A Comparative Climatology Symposium held at NASA Headquarters on May 7 focused on new approaches to climate research by highlighting the similarities and contrasts between the environments of the rocky worlds Venus, Earth, Mars and Saturn’s smoggy moon Titan.
The symposium also included discussions about exoplanets, the sun and past, present and future space missions.
John Grunsfeld, Associate Administrator for NASA’s Science Mission Directorate, said that the upcoming James Webb Space Telescope will be able to make important observations of the atmospheres of exoplanets. [Photos: The James Webb Space Telescope]
He said JWST won’t be able to locate the exoplanets, only study them, but the recently selected TESS mission could act as a planet scout for JWST targets. It is estimated that TESS will discover around 300 “super-Earth” alien planets, many of them in the habitable zone.
But the number one challenge, Grunsfeld noted, is figuring out the climate of our own planet.
Understanding climate change
Jim Green, NASA’s Planetary Science Division Director, said that one goal is to examine a variety of planetary bodies as a system, to see if there are trends or similarities. He also pointed out that from a planetary scientist’s perspective, climate change on our planet is not a new thing.
“Earth’s climate has done nothing but change,” Green said.
Green said that three Earth-observing satellites will be launched this year, and they will help us better understand how the climate is currently changing and the implications that has for our planet’s environment.
David Grinspoon, holder of the first Baruch S. Blumberg NASA/Library of Congress chair in Astrobiology, talked about Mars’ “ferocious and interesting” meteorology, and how Martian global dust storms may help unravel what happened on our planet during the K-T extinction 65 million years ago, when an asteroid hitting the Yucatan Peninsula is thought to have eradicated 75 percent of animals and plants on Earth, including the dinosaurs. [Wipeout: History's Most Mysterious Extinctions]
As for Venus, Grinspoon said scientists believe current-day volcanism on Venus is thought to be necessary to sustain the planet’s thick clouds. He added that the active surface has eradicated most ancient rocks, preventing us from easily understanding Venus’ early history.
Grinspoon also discussed the unique climate of Titan, noting that the methane cycle on this moon of Saturn is “like Earth’s hydrological cycle on steroids.”
Studying the climates of Mars, Venus, Titan and even exoplanets could help us refine our climate models of the Earth. However, Grinspoon said that “clouds are the biggest uncertainty in understanding the past of Venus and predicting the future of Earth.”
Tying climatology to astrobiology, Grinspoon said that our expectations of the other planets, in the absence of data, were that they’d be much more Earth-like than they actually are. We still haven’t found a planet quite like our own, although astronomers are zeroing in on exoplanets that should have habitable conditions.
But, Grinspoon said, “it may be that conditions for life’s origin aren’t rare, but the hard part is the persistence of habitable conditions.”
Venus was a popular topic during the symposium. Roald Sagdeev, University of Maryland professor and former director of the Space Research Institute of the USSR Academy of Sciences, said during an overview of the Russian missions to Venus that “from the point of view of habitability, Venus is like having a dead body to study, which is of course very useful for learning anatomy.”
David Crisp, Senior Research Scientist at the Jet Propulsion Laboratory/California Institute of Technology, said that sending weather balloons to Venus taught us a lot about atmospheric physics. And Roger Bonnet, Executive Director of the International Space Science Institute, said there was no chance for a big “flagship” mission to Venus, since the viewpoint among many amounts to “Who cares about clouds and wind on Venus, when we have so much of that on Earth? We want to see little green men!”
One participant noted the presence of “the Venus mafia” at the symposium, inferring that the focus on Earth’s “twin planet” had muscled out discussion of other places of interest.
But in addition to studies of Venus and other terrestrial worlds, there was a talk about our sun and its influence on space weather, and general discussions about refining climate models, defining habitable zones, and the importance of basic research.
The participants seemed to agree that, most importantly, planetary climate studies needed to be interdisciplinary, with scientists from different fields communicating and collaborating.
Michael Meyer, lead scientist for the Mars Exploration Program at NASA Headquarters, also pointed out that we should never become complacent in our scientific understanding. For instance, he said that while climate models have not been able to make early Mars warm enough to sustain liquid water on its surface, the same can be true for models of the young Earth.
And when it comes to understanding where a planet needs to reside in its solar system to be habitable — the so-called Goldilocks Zone where the temperature is just right for water to be liquid rather than ice or gas — he commented that “the approach [to the habitable zone] is very Goldilocks in that it’s almost a fairy tale.”
Finally, Meyer noted, just when we thought we understood how planets are made, we discovered hot Jupiters and other unusual exoplanets that “turned all of our planet formation models on their head.”
“And that’s a good thing,” he added.
The newfound world — nicknamed “Einstein’s planet” by the astronomers who discovered it — is the latest of more than 800 planets known to exist beyond our solar system, and the first to be found through this method.
The planet, officially known as Kepler-76b, is 25 percent larger than Jupiter and weighs about twice as much, putting it in a class known as “hot Jupiters.” The world orbits a star located about 2,000 light-years from Earth in the constellation Cygnus. [7 Ways to Discover Alien Planets]
The researchers capitalized on subtle effects predicted by Albert Einstein’s special theory of relativity to find the planet. The first is called the “beaming” effect, and occurs when light from the parent star brightens as its planet tugs it a nudge closer to Earth, and dims as the planet pulls it away. Relativistic effects cause light particles, called photons, to pile up and become focused in the direction of the star’s motion.
“This is the first time that this aspect of Einstein’s theory of relativity has been used to discover a planet,” research team member Tsevi Mazeh of Tel Aviv University in Israel said in a statement.
Additionally, gravitational tides from the orbiting planet caused its star to stretch slightly into a football shape, causing it to appear brighter when its wider side faces us, revealing more surface area. Finally, the planet itself reflects a small amount of starlight, which also contributed to its discovery.
“We are looking for very subtle effects,” said team member David Latham of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “We needed high quality measurements of stellar brightnesses, accurate to a few parts per million.”
The researchers used data from NASA’s Kepler spacecraft, which provided the extremely detailed observations necessary. While Kepler was designed to hunt for alien planets, it normally does so using the transit method, which looks for stars that dim periodically as planets pass in front of them.
“This was only possible because of the exquisite data NASA is collecting with the Kepler spacecraft,” said study leader Simchon Faigler of Tel Aviv University.
The other most popular planet-hunting tactic is called the wobble method, and searches for slight signs of movement in stars’ radial velocities caused by tugging planets.
