.
Originally, he hoped to target Jupiter-size and larger planets. In the solar system, the only moons considered potentially habitable orbit gas giants. Earth-sized moons could lay outside of the habitable zone of a star but still hold liquid water due to tidal heating from their planet. Such moons would be minimally affected by their orbit around a binary star system instead of a single star.
But it was not to be.
“One of the most staggering discoveries from Kepler is that Jupiter-like planets are rare,” Kipping said. “This is kind of a shame for the exomoon hunt, because those are the planets easiest to find a moon around.”
Instead, Kipping and his team have turned to the slightly less massive sub-Neptunes, which abound in Kepler’s field of view.
Although moons around Earth-sized planets may not be habitable by themselves, they could have an enormous impact on their parent body. Born of a collision early in the life of the solar system, Earth’s moon is far larger by comparison to its planet than other moons in the solar system. The collision might have kicked off volcanism and plate tectonics on the early Earth, while the moon stabilizes the planet’s tilt and controls the tides. Biologists consider all four actions important to the evolution of life.
“There are many beneficial qualities to having a big moon nearby,” Kipping said. “If we find Earth 2.0, one of the first things we’ll be asking is if it has a Moon 2.0.”
Because our moon is unique in the solar system, scientists don’t yet understand whether its formation was what Kipping called “a freak event” or something that’s very common. Detecting different kinds of moons in a variety of orbits will help scientists narrow down how unique the solar system and the Earth-moon system are.
As scientists have used Kepler to hunt for exoplanets, they have learned a great deal about the variety of planetary systems in the galaxy. According to Kipping, circumbinary systems were once thought to be “the more exotic type” of binary systems. But Kepler revealed several cases, showing them to be fairly common.
Though the mission goal is to detect planets, Kepler is an important tool when it comes to looking for moons outside of the solar system.
“Kepler is really the ideal instrument for detecting exomoons,” Kipping said.
He pointed out that while NASA’s upcoming James Webb Space Telescope, to be launched in October 2018, will be ideal for follow-up observations, it will be in too high of a demand by the astronomical community to stare at a patch of sky for years on end the way Kepler did. If Kepler’s second run, K2, is funded, it will stare at a different region of the sky than the original Kepler mission, and will provide greater insight into planet populations.
Though he expressed excitement about the exoplanet discoveries that will come from both upcoming missions, he said, “My feeling is that Kepler is still probably the best resource for discovering exomoons.”
As to how long it might be before the first exomoon is confirmed around a distant planet, Kipping said that it depends on how common larger moons are.
“If Nature builds big moons—Earth-sized moons—very frequently throughout the cosmos, then they’re in the Kepler data,” he said. “They’re lurking there, and we will find them in the next year or two.”
If, however, most of the moons are small, like the ones in orbit around Neptune and Uranus, they may never be seen.
If moons large enough to be detected exist, Mason is confident that the most habitable of them can be found in circumbinary systems.
“Exomoons in binary systems may be more habitable than around single stars,” he said in his presentation at the AAS meeting. “Maybe less common, but potentially more habitable.”
The stars in the night sky shine in myriad hues and brightnesses—piercing blues, clean whites, smoldering crimsons. Every star has a different mass, the basic characteristic that determines its size, lifespan, light output and temperature (which we discern as a particular color).
Yet when it comes to the existence of life, we know with certainty of only a single star—a toasty, yellow-whitish one, our Sun—that has permitted the rise of life on an encircling world. Astrobiologists are quite convinced, though, that life can also develop on planets orbiting smaller, cooler stars.
But what about stars with light more intense than our Sun’s? A new paper, accepted for publication in the International Journal of Astrobiology, examines some of the fundamentals for life arising around a class of slightly heftier, hotter stars known as F-type main-sequence stars. (Stars in the main-sequence are in “full bloom,” so to speak, and like our Sun, fuse hydrogen into helium in their cores.) Procyon, a bright white star and the brightest star in the constellation Canis Minor, is a well-known F-type main-sequence star. These bigger cousins to the Sun differ from our home star in many important ways when it comes to astrobiology. [The Strangest Alien Planets Ever (Gallery)]
The new study specifically considers how the higher levels of ultraviolet (UV) radiation cranked out by F-type stars could hinder the development of alien life. UV rays can alter or destroy the molecules, such as DNA, that are deemed necessary for carbon-based biochemistry. Another drawback of F-type stars, as the study conveys, is that they live shorter lives than slower-burning stars like the Sun as well as orange (K-type) and red dwarf (M-type) stars. This variable is important because life, as we understand it, needs a lot of time to get going and to eventually evolve complexity. Furthermore, the more massive a star is, the rarer it is; tiny M-type stars vastly outnumber G-type stars, which in turn outnumber heavier F-type stars.
However, the new paper cautions completely writing off the rare, ephemeral, UV-blasting F-type stars as incubators of life. Instead, F-types likely represent the brightest and hottest main-sequence stars that could plausibly allow life to form.
