Black holes do indeed come in three sizes: small, medium and extra large, a new study suggests.
Astronomers have studied many black holes at either size extreme — “stellar-mass” black holes, which are a few dozen times as weighty as the sun, and supermassive black holes, which can contain millions or billions of times the mass of the sun and lurk at the heart of most, if not all, galaxies.
Researchers have spotted hints of much rarer medium-size black holes, which harbor between 100 and several hundred thousand solar masses. But it’s tough to weigh these objects definitively — so tough that their existence has been a matter of debate.
But that debate can now be put to rest, says a research team that has measured an intermediate black hole’s mass with unprecedented precision. A black hole in the nearby galaxy M82 weighs in at 428 solar masses, give or take a hundred suns or so, they report today (Aug. 17) in the journal Nature.
“Objects in this range are the least expected of all black holes,” study co-author Richard Mushotzky, an astronomy professor at the University of Maryland, said in a statement. “Astronomers have been asking, ‘Do these objects exist, or do they not exist? What are their properties?’ Until now, we have not had the data to answer these questions.”
Patterns in the light
Black holes famously gobble up anything that gets too close, including light. But that doesn’t mean astronomers can’t see them; bright X-ray light streams from the superhot disk of material spiraling into a black hole’s mouth.
About 15 years ago, NASA’s Chandra X-ray Observatory spacecraft spotted such emissions coming from a source in the galaxy M82, which lies about 12 million light-years away from Earth. For a long time, Mushotzky and some other scientists suspected that the object, called M82 X-1, was a medium-size black hole. But those suspicions were tough to confirm.
“For reasons that are very hard to understand, these objects have resisted standard measurement techniques,” Mushotzky said.
In the new study, a team led by University of Maryland doctoral student Dheeraj Pasham took a closer look at M82 X-1. They studied observations made from 2004 to 2010 by NASA’s Rossi X-ray Timing Explorer (RXTE) satellite, which ceased operations in 2012.
The RXTE data revealed a pair of repeating oscillations in M82 X-1′s X-ray emissions. These oscillations occurred 5.1 times per second and 3.3 times per second, respectively — a ratio of three to two. This fact allowed the team to determine the black hole’s mass.
“In essence, [the] frequency of these 3:2 ratio oscillations scales inverse[ly] with black hole mass,” Pasham told Space.com via email. “Simply put, if the black hole is small, the orbital periods at the innermost circular orbit are shorter, but if the black hole is big, the orbital periods are longer (smaller frequencies).”
The researchers calculated M82 X-1′s mass at 428 suns, plus or minus 105 solar masses.
“In our opinion, and as the paper’s referees seem to agree, this is the most accurate mass measurement of an intermediate-mass black hole to date,” Pasham said.
Learning about black-hole growth
Confirming the existence of intermediate black holes could help researchers better understand the supermassive monsters at the cores of galaxies.
Such behemoths apparently first formed in the universe’s very early days, just a few hundred million years after the Big Bang. They could not have grown so big so fast if their “seeds” were small stellar-mass black holes (which result from the collapse of giant stars), Pasham said.
“Many theories, therefore, have suggested that these initial seed black holes had to have been a few 100 -1,000 times our sun,” he said. “But we did not have firm evidence for such intermediate-mass black holes.”
Stellar-mass black holes also often feature paired X-ray oscillations that occur in a 3:2 frequency ratio. Therefore, the new observations suggest that medium-size black holes may behave like scaled-up versions of stellar-mass black hole systems, Pasham added.
These compounds may reveal that extraterrestrials have disastrously altered their planets, scientists added.
To detect biomarkers, or signs of life, on distant worlds, scientists have often focused on molecules such as oxygen, which theoretically disappears quickly from atmospheres unless life is present to provide a constant supply of the gas. By looking at light passing through atmospheres of alien worlds, past studies have suggested future instruments such as NASA’s James Webb Space Telescope could detect telltale traces of oxygen.