The new Einstein-based method is best for larger worlds, and is currently incapable of finding Earth-sized planets, the scientists said. Still, it offers some benefits, as it does not require high-precision measurements of a star’s velocity, or for a star and its planet to align perfectly as viewed from Earth — the two main drawbacks of the most common methods.
“Each planet-hunting technique has its strengths and weaknesses. And each novel technique we add to the arsenal allows us to probe planets in new regimes,” said Avi Loeb, also from the Center for Astrophysics.
A paper detailing the planet discovery will be published in an upcoming issue of The Astrophysical Journal.
Where it can exist on a planet’s or moon’s surface, so the thinking goes, life as we know it has a chance. Much of the observational and theoretical work in astrobiology therefore concerns the “habitable zone,” the orbital band around stars where a rocky world’s water neither freezes away nor boils off.
In a new contribution to this effort, a recent study has looked at a little-explored influencer on the ability of water to remain liquid on a world’s surface: atmospheric pressure. [Habitable Super-Earths Ideal for Live (Gallery)]
“Atmospheric pressure affects the liquid water temperature range that is commonly used to define planetary habitability,” said Giovanni Vladilo of the Trieste Astronomical Observatory in Italy and lead author of the paper published in the Astrophysical Journal. “So, if you wish to estimate habitability, you should explicitly take into account pressure in your problem.”
On Earth, the space around us is filled with air molecules that collectively weigh on our bodies. Although you cannot feel it, Earth’s atmosphere presses down with the force of one kilogram per square centimeter, or 14.7 pounds per square inch. That pressure works out to about a ton per square foot. Our terrestrial biology evolved to operate in this pressure, which, while startling-sounding, pales when compared to underwater creatures’ bodies in the deep seathat can withstand dozens of tons per square foot.
Atmospheric pressure has an impact on water’s boiling point when it transitions from a liquid to a gas. As anyone who has cooked at high altitudes has experienced, water boils there at a lower temperature than the typical 100 degrees Celsius (212 degrees Fahrenheit). The reason: Atmospheric pressure is lower at high altitudes than at sea level; there is simply less atmosphere pressing down the higher you go up in the mountains. Lower pressure is why it takes a few minutes longer to make pasta in mile-high Denver than below-sea-level New Orleans—the pasta has to soak and soften in the water longer to become al dentein Denver because the water is boiling at a slightly lower temperature.
Vladilo explained why pressure has this effect. “Temperature is an indicator of the speed of molecular motions. The boiling point occurs when molecular motions are sufficiently fast to allow most molecules to escape from each other,” and thus turn into gas, he said. “Pressure keeps molecules tight, so the higher the pressure, the faster the molecules must move—that is, the higher the temperature must be—for evaporation to occur.”
Like Earth, give or take an atmosphere
In Vladilo’s new paper, he and his colleagues modeled a planet just like Earth in size and atmospheric composition. They ran over 4,000 computer simulations that varied the model planet’s atmospheric pressure from one-hundredth to six times the atmospheric pressure of Earth. The researchers also varied the virtual planet’s orbital distance from its sun-like star from about two-thirds of the Earth-sun distance to around an additional third. To get a sense of these orbital parameters, the former is a bit tighter than Venus and the latter more than half the distance out to Mars.
The researchers’ model estimated the global habitability of these Earth-like exoplanets by gauging the extent of the planet’s latitudes that could possess liquid surface water.
Through their modeling, Vladilo and colleagues saw that the habitable zone expanded in width as the atmospheric pressure increased. At a tenth of Earth’s atmospheric pressure, the outer edge of the habitable zone reached just two percent farther out than Earth; not a lot of wiggle room for a low-pressure, Earth-like world, in other words, when it comes to habitability. But as the atmospheric pressure increased to threefold that of Earth’s, the habitable zone extended out a farther 18 percent.
For the same pressure interval, low-to-high, the inner edge of the habitable zone ranged from 87 percent of the Earth-sun distance to 77 percent. In this model, for a planet with Earth’s atmospheric pressure, cloudiness, and humidity, the inner edge of the habitable zone is smack dab in the middle of this range, at 82 percent of the Earth-sun distance.
The results indicate that an exoplanet just like Earth in all other respects but with a higher atmospheric pressure could be considered habitable about five percent closer to its sun-like star. Conversely, a low-pressure Earth would not be considered habitable unless placed in an orbit five percent farther out than a standard-pressure Earth.
The movement of heat
A main factor behind the expanded orbital range of habitability at higher pressures is that higher pressure atmospheres are denser. Denser atmospheres, in turn, transport heat better than thin atmospheres, and promote a stronger “greenhouse effect”, whereby atmospheric gases absorb heat.
For exoplanets farther from their star than Earth is from our sun, and therefore receive less sunlight, a high-pressure atmosphere traps heat better and distributes the greater warmth received at the equator. Polar zones that would otherwise freeze instead retain liquid water. A high-pressure planet can remain warmer at farther distances from its star accordingly.
With regards to low-pressure worlds — hearkening back to the pasta cooking analogy — water boils at lower temperatures than it does on higher atmospheric pressure worlds. In a low-pressure scenario, a world closer to its star than Earth that would otherwise be broadly habitable with Earth’s atmospheric pressure would have its water boil off.
For closer-in exoplanets with a high atmospheric pressure, however, the sun-scorched equatorial zones would not heat to a boiling level as readily as in a normal- or low-pressure situation, and thus could still be habitable.
In addition to these general findings, the researchers’ model offers intriguing insights. For example, much of the gain in survivability on the closer-to-the-star side of the habitable zone for high-pressure worlds is for organisms that, at least by our Earthly standards, are extreme.
The global temperatures on these inner-edge worlds made habitable by their high atmospheric pressures would be too high for complex life forms such as ourselves. So-called thermophiles, however — bacteria that thrive at temperatures more than 45 degrees Celsius (113 degrees Fahrenheit) or so and on up to considerably higher temperatures — might find such heat-blasted worlds quite comfortable.
Overall, the habitable zone for creatures like us that require relatively moderate temperatures actually moves outward somewhat from a sun-like star in high-pressure scenarios.
Atmospheric pressure could also have a profound effect on biodiversity. Compared to low-pressure worlds, high-atmospheric pressure exoplanets would have rather uniform global surface temperatures, again owing to the efficient transfer of heat amongst their latitudes. These heavy-atmosphere planets might host a fairly narrow range of life forms, since all would be adapted to the same slim temperature regime.