“It has been argued that the most likely host candidates for exobiology should be K-type or even M-type stars based on their relatively long life spans and high frequency compared to the other types of main sequence stars,” said paper co-author Manfred Cuntz, a professor of physics at the University of Texas at Arlington, and lead scientist of the project. “But the only case known for life to exist is the environment of our Sun, identified as a relatively hot and massive G-type star.”
“Therefore,” Cuntz continued, “it appears to be fully appropriate to explore the possibility of exobiology for stars even hotter and more massive than the Sun.”
Star light, star bright
Astronomers classify stars by the sort of the light they emit. Underlying this characteristic is the essential property of mass. The amount of matter stars contain determines not only their surface temperature, but their size and lifetime as well.
More massive stars generate stronger gravity; stronger gravity creates higher inside temperatures and pressures, therefore making the stars’ nuclear fusion furnace much more efficient. Atomic nuclei fuse at higher rates and release more energy in compressed conditions. As a result, more massive stars put out more light and heat as they burn their nuclear fuel at a faster pace than cooler, less massive stars.
F-type main sequence stars have a mass about 10 to 60 percent greater than the Sun’s. That added mass gives the star a surface temperature ranging from about 10,500 to 12,500 degrees Fahrenheit; for comparison, our Sun’s surface is a bit shy of 10,000 degrees Fahrenheit.
Stars’ hotness determines their color and thus their classification according to the standard Morgan-Keenan system. This letter-based classification system runs, from the bluest and hottest stars to the reddest and coolest, as follows: O, B, A, F, G, K, M.
The first two types blaze at too high a temperature and live far too short a time (mere millions to hundreds of millions of years) for life to stand a chance, and the third type is often considered irrelevant as well. Life took an estimated half a billion years to establish itself on Earth, and another two to three billion years to develop multicellular complexity. (Earth and the Sun are presently about 4.6 billion years old.) F-type main-sequence stars are expected to remain stable for about 2 to 4 billion years as detailed models suggest. (Our Sun should have a correspondingly stable lifetime of approximately 10 billion years.)
Accordingly, Cuntz and colleagues see no reason why life would not have a sufficient “window of opportunity” to arise on worlds bathed in an F-type star’s light.
“The lifetimes of F-type stars on the main-sequence appear to be sufficiently long for life having a chance to start and to flourish,” Cuntz said.
Too harsh a glare?
The light from F-type stars, however, is more intense than that from our Sun. Hotter stars churn out more higher-energy forms of light, such as UV, than their less massive cousins, which can make a profound difference when considering habitability.
The study by Cuntz and colleagues considered F-type stars in the mass range of 1.2 to 1.5 times that of the Sun. The researchers gauged the damage that these representative stars’ UV rays would cause to a DNA molecule. The amount of radiation received by the molecule was pegged to a hypothetical planet situated at a position equivalent to Earth’s orbit (thus, the model planet’s orbit was a bit farther out from its hotter, F-type star than Earth’s). The scientists calculated the DNA damage through what is known as an action spectrum, which is a measure of the degree of destruction or alteration of molecules under the influence of radiation.
The results: DNA molecules under the glare of an F-type star would suffer 2.5 to 7.1 more damage from UV light compared to that inflicted by the Sun. Life-friendly hydrocarbon molecules would suffer serious degradation, potentially enough to wreck the delicate chemistry that underlies biology.
A shaded, safe place
Game over for life? Not quite. The preceding figures did not take into account the extent to which shielding of some sort—say, an atmosphere or submersion in water—could block some harmful UV rays. Most biologists think life arose on primordial Earth in an aqueous environment anyhow and perhaps well beyond the reach of UV rays at hydrothermal ventson ancient ocean floors. Similarly, by living underwater, or underground, Cuntz said, primitive single-celled creatures could survive on a world awash in heavy UV light from an F-type star.
The development of multi-cellular, complex life on a planet around an F-type star, though, would require the sort of UV protection afforded by an ozone layer. Earth has just such a layer, high up in the atmosphere, of ozone molecules composed of three oxygen atoms. The layer absorbs nearly all of the highest-energy and thus most dangerous kind of UV rays, dubbed UVC. These rays would otherwise penetrate to our planet’s surface and could kill off exposed life forms. Some less-energetic UVA and UVB rays still reach the ground, where they damage our skin, causing sunburns and skin cancers.
In a seeming paradox, the Earth’s protective ozone layer is actually thought to have arisen thanks to life. Early in our planet’s history, photosynthetic life—partially shielded from UV rays by water—loaded the atmosphere up with the oxygen gas that then reacted with sunlight to produce ozone. Accordingly, simple life paved the way for more complex life as an ozone layer took hold; such a stepwise evolutionary process could transpire on an F-type star’s planetary bodies.
Cuntz pointed out that extra UV light exposure should not be construed as a wholly bad actor. Although it can damage biomolecules, UV could also give burgeoning life a handy spark, providing a source of energy. It could also be helpful to jumpstart the origin of life by providing a highly reactive biochemical environment.