But the search for extraterrestrial intelligence (SETI) has mostly concentrated on “technosignatures,” such as radio and other electromagnetic signals that alien civilizations might give off. Now researchers suggest that searches for atmospheric biomarkers could also look for industrial pollutants as potential signs of intelligent aliens.
Astronomers at Harvard University focused on tiny, superdense stars known as white dwarfs. More than 90 percent of all stars in the Milky Way, including our own sun, will one day end up as white dwarfs, which are made up of the dim, fading cores of stars.
Though white dwarfs are quite cold for stars, they would still be warm enough to possess so-called habitable zones — orbits where liquid water can exist on the surfaces of circling planets. These zones are considered potential habitats for life, as there is life virtually everywhere there is liquid water on Earth.
The scientists examined how Earth-size planets in the habitable zones of white dwarfs might look if they possessed industrial pollutants in their atmosphere. They focused on chlorofluorocarbons (CFCs), which are entirely artificial compounds, with no known natural process capable of creating them in atmospheres.
CFCs are nontoxic chemicals that were once used in hairspray and air conditioners, among many other products, before researchers discovered they were causing a hole in Earth’s ozone layer, which protects the planet from dangerous ultraviolet radiation.
“Very hairy extraterrestrials may be a little easier to detect,” joked lead study author Henry Lin, a physicist at Harvard.
CFCs are strong greenhouse gases, meaning they are very effective at absorbing heat. This means that if CFCs are in the atmosphere of a distant Earth-size planet, they could alter a white dwarf’s light when that world passes in front of that star — enough for the $8.8 billion James Webb Space Telescope (JWST), which is due to launch in 2018, to detect them.
In addition, the researchers noted that CFCs are long-lived molecules, capable of lasting up to about 100,000 years in atmospheres. This means they could even serve as markers of long-dead alien civilizationsThe investigators simulated the amount of time it would take JWST to detect the fluorocarbon CF4 and the chlorofluorocarbon CCl3F in the atmosphere of an Earth-size planet in the habitable zone of a white dwarf. They modeled concentrations of these gases 100 times greater than the highs currently seen on Earth.
The scientists found it would take JWST three days of looking at such a white dwarf to detect signs of CF4, and only a day and a half for CCl3F.
“The most exciting aspect of the results is that within the next decade we might be able to search for excessive industrial pollution in the atmospheres of Earth-like planets,” study co-author Abraham Loeb, a theoretical astrophysicist and chair of Harvard’s astronomy department, told Space.com.
Ironically, “aliens are often referred to as green little creatures, but ‘green’ also means ‘environmentally friendly,’” Loeb said. “Detectable CFC-rich civilizations would not be ‘green.’”
The scientists did caution that it would take much longer to detect these industrial pollutants than it would biomarkers such as oxygen, which JWST could find after about three hours of looking at such a planet. Astronomers should only attempt to discover technosignatures such as CFCs if initial searches for fundamental biomarkers like oxygen were successful, the research team suggested.
The astronomers cautioned it would be 100 times more difficult to detect industrial pollutants on planets orbiting yellow dwarf stars like the sun, making such searches beyond the capabilities of JWST. It would also take an unrealistically long time to detect CFC levels on alien planets that match those currently found on Earth, Loeb said.
One potentially sobering future discovery might be of alien worlds that possess long-lived industrial pollutants such as CFCs but no longer have any short-lived biomarkers such as oxygen.
“If we find graveyards of other civilizations, most rational people would likely get engaged in protecting the Earth from a similar catastrophe,” Loeb said.
“We call industrial pollution a biomarker for intelligent life, but perhaps a civilization much more advanced than us with their own exoplanet program will classify industrial pollution as a biomarker for unintelligent life,” Lin said.