Planets with lower atmospheric pressures than Earth, though, would have even more varied temperatures than our planet. These abodes might then provide an even wider range of habitats than our world, with organisms exotically adapted to their considerably more intensely varied polar-temperate-tropical bands.
For now, research on a “pressure-dependent habitable zone” is somewhat purely academic, given that atmospheric pressure is not a property of exoplanets that we can yet measure. But Vladilo believes that work with planets several times larger than Earth, dubbed super-Earths, could be where the atmospheric pressure insights are first able to be applied.
“At the present time, observations are able to determine only a few properties of planetary atmospheres, such as their chemical composition, and mostly for giant planets rather than terrestrial ones,” Vladilo said. “However, I’m confident that technological improvements will allow us to characterize, to some extent, the atmospheres of super-Earths, which are reasonable candidates for studies of planetary habitability. If we will be able to estimate some basic planet parameters with observations, such as the planetary albedo [the amount of light reflected by the surface] and infrared flux [the amount of infrared light emitted], then our models will be sufficiently constrained to yield a reasonable estimate of the planet surface pressure.”
A major issue for assessing exoplanetary atmospheric pressures is the fact that the formation of atmospheres and the densities they develop is not well understood. Saturn’s moon Titan, for instance, has a thick atmosphere with a pressure about 50 percent greater than that of Earth’s. Yet similar bodies in the outer solar system, such as Jupiter’s moons Ganymede and Callisto, cling to only very tenuous envelopes of gas.
“It’s embarrassing that we have almost no idea of atmospheres and where they come from,” Sara Seager, a professor of planetary science and physics at the Massachusetts Institute of Technology who was not involved in the new study said. “It’s one of those thing we can hope and wait to learn about.”
With regards to Vladilo’s research, Seager said “it’s refreshing to see that a wide range of surface pressures could enable a habitable surface.”
Vladilo and his colleagues plan a number of follow-ups with their model. Other subtle aspects of exoplanetary habitability remain to be examined in the ever-expanding scientific literature that matches the growing excitement for possibly detecting alien life in the near future.
“The uptick in papers on planetary habitability is a telling sign of what’s to come,” Seager said.
“We have identified chemical evidence for the building blocks of rocky planets,” researcher Jay Farihi of the University of Cambridge said in a statement Thursday (May 9). “When these stars were born, they built planets, and there’s a good chance that they currently retain some of them. The signs of rocky debris we are seeing are evidence of this — it is at least as rocky as the most primitive terrestrial bodies in our solar system.”
The discovery came after the researchers used the Hubble telescope to study two dead white dwarf stars in the Hyades star cluster. Most stars, including our own sun, will end their lives as dense and dim stellar cores called white dwarfs. Farihi and his team sought out signs of planet formation in these types of retired stars in the Hyades cluster, a 625-million-year-old grouping of stars in the constellation of Taurus. [See how the white dwarf stars collect planet debris (Video)]
White dwarf atmospheres are typically quite “clean,” with heavier elements clumping in the core, as Ben Zuckerman, a physics and astronomy professor at UCLA, told scientists at the American Astronomical Society meeting earlier this year.
But using Hubble’s spectroscopic observations, Farihi and his fellow researchers saw that silicon — a major ingredient in the rocky material that formed Earth — was dirtying up the atmospheres of two white dwarfs. The researchers also identified low levels of carbon with Hubble’s powerful Cosmic Origins Spectrograph. (Carbon levels are expected to be very low in rocky, terrestrial material.)
“The one thing the white dwarf pollution technique gives us that we won’t get with any other planet detection technique is the chemistry of solid planets,” Farihi said in a statement from the European Space Agency (ESA). “Based on the silicon-to-carbon ratio in our study, for example, we can actually say that this material is basically Earth-like.”
The material is thought to be leftover from terrestrial planets that formed when these stars were first born. After the stars collapsed into white dwarfs, relics from their asteroid belts may have been knocked into dangerous, star-grazing orbits. Torn apart by the white dwarfs’ gravity, debris from these asteroid-like objects was sent swirling around the dead stars in a ring that then funneled the material inwards, the researchers say.
Star clusterswere thought to be unlikely hosts for alien planets. Of the 800 exoplanets known today, just four of them circle stars in these crowded stellar neighborhoods, including one in the Hyades cluster, researchers say. The new findings suggest planet formation in star clusters may be more common than previously believed.
The research is detailed in the Monthly Notices of the Royal Astronomical Society. The science team hopes to detect more material around white dwarfs that could tell them about their parent bodies.
The Hubble Space Telescope launched in 1990 and is overseen by NASA and the European Space Agency.
Scientists searching for signs of life beyond our solar system should keep an open mind, for planets very different than Earth may well be habitable, a prominent researcher says.
While it may seem natural to zero in on “alien Earths,” such a narrow focus would exclude many potentially life-supporting exoplanets, whose diversity continues to astound astronomers, says Sara Seager of MIT.
And researchers can’t afford to be so picky, she adds, since they’ll be able to get in-depth looks at just a handful of alien worlds for the foreseeable future. [9 Exoplanets That Could Host Alien Life]
“The number of planets that we’re going to be able to see in our lifetime — and look at their atmospheres for signs of life— is so small that we’re forced to be open-minded,” Seager told SPACE.com.
Seager discusses exoplanet habitability in a review article published online today (May 2) in the journal Science.
A dazzling diversity of alien worlds
Scientists discovered the first alien planet around a sunlike star in 1995. Since then, the tally has grown to more than 700 (or more than 800, depending on whose list is consulted), with thousands more candidates waiting to be confirmed by follow-up observations.
Some of these alien worlds are broadly similar to planets in our own solar system. But many others are truly alien — enormous “hot Jupiters” that whip around their parent stars at extremely close range, for example, or “rogue planets” that cruise through the cold depths of space alone, with no parent star.
“If there is one important lesson from exoplanets, it is that anything is possible within the laws of physics and chemistry,” Seager writes in the Science article. “Planets of almost all masses, sizes and orbits have been detected, illustrating not only the stochastic nature of planet formation but also a subsequent migration through the planetary disk from the planet’s place of origin.” [The Strangest Alien Planets]
Intriguingly, a number of planets have been spotted orbiting within the so-called “habitable zone” — that just-right range of distances from a star where liquid water is possible on a world’s surface. (Water is required for life as we know it here on Earth and has thus spurred astrobiologists to “follow the water” on other planets, Seager writes.)