“Broadly speaking, UV should be considered both a friend and a foe to the principal possibility of life,” Cuntz said. “At the present stage of Earth in regard to many of its life forms, the lack of an ozone layer would be truly harmful to most types of surface life. On the other hand, in regard to life’s origin and early stages of evolution, a notable UV intensity could be important for facilitating the onset of life by triggering relevant early-stage biochemical reactions.”
More, but not too much more UV exposure could even provide a biological bonus by causing DNA to mutate more rapidly—the very essence of evolution, especially during its early stages.
“UV could also support accelerated evolution through initiating mutations,” Cuntz said.
More room to live
Another boon to F-type main sequence stars is its expanded climatological “habitable zone,” the temperate band around stars where orbiting planets can maintain liquid water on their surfaces. Habitable zones are usually measured in astronomical units (AU), the average Sun-Earth distance of 93 million miles, equaling 1.0 AU.
A bit more nomenclature: Our Sun is technically a G2 star; in the MK stellar classification system. The digit after the lettered spectral class refers to how far along a continuum, numbered 0 through 9, that the particular star ranks, with 0 being closest to the next-hotter spectral class and 9 the farthest. Thus, a G2 star is warmer than a G7 star.
The paper cites prior work gauging a relatively hot F0 star as possessing a habitable zone extending from about 2.0 AU to 3.7 AU—a band more than twice that of the Sun’s, reckoned span of 0.8 AU to 1.5 AU. The habitable zone for a comparatively cool F-type star, an F8, meanwhile, extends between 1.1 and 2.2 AU.
It follows that exoplanets placed at the outer edges of their respective F-type star’s habitable zones, and orbiting lower-end F-type stars rather than those roasters bordering on the A-type star category, would receive less UV light.
Ultimately, given the various considerations in the paper, life should stand a reasonably good chance of developing in the cooler, less-severe-UV realms about F-type stars. The cons of F-type main sequence stars—more UV, shorter stellar lifetimes—do not eliminate them from astrobiological contention along with their smaller cousins, G-type, K-type and M-type stars.
“When it comes to evaluating F-type star planetary habitability, it is necessary to keep an open mind on the various important and intertwined topics of exobiology, and to promote future research,” Cuntz said.
Like its famous neighbor Europa, the huge Jovian satellite Ganymede might have an ocean of liquid water in contact with a rocky seafloor, a new study reports. This arrangement would make possible all sorts of interesting chemical reactions — including, perhaps, the kind that led to the rise of life on Earth.
Astronomers have known since the 1990s that the 3,275-mile-wide (5,270 kilometers) Ganymede, the largest moon in the solar system, harbors a deep, salty ocean of liquid water beneath its icy shell. But they had thought another layer of ice bounded this global sea on the bottom, ruling out any water-rock interactions. [Ganymede, Jupiter's Largest Moon, Explained (Video)]
The new modeling study, however, suggests that the moon’s internal composition is much more complicated, with several layers of ice and water stacked on top of each other, and liquid water perhaps in contact with rock on the bottom.
“Ganymede’s ocean might be organized like a Dagwood sandwich,” study lead author Steve Vance, of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., said in a statement, referring to the many-layered sandwiches regularly eaten by Dagwood Bumstead, a character in the long-running comic strip “Blondie.”
“This is good news for Ganymede,” Vance added. “Its ocean is huge, with enormous pressures, so it was thought that dense ice had to form at the bottom of the ocean. When we added salts to our models, we came up with liquids dense enough to sink to the seafloor.”
Vance and his team performed lab experiments that showed how much salt increases the density of water inside bodies such as Ganymede — an effect that previous modeling work hadn’t fully appreciated.
The model developed by Vance and his colleagues also takes into account the different types of water-ice that occur at different pressures. Inside Ganymede, for example, ice crystals should be greatly condensed, creating a form of ice that’s heavier than water and thus sinks to the bottom.
“It’s like finding a better arrangement of shoes in your luggage — the ice molecules become packed together more tightly,” Vance said.
The team’s model suggests that Ganymede’s interior contains an ocean separated by as many as three ice layers, with a rocky seafloor underlying everything. But the researchers can’t say for certain that this model is an accurate representation of reality.
“We don’t know how long the Dagwood-sandwich structure would exist,” said co-author Christophe Sotin, also of JPL. “This structure represents a stable state, but various factors could mean the moon doesn’t reach this stable state.”
Scientists think that Ganymede and four other moons in the solar system possess oceans of liquid water beneath their frigid surfaces. The others are the Jovian moons Callisto and Europa and the Saturn satellites Titan and Enceladus. Scientists think the oceans of Europa and Enceladus are in contact with rock, making these two moons high-priority targets for astrobiology missions in the future.
Indeed, the European Space Agency plans to launch a mission called JUICE (JUpiter ICy moons Explorer) in the early 2020s to study Europa, Callisto and Ganymede. And NASA officials have said they want to send a relatively low-cost (less than $1 billion) probe toward Europa by 2025 or so.
The new study appears in the journal Planetary and Space Science.