However, if astronomers discover a world heavy with CFCs that exists outside the habitable zone of its star, that could mean an extraterrestrial civilization may have intentionally “terraformed” that planet, making it livably warmer “by polluting it with greenhouse gases,” Loeb said. Scientists have previously suggested terraforming Mars by warming and thickening the Red Planet’s atmosphere so that humans can roam its surface without having to wear spacesuits.
The agency’s Nuclear Spectroscopic Telescope Arrayprobe, or NuSTAR, looked on as a mysterious X-ray source, called a corona, moved closer to a supermassive black hole. The black hole’s immense gravity pulled harder on the corona the closer it came, stretching and blurring the X-ray light in the process, researchers said.
“The corona recently collapsed in toward the black hole, with the result that the black hole’s intense gravity pulled all the light down onto its surrounding disk, where material is spiraling inward,” study lead author Michael Parker, of the Institute of Astronomy in Cambridge, England, said in a statement.
NuSTAR’s observations provide the most detailed look yet at such events, researchers said.
While light cannot escape once it passes the “event horizon” of a black hole, high-energy emissions do stream from the vicinity of these objects — from the corona, for example, and from the superhot disk of material spiraling into a black hole’s maw.
Astronomers think supermassive black holes — which can contain millions or billions of times the mass of the sun — reside at the cores of most, if not all, galaxies. The black hole observed by NuSTAR, called Markarian 335 (Mrk 335), is 10 million times more massive than the sun and lies 324 million light-years away, researchers said.
NASA’s Swift satellite recently observed a change in Mrk 335′s X-ray brightness, so scientists pointed NuSTAR at the supermassive black hole to take a closer look.
NuSTAR’s observations revealed that the black hole’s gravity pulled the coronal X-ray light onto the inner regions of Mrk 335′s accretion disk. Further study has shown that the corona remains close to the black hole, months after it originally moved inward. (This inward migration was rapid, occuring over a period of days rather than weeks or months, researchers said.)
The new study could shed light on the nature of black hole coronas and the extreme conditions near the cores of supermassive black holes, NASA officials said.
“We still don’t understand exactly how the corona is produced or why it changes its shape, but we see it lighting up material around the black hole, enabling us to study the regions so close in that effects described by Einstein’s theory of general relativity become prominent,” said co-author and NuSTAR principal investigator Fiona Harrison, of the California Institute of Technology in Pasadena.
“NuSTAR’s unprecedented capability for observing this and similar events allows us to study the most extreme light-bending effects of general relativity,” she added.
Black holes may have grown incredibly rapidly in the newborn universe, perhaps helping explain why they appear so early in cosmic history, researchers say.
Black holes possess gravitational pulls so powerful that not even light can escape their clutches. They are generally believed to form after massive stars die in gargantuan explosions known as supernovas, which crush the remaining cores into incredibly dense objects.
Supermassive black holes millions to billions of times the mass of the sun occur at the center of most, if not all, galaxies. Such monstrously large black holes have existed since the infancy of the universe, some 800 million years or so after the Big Bang. However, it remains a mystery how these giants could have grown so big in the relatively short amount of time they had to form. [Images: Black Holes of the Universe]
In modern black holes, features called accretion disks limit the speed of growth. These disks of gas and dust that swirl into black holes can prevent black holes from growing rapidly in two different ways, researchers say. First, as matter in an accretion disk gets close to a black hole, traffic jams occur that slow down any other infalling material. Second, as matter collides within these traffic jams, it heats up, generating energetic radiation that drives gas and dust away from the black hole.
“Black holes don’t actively suck in matter — they are not like vacuum cleaners,” said lead study author Tal Alexander, an astrophysicist at the Weizmann Institute of Science in Rehovot, Israel.
“A star or a gas stream can be on a stable orbit around a black hole, exactly as the Earth revolves around the sun, without falling into it,” Alexander told Space.com. “It is actually quite a challenge to think of efficient ways to drive gas into the black hole at a high enough rate that can lead to rapid growth.”