Just where this habitable zone lies for each planet depends on a number of factors, most crucially its host star’s brightness and the planet’s atmospheric makeup.
“It’s really all about the greenhouse gases,” Seager told SPACE.com. “The greenhouse gases are like a blanket that moderates the temperature at the surface.”
The conventional definition of the habitable zone assumes a roughly Earth-like atmosphere, dominated by nitrogen, carbon dioxide and water vapor. But the huge diversity of alien worlds argues for a new way of thinking, writes Seager, who literally wrote the book on exoplanet atmospheres (“Exoplanet Atmospheres: Physical Processes,” Princeton University Press, 2010).
For example, large and/or chilly alien worlds could conceivably hang onto their gaseous molecular hydrogen, which long ago escaped from small planets such as Earth, Venus and Mars.
Hydrogen is a powerful greenhouse gas that could make liquid water possible on a number of worlds far beyond the outer edge of the traditional habitable zone — and perhaps even on seemingly frigid rogue planets, Seager writes.
Similarly, the habitable zone may extend inward, toward the host star, on “dry” rocky planets whose atmospheres have much less water vapor than Earth’s does. So it’s best to consider alien planets’ potential to support life individually, on a case-by-case basis, Seager says.
Seager and others stress that a better understanding of exoplanet habitability is key to the next phase of the alien life hunt, which seeks to search promising candidates’ atmospheres for water vapor and gases that may have been produced by life.
Astronomers have already scanned the air of a few dozen planets using NASA’s Hubble Space Telescope and other instruments, Seager said. But those were hot Jupiters with big, puffy atmospheres — relatively easy targets that aren’t intriguing from an astrobiological perspective.
Scientists plan to do the same with smaller, potentially habitable worlds soon, Seager said. They’ll use the Transiting Exoplanet Survey Satellite, which NASA recently approved for a 2017 launch, to identify promising candidates relatively close to Earth. Then NASA’s James Webb Space Telescope (which is scheduled to blast off in 2018) will follow up, getting an in-depth look at these worlds’ air.
Though JWST is designed to be incredibly powerful, the $8.8 billion instrument will probably only be able to investigate the atmospheres of exoplanets that lie within a few tens of light-years from Earth, Seager added.
Seager said she hopes her review article in Science helps her fellow astronomers make the most of this small pool of observable candidates.
“I hope it gets people to realize that so many types of worlds could be habitable, and that our chance of finding one is higher when we accept that,” she told SPACE.com.
With more and more Earth-like alien planets being discovered around the galaxy, humanity should now start planning out the next steps in its hunt for far-flung alien life, researchers say.
Last week, scientists announced the discovery of three more potentially habitable exoplanets — Kepler-62e, Kepler-62f and Kepler-69c — further suggesting that the cosmos is jam-packed with worlds capable of supporting life as we know it.
So the time is right to get the ball rolling beyond mere discovery to the detailed study and characterization of promising alien planets, researchers said — a task that will require new and more powerful instruments. [Habitable Super-Earths Ideal for Life (Gallery)]
“You really want to collect the light from these planets, to figure out — take the data, not just infer —whether or not there’s water, and even signs of life, on these planets,” Lisa Kaltenegger of the Max Planck Institute for Astronomy and the Harvard-Smithsonian Center for Astrophysics, who was part of the team that discovered Kepler-62e and f, said during a press conference Thursday.
Billions of Earth-like planets
As their names suggest, the three newfound planets were discovered by NASA’s prolific Kepler space telescope, which has spotted more than 2,700 potential alien worlds since its March 2009 launch. Just 122 have been confirmed to date, but mission scientists expect more than 90 percent will end up being the real deal.
The $600 million Kepler mission was designed to determine how common Earth-like planets are around the Milky Way galaxy. Its observations so far suggest our home planet may not be so special.
For example, astronomers recently used Kepler data to estimate that 6 percent of the galaxy’s 75 billion or so red dwarfs — stars smaller and dimmer than the sun — likely host habitable, roughly Earth-size planets.
That works out to a minimum of 4.5 billion “alien Earths,” the closest of which may be just 13 light-years or so away, according to the study.
While Kepler’s work is not done, the instrument has already laid the foundation for the next generation of exoplanet missions, mission team members said.
“In many ways, Kepler was a scout. It scouted deep into the galaxy to find out what the frequencies were, and to show there were a lot of planets to find. It’s accomplished that,” Kepler science principal investigator Bill Borucki of NASA’s Ames Research Center in Moffett Field, Calif., who led the team that found Kepler-62e and f, said at Thursday’s press conference.
“And now these new missions will come online and give us more information about these planets,” Borucki added, referring to efforts such as NASA’s Transiting Exoplanet Survey Satellite, which will launch in 2017 to search for nearby alien worlds. “But the big step is that step where we first start measuring the composition of the atmospheres, and that will be a very technologically difficult task.”
Scanning exoplanet air
Borucki and other researchers are keen to get a look at exoplanet atmospheres because the gases present in them can reveal a great deal about the worlds’ potential to host life.
Finding carbon dioxide, water and oxygen would bolster the case for a planet’s habitability, for example, while spotting extremely complex compounds could make headlines around the world.
“If there are freons, I mean, you’ve got it made,” Borucki said. “Obviously, intelligent life is there.”
Studying exoplanets’ air will require blocking out the overwhelming glare of their parent stars, which are a billion times brighter than the planets themselves, Borucki said.
That’s a daunting task but not an impossible one. A decade ago, in fact, a proposed NASA mission called the Terrestrial Planet Finder (TPF) devised two different techniques to study exoplanet atmospheres, with a possible maximum range of 30 light-years or more.
Funding for TPF never materialized, and the project is now regarded as cancelled. But Borucki expressed confidence that the ongoing exoplanet revolution sparked in large part by Kepler will bring the project back, though not necessarily under the same name.
“Undoubtedly, it at some point will be reinstated,” he told SPACE.com. “As we progress in the exploration of the galaxy, looking for life, we must start looking at the atmospheres. Everybody recognizes that.”
NASA’s Kepler space telescope has discovered three exoplanets that may be capable of supporting life, and one of them is perhaps the most Earth-like alien world spotted to date, scientists announced today (April 18).
That most intriguing one is called Kepler-62f, a rocky world just 1.4 times bigger than Earth that circles a star smaller and dimmer than the sun. Kepler-62f’s newfound neighbor, Kepler-62e, is just 1.6 times larger than Earth, making the pair among the smallest exoplanets yet found in their star’s habitable zone — the just-right range of distances where liquid water can exist on a world’s surface.
Kepler-62e and f, which are part of a newly discovered five-planet system, “look very good as possibilities for looking for life,” said Kepler science principal investigator Bill Borucki, of NASA’s Ames Research Center in Moffett Field, Calif. [Three Possibly Habitable Super-Earths Found (Gallery)]
The third potentially habitable planet, called Kepler-69c, is 1.7 times bigger than Earth and orbits a star similar to our own. It’s the smallest world ever found in the habitable zone of a sunlike star, researchers said, and represents a big step toward discovering the first-ever “alien Earth.”
“We’re moving very rapidly toward finding an Earth analogue around a star like the sun,” Borucki told SPACE.com.
Researchers announced these newfound planets — all three of which are “super-Earths,” or worlds slightly larger than our own — today at a NASA news conference. The Kepler-62 discovery paper, led by Borucki, was also published today in the journal Science; the Kepler-69 study, led by Thomas Barclay of the Bay Area Environmental Research Institute in Sonoma, Calif., appeared today in The Astrophysical Journal.
The three potentially habitable worlds are part of a larger haul. All told, the scientists rolled out seven new exoplanets today — five in the Kepler-62 system and two in Kepler-69.
Alien water worlds?
The five newfound planets in the Kepler-62 system, which is located about 1,200 light-years away in the constellation Lyra, range from 0.54 to 1.95 times the size of Earth. Only Kepler-62e and f are potentially habitable; the other three zip around the star at close range, making them too hot to support life as we know it, researchers said.
Kepler-62e and f take 122 and 267 days, respectively, to complete one orbit around their star, which is just 20 percent as bright as the sun. While nobody knows what the two exoplanets look like, a separate modeling study suggests they’re both probably water worlds covered by endless, uninterrupted global oceans. [Two Habitable Alien Water Worlds? (Video)]
“There may be life there, but could it be technology-based like ours? Life on these worlds would be under water with no easy access to metals, to electricity, or fire for metallurgy,” lead author Lisa Kaltenegger, of the Max Planck Institute for Astronomy and the Harvard-Smithsonian Center for Astrophysics, said in a statement.
“Nonetheless, these worlds will still be beautiful blue planets circling an orange star — and maybe life’s inventiveness to get to a technology stage will surprise us,” she added.
Not surprisingly, Kepler-62e should be warmer than its more distantly orbiting neighbor. In fact, Kepler-62f may require a greenhouse effect to keep its ocean from freezing over, researchers said.
“Kepler-62e probably has a very cloudy sky and is warm and humid all the way to the polar regions,” co-author Dimitar Sasselov of Harvard said in a statement. “Kepler-62f would be cooler, but still potentially life-friendly.”
The new modeling study has been accepted for publication in The Astrophysical Journal.
Searching for Earth’s twin
The $600 million Kepler observatory launched in March 2009 to hunt for Earth-size exoplanets in the habitable zone of their parent stars. Kepler finds alien worlds by detecting the tiny brightness dips caused when they transit, or cross the face of, their stars from the instrument’s perspective.
Kepler has used this technique to great effect, spotting more than 2,700 potential planets since its March 2009 launch. While just 120 or so of these candidates have been confirmed to date, mission scientists estimate that more than 90 percent will end up being the real deal.
While Kepler has yet to discover a true Earth twin, it’s getting closer and closer, Borucki said, pointing to the confirmation of Kepler-69c as an example. (That planet lies 2,700 light-years away, in the constellation Cygnus. Kepler-69c’s neighbor Kepler-69b, which is about twice the size of Earth and too hot to host life, was also announced today.)
“I think we’re making excellent progress in that direction,” Borucki said. “We have a number of candidates that look good.”
Such steady progress makes sense, since Kepler will of course spot more transits the longer it looks. The telescope needs to observe three transits to flag a planet candidate, so detecting a potentially habitable world in a relatively distant orbit can take several years.
Kepler cannot search for signs of life on worlds like Kepler-62e, Kepler-62f and Kepler-69c, but the telescope is paving the way for future missions that should do just that, Borucki said.
“This is one of the early steps, but there’s no mistake — we are on our way to explore the galaxy, to learn about life in the galaxy,” he said.
It is time for the private sector to aid in the search for potentially city-destroying asteroids and meteors, lawmakers said during a hearing Wednesday (April 10).
The House Committee on Science, Space and Technology made the call while hearing from NASA scientists and private-sector asteroid hunters during a hearing entitled “Threats from Space,” with both groups agreeing that something more needs to be done.
“Detecting asteroids should not be the primary mission of NASA,” Rep. Lamar Smith (R-Texas), chairman of the House Committee on Science, Space and Technology, said at the hearing. “No doubt the private sector will play an important role as well. We must better recognize what the private sector can do to aid our efforts to protect the world.” [Meteor Streaks over Russia, Explodes (Photos)]
The meeting Wednesday was the second of three aimed at understanding the threat to Earth posed by asteroids in space. The first hearing took place in late March, and addressed the ways governmental entities, like NASA and the Air Force, are mitigating the risks posed by close-flying space rocks. The meetings were scheduled in response to a surprise meteor explosion over Russia and the close flyby of asteroid 2012 DA14 — both of which occurred on Feb. 15.
Astronomers have mapped the orbits of more than 90 percent of the potentially world-ending asteroids in close proximity to the Earth; however, tracking anything smaller than 0.6 miles (1 kilometer) in diameter is more difficult, said Ed Lu, the CEO of the B612 Foundation, a nonprofit organization in the early stages of building a near-Earth-object-hunting space telescope scheduled for launch in 2018.
“NASA has not even come close to finding and tracking the 1 million smaller asteroids that might only just wipe out a city, or perhaps collapse the world economy if they hit in the wrong place,” Lu said at the hearing.
B612′s space telescope, dubbed Sentinel, will be built to aid in the search for smaller asteroids near Earth. Less than 10 percent of asteroids measuring around 459 feet (140 meters) in diameter have been found, while only 1 percent of all asteroids measuring around 131 feet (40 meters) — or “city killer” range — have been tracked, Lu said.
These city-destroying asteroids are notoriously difficult to track with the ground-based methods used by NASA today because the space rocks are relatively small and dark, said Don Yeomans, the head of NASA’s Near-Earth Object Program.