Alexander and his colleague Priyamvada Natarajan may have found a way in which early black holes could have grown to supermassive proportions — in part, by operating without the restrictions of accretion disks. The pair detailed their findings online today (Aug. 7) in the journal Science.
The scientists began with a model of a black hole 10 times the mass of the sun embedded in a cluster of thousands of stars. They fed the simulated black hole continuous flows of dense, cold, opaque gas.
“The early universe was much smaller and hence denser on average than it is today,” Alexander said.
This cold, dense gas would have obscured a substantial amount of the energetic radiation given off by matter falling into the black hole. In addition, the gravitational pull of the many stars around the black hole “causes it to zigzag randomly, and this erratic motion prevents the formation of a slowly draining accretion disk,” Alexander said. This means that matter falls into the black hole from all sides instead of getting forced into a disk around the black hole, from which it would swirl in far more slowly.
The “supra-exponential growth” observed in the model black hole suggests that a black hole 10 times the mass of the sun could have grown to more than 10 billion times the mass of the sun by just 1 billion years after the Big Bang, researchers said.
“This theoretical result shows a plausible route to the formation of supermassive black holes very soon after the Big Bang,” Alexander said.
Future research could examine whether supra-exponential growth of black holes could occur in modern times as well. The high-density and high-mass cold flows seen in the ancient universe may exist “for short times in unstable, dense, star-forming clusters, or in dense accretion disks around already-existing supermassive black holes,” Alexander said.
Because Pluto is so far away — it orbits the sun at an average distance of 3.65 billion miles (5.87 billion kilometers) — many questions about the dwarf planet’s composition and activity remain unanswered. Researchers hope New Horizons will lay some of those questions to rest when it flies by Pluto on July 15, 2015.
“Many predictions have been made by the science community, including possible rings, geyser eruptions, and even lakes,” Adriana Ocampo, program executive for NASA’s New Frontiers program, said in a statement. “Whatever we find, I believe Pluto and its satellites will surpass all our expectations and surprise us beyond our imagination.” [New Horizons' Flight to Pluto in Pictures]
Orbiting the sun once every 248 years, Pluto lies outside the reach of most visible instruments. The best images from NASA’s famous Hubble Space Telescope simply show Pluto’s spherical shape and reddish color. Changes in the dwarf planet’s color patterns over the years hint that something is happening there, but no one knows exactly what.
By late April 2015, New Horizons will be close enough to Pluto and its moons to capture pictures rivaling those of Hubble. On July 14, 2015, the craft will make a close flyby of the icy world, ultimately zooming within about 6,200 miles (10,000 kilometers) of its surface. If it cruised past Earth at that range, New Horizons would be able to recognize individual buildings and their shapes.
“Because Pluto has never been visited up-close by a spacecraft from Earth, everything we see will be a first,” Ocampo said. “I know this will be an astonishing experience full of history-making moments.”
New Horizons principal investigator Alan Stern, of the Southwest Research Institute in Colorado, likened the upcoming visit to the way Mariner 4revolutionized understanding of Marsin July 1965. At the time, many people thought the Red Planet was a life-friendly world possibly harboring liquid water and even plants. The New Horizons flyby could change perceptions of Pluto just as dramatically, Stern said.
The flight in won’t be without its challenges. Since New Horizons launched in 2006, two new moons have been discovered orbiting Pluto, upping the total known satellite countto five: Charon, Nix, Kerberos, Styxand Hydra. As many as 10 other moonscould still await detection in the system, one study suggested.
According to simulations, meteorites striking Pluto’s moons could send tiny rocks flying into space, where many of them would enter orbit around the dwarf planet. The debris field likely changes with time as it orbits, growing larger as new material is added. As the New Horizons probe gets closer and closer to Pluto, the mission team will need to keep watch on the system in case evasive maneuvers are required.
“The New Horizons team continues to do a magnificent job in keeping the spacecraft healthy and ready for this incredible rendezvous,” said Ocampo. “The spacecraft is in good hands.”
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.