“A dramatic increase in near-Earth asteroid-discovery efficiencies is achievable using space-based infrared telescopes,” Yeomans said at the hearing.
Searching for space rocks in infrared light — as the $240 million Sentinel is expected to do — could allow astronomers to find a larger number of smaller objects that are too dark to be seen in visible light, Yeomans said.
A space-based asteroid hunter is also helpful because it can seek out space rocks at all hours of the day, as opposed to just at night, Yeomans added.
All of these hunting efforts should be put in place to find near-Earth objects well before they could hit the Earth, the panelists said.
At the moment, we have the technology to deflect an asteroid, but scientists won’t be able to use those methods without ample time to implement them, Michael A’Hearn, an astronomer working with the National Research Council, said at the hearing.
But first, the asteroids have to be found, Lu said.
“You can’t deflect an asteroid that you haven’t yet tracked,” Lu said. “Our technology is useless against something we haven’t yet found.”
NASA’s bold plan to drag an asteroid into orbit around the moon may sound like science fiction, but it’s achievable with current technology, experts say.
President Barack Obama’s 2014 federal budget request, which will be unveiled today (April 10), likely includes about $100 million for NASA to jump-start an asteroid-capture mission, U.S. Senator Bill Nelson (D-FL) said last week.
The plan aims to place a roughly 23-foot-wide (7 meters) space rock into a stable lunar orbit, where astronauts could begin visiting it as soon as 2021 using NASA’s Space Launch System rocket and Orion capsule, Nelson said.
While challenging, the mission is definitely doable, said Chris Lewicki, president and chief engineer of billionaire-backed asteroid-mining firm Planetary Resources. [NASA's Asteroid-Capture Plan (Video)]
“Return of a near-Earth asteroid of this size would require today’s largest launch vehicles and today’s most efficient propulsion systems in order to achieve the mission,” Lewicki, who served as flight director for NASA’s Spirit and Opportunity Mars rovers and surface mission manager for the agency’s Phoenix Mars lander, wrote in a blog post Sunday (April 7).
“Even so, capturing and transporting a small asteroid should be a fairly straightforward affair,” Lewicki added. “Mission cost and complexity are likely on par with missions like the [$2.5 billion] Curiosity Mars rover.”
Spurring solar system exploration
NASA’s idea is similar to one proposed last year by scientists based at Caltech’s Keck Institute for Space Studies in Pasadena.
The Keck study estimated that a robotic spacecraft could drag a 23-foot near-Earth asteroid (NEA) — which would likely weigh about 500 tons — into a high lunar orbit for $2.6 billion. The returns on this initial investment are potentially huge, the researchers said.
“Experience gained via human expeditions to the small returned NEA would transfer directly to follow-on international expeditions beyond the Earth-moon system: to other near-Earth asteroids, [the Mars moons] Phobos and Deimos, Mars and potentially someday to the main asteroid belt,” the Keck team wrote in a feasibility study of their plan.
The mission would also help develop asteroid-mining technology, advocates say, and advance scientists’ understanding of how our solar system took shape more than 4.5 billion years ago.
Asteroids “probably represent samples of the earliest matter that was made available to form our solar system and our Earth,” Caltech’s Paul Dimotakis, a member of the Keck study team, told SPACE.com in February.
“We learned a lot about the moon by analyzing the moon rocks that Apollo astronauts brought back,” he added. [NASA's 17 Apollo Moon Missions in Pictures]
Asteroids are fascinating for lots of reasons. They contain a variety of valuable resources and slam into our planet on a regular basis, occasionally snuffing out most of Earth’s lifeforms. How much do you know about space rocks?
Unmanned probes have successfully rendezvoused with asteroids in deep space multiple times. Japan’s Hayabusa craft even snagged pieces of the near-Earth asteroid Itokawa in 2005, sending them back to our planet for study.
But bagging an entire asteroid and dragging it to our neck of the cosmic woods is unprecedented, and it presents several daunting challenges.
For example, the target asteroid will be spinning, which doesn’t make for a smooth ride to lunar orbit. After the spacecraft captures the asteroid and brings it into a hold of sorts, the space rock will have to be de-spun, likely with thrusters, Dimotakis said.
“You might use reaction jets to take out most of it [the spin],” he said. “You would give you yourself a lot of time to do this, because there’s no second chance in any of this.”
Further, bringing the asteroid onboard greatly increases the spacecraft’s mass, making propulsion and navigation much more difficult. And precise navigation will definitely be required to deliver the space rock to its desired orbit, Dimotakis said (though he also stressed that any asteroid chosen would pose no danger to humanity even if it somehow struck our planet).
But ion thrusters like the ones powering NASA’s Dawn mission to the huge asteroid Vesta and dwarf planet Ceres should be muscular enough to make the journey, likely taking a few years to reach the asteroid and somewhat longer to come back. And the asteroid-laden probe could probably still be guided with great care, he added.
“My guess is that all of these are not insurmountable challenges, and you would be able to calibrate yourself after you snagged it and adjust your controls,” Dimotakis said.
Choosing a target
Perhaps the biggest challenge of the entire mission is picking a suitable space rock to retrieve, Lewicki wrote in his blog post.
The Keck study recommends going after a carbonaceous asteroid packed full of water and other volatiles. Carbonaceous asteroids can be very dark, and it’s tough to spot and characterize a 23-foot asteroid in the vast depths of space whatever its color.
So both Lewicki and Dimotakis stressed the importance of searching for potential asteroid targets sooner rather than later. Planetary Resources plans to begin launching a line of small prospecting space telescopes in 2014 or 2015, and these “Arkyd-100″ craft could aid NASA’s mission, Lewicki wrote.
Dimotakis, for his part, is engaged in a follow-up to the Keck study that’s looking for potential targets in observations made by current telescopes.
“We are developing software in collaboration with JPL [NASA's Jet Propulsion Laboratory] that is going to exploit the observational digital record and essentially flag things that could be of interest and might be in this class,” he said. “This has never happened before.”
Still, mission scientists and engineers shouldn’t just sit on their hands until an asteroid selection is made, he added.
It’s important “to start developing the spacecraft before you even know where you’re going,” Dimotakis said. “If you do these things in parallel, then the mission timeline shrinks.”
When it comes to life across the cosmos, the universe might just be an “awful waste of space” after all.
A new theory presented at a conference this week would confirm the worry of Ellie Arroway, Jodie Foster’s character in the film “Contact,” that life might not exist on other worlds.
Some scientists think that just because exoplanets could have habitable environments, that does not mean that life evolved there.
“The pervasive nature of life on Earth is leading us to make this assumption,” Charles Cockell, the director of the U.K. Center for Astrobiology at the University of Edinburgh, said in a statement.”On our planet, carbon leaches into most habitat space and provides energy for microorganisms to live. There are only a few vacant habitats that may persist for any length of time on Earth, but we cannot assume that this is the case on other planets.”
Cockell’s hypothesis states that, although habitable alien planets might abound in solar systems around the universe, it does not mean these locales harbor extraterrestrial life.
“It is dangerous to assume life is common across the universe. It encourages people to think that not finding signs of life is a ‘failure,’ when in fact it would tell us a lot about the origins of life,” added Cockell.
It is also possible that scientists will not be able to detect alien signs of life, even if it exists, Cockell said. Life might be markedly dissimilar from planet to planet, making it unlikely that astronomers on Earth will see recognizable signatures of life. But not all hope is lost.
“Professor Cockell explains that in coming decades, increasingly powerful telescopes and developments in spectroscopy may allow us to look for the signals of life on planets beyond our solar system,” officials from the Royal Society, the United Kingdom’s national academy of science, said in a statement.”However, regardless of this, our view is still going to be heavily influenced by our knowledge of life on Earth.”
Astronomers have their fingers crossed that within the haul of data collected by NASA’s Kepler mission, which has already detected nearly 3,000 possible exoplanets, hide the signatures of the very first exomoons.
The discovery of alien moons will open up an exciting new frontier in the continuing hunt for habitable worlds outside the solar system. With the confirmation of exomoons likely right around the corner, researchers have begun addressing the unique and un-Earthly factors that might affect their habitability.
Because exomoons orbit a larger planetary body, they have an additional set of constraints on their potential livability than exoplanets themselves. Examples include eclipses by their host planet, as well as reflected sunlight and heat emissions. Most of all, gravitationally induced tidal heating by a host planet can dramatically impact a moon’s climate and geology.
In essence, compared to planets, exomoons have additional sources of energy that can alter their “energy budgets,” which, if too high, can turn a temperate, potential paradise into a scorched wasteland. [9 Exoplanets That Could Host Alien Life]
“What discriminates the habitability of a satellite from the habitability of a planet in general is that it has different contributions to its energy budget,” said René Heller, a postdoctoral research associate at the Leibniz Institute for Astrophysics in Potsdam, Germany.
The ‘habitable edge’
In a series of recent papers, Heller and his colleague Rory Barnes from the University of Washington and the NASA Astrobiology Institute tackled some of the big-picture problems to habitability posed by the relationship between exomoons and their host planets.
Heller and Barnes have proposed a circumplanetary “habitable edge,” similar to the well-established circumstellar “habitable zone.” This zone is the temperature band around a star within which water neither boils off nor freezes away on a planet’s surface — not too hot, not too cold, thus earning it the nickname “the Goldilocks zone.”
The habitable edge is rather different. It is defined as the innermost circumplanetary orbit in which an exomoon will not undergo what is known as a runaway greenhouse effect. “To be habitable, moons must orbit their planets outside of the habitable edge,” Heller said.
A runaway greenhouse effect occurs when a planet’s or moon’s climate warms inexorably due to positive feedback loops. This phenomenon is thought to have taken place on Earth’s so-called “sister planet,” Venus.
On Venus, the heat from a young, brightening sun could have increasingly evaporated a primordial ocean. This evaporative process put ever more heat-trapping water vapor in the atmosphere, which led to more evaporation, and so on, eventually drying the planet out as the water was broken apart into hydrogen and oxygen by the sun’s ultraviolet radiation. The atmospheric hydrogen on Venus escaped into space, and without hydrogen, no more water could form. [Runaway Greenhouse Effect on Venus (Video)]
Moons situated in fairly distant orbits from their planets should be safely beyond the habitable edge wherein this desiccation takes place.
“Typically, and especially in the solar system, stellar illumination is by far the greatest source of energy on a moon,” Heller said. “In wide planetary orbits, moons will be fed almost entirely by stellar input. But if a satellite orbits its host planet very closely, then the planet’s stellar reflection, its own thermal emission, eclipses and tidal heating in the moon can become substantial.”
The cumulative effects of the non-tidal heating effects are small, but could be the difference between an exomoon being inside or outside the habitable edge.
Basking in the glow
Here on Earth, we get a little extra energy from the moon in the form of moonlight, which is reflected light from the sun.
Moons, though, get bathed in a lot more sunlight from their planetary neighbors; Earth shines almost 50 times as brightly in the lunar sky as the moon does in our night sky. In addition to reflected sunlight, planets also emit absorbed sunlight as thermal radiation onto their exomoons.
This “planetshine” can add a not-insubstantial amount of energy to an exomoon’s overall intake. Imagine a gas giant planet orbiting a sun-like star at about the same distance that Earth orbits our sun. For a moon with a relatively close orbit around this planet, like Io’s orbit around Jupiter, Heller calculates that the moon could absorb an additional seven or so watts per square meter of power. (Earth absorbs about 240 watts per square meter from the sun).
Periodic plunges into darkness
Eclipses can potentially offset some of the extra energy input from planetshine. For eclipses, Heller calculated that lost stellar illumination for an exomoon in a close orbit (similar to the closest found in our solar system) is up to 6.4 percent.
Interestingly, because most moons (including ours) are tidally locked to their planet — that is, one side of the moon constantly faces the planet — eclipses, as well as planetshine, would only darken and lighten one hemisphere. This phenomenon could modify the climate, as well as the behavior of life forms, in ways not seen on Earth.
“Asymmetric illumination on the moon could induce wind and temperature patterns, both in terms of geography and in time, which are unknown from planetary climates,” Heller noted. “Life on a moon with regular, frequent eclipses would surely have to adapt their sleep-wake and hunt-hide rhythms as well, but only those creatures on the planet-facing hemisphere.” [5 Bold Claims of Alien Life]
Although the eclipse-related loss of several percentage points of illumination is not a huge loss of energy, a moon-planet duo might need to be closer to its star to compensate for this deficit if the moon were still to be considered habitable from a Goldilocks zone perspective.
However, this situation introduces another hurdle to habitability: The closer a planet is to its star, the stronger the star’s gravitational pull is on the planet’s moons. This extra pull can tug moons into non-circular, or eccentric, orbitsabout their planets. Eccentric orbits, in turn, result in varying amounts of gravitational stress exerted on the moon as it orbits.
These “tidal forces,” as they are called, cause heating due to friction. The ocean tides we experience on Earth occur partly as a result of the moon’s gravity tugging more on the water and land nearest it, which distorts Earth’s shape. The effect goes both ways, of course, but not equally, with planets inducing significantly greater tidal heating within their much smaller moons.
If an exomoon’s orbit takes it too close to its planet, tidal heating could push the energy budget too high, culminating in a runaway greenhouse effect. At the extremes, the tidal heating could unleash massive volcanic activity, leaving the satellite covered in magma and distinctly inhospitable, like the “pizza moon” Io.
On the other hand, it should be noted, tidal heating might be a savior for life. Tidal heating could help sustain a subsurface ocean, like the one suspected to exist within Jupiter’s moon Europa, alternatively making an otherwise unwelcoming exomoon outside the traditional habitable zone potentially livable. [Photos: Europa, Mysterious Icy Moon of Jupiter]
Small stars, dead moons
Another factor comes into play as eclipses rob a bit of energy from an exomoon and require the moon-planet pair to be closer to their star. To remain gravitationally bound to a planet and not be ripped away by the star’s gravity, a moon must fall within a so-called “Hill radius” — the planet’s sphere of gravitational dominance. This radius shrinks with greater proximity to the host star. The closer the planet and moon are to their star, the less space is available outside the habitable edge.
For planets and attendant moons around dim, cool, low-mass stars called red dwarfs, this dynamic becomes important. The habitable zone around red dwarf stars is very tight; for a star with a quarter of the sun’s mass, for instance, the Goldilocks zone is thought to be around just 13 percent the sun-Earth distance – in other words, a third of Mercury’s orbital distance from the sun.
In a red dwarf solar system, not only must a moon then be closer to its habitable zone planet, but given the planet’s necessary proximity to its star, the moon’s orbit will tend to be eccentric. These qualities increase the chances that the moon will fall within the habitable edge.
Heller calculated that for many red dwarf stars, the odds of them hosting habitable moons is accordingly slim.
“There is a critical stellar mass limit below which no habitable moon can exist,” Heller said. “Around low-mass stars with masses of about 20 percent the mass of the sun, a moon must be so close to its habitable zone planet to remain gravitationally bound that it is subject to intense tidal heating and cannot under any circumstances be habitable.”
A little here, a little there
Many factors beyond habitable edge considerations, of course, ultimately determine an exomoon’s habitability.
To be considered broadly habitable by creatures other than, say, subsurface bacteria, an exomoon must meet some of the same basic criteria as a habitable, Earth-like exoplanet: It must have liquid surface water, a long-lived substantial atmosphere and a magnetic field to protect it from solar radiation (and, in the case of exomoons around gas giants like Jupiter, from the charged particles created in the giant exoplanet’s magnetosphere).
To possess these qualities, which scientists say grow likelier with increasing mass, a habitable exomoon will likely be quite large compared to those in the solar system – more on the order of the size of Earth itself. The biggest moon in our solar system, Jupiter’s Ganymede, is just 2.5 percent of Earth’s mass. But previous studies have suggested that monstrous moons by the solar system’s standards are indeed possible.
NASA’s Kepler mission is expected to be able to detect exomoons down to about 20 percent of the mass of the Earth. The data, which consists of measuring the extremely small dips in the amount of starlight as their planets (or moons) block it from our point of view – should reveal a moon’s mass and orbital parameters as well.
Armed with this information — and now with habitable edge considerations — astronomers can thus hope to make some ballpark speculations on any soon-to-be-discovered exomoon’s propensity to support living beings.
Heller hopes that there will be a list of candidate exomoons ready for observing by next-generation instruments, such as NASA’s James Webb Space Telescope and various 30-meter-class ground telescopes. These observatories, coming online in the next decade, could be able to characterize exomoon atmospheres and offer tantalizing evidence of life.
“The first exomoons that we find will be large — maybe Mars- or even Earth-sized — and therefore intrinsically more likely to be habitable than small moons,” Heller said. “With Kepler finding many more giant planets than terrestrial planets in stellar habitable zones, it’s really important that we try to figure out what conditions might be like on the moons of these giants to gauge if they can host extraterrestrial life.”
A white dwarf is a dead star that slowly cools down until it fades into oblivion. Yet it has been predicted that habitable planets can orbit a white dwarf. If we can somehow detect these planets, would we also be able to spot signs of life?
Scientists have created an artificial spectrum showing that the upcoming James Webb Space Telescope (JWST) will be capable of detecting oxygen and water on an Earth-like planet orbiting a white dwarf.
A white dwarf is the end stage of evolution of a low mass star, and it is tiny compared to its former self. The habitable zone around a white dwarf would therefore have once been located deep within the region of space the star once inhabited, requiring planets to migrate inwards to experience temperatures that are just right for surface liquid water. Infrared observations also have revealed disks of dust surrounding some white dwarfs, which could be the birthplace of a new generation of planets.
Searching for signs of life on a white dwarf exoplanet will involve inspecting the spectral fingerprint of the planet’s atmosphere. Exoplanet atmospheres can be detected and analyzed during a planetary transit, when a planet passes in front of a star from our point of view, as the background starlight shines through the planet’s atmosphere. Elements in the atmosphere will absorb some of the starlight, meaning that more light than normal will be blocked at the particular wavelengths associated with that element, giving us a spectrum of the planet. [Photo Tour: Building NASA's James Webb Space Telescope]
This technique, called transmission spectroscopy, is difficult to utilize because the parent star is incredibly bright and thus washes out most of the planetary signal. However, if the host star is a white dwarf instead of a main sequence star, then the small stellar radius will result in a very prominent transit signal. The diameter of the average white dwarf is around 17,000 kilometers, which isn’t much bigger than the Earth’s diameter of 12,800 kilometers. Therefore, although white dwarfs are dim and hard to detect, it should still be possible to see the signal of an Earth-like planet transiting one.
A sign of life
Certain elements in a planet’s atmosphere may indicate the presence of life. Such “biomarkers” include oxygen and methane, gases that are produced by different forms of life on Earth and would quickly degrade if they weren’t constantly being generated.
Some of Earth’s biomarkers are prominent in the infrared region of the spectrum, making JWST ideal to search for signs of life on other planets. The JWST, due to launch in a few years time, will be looking in the infrared part of the light spectrum, and it will be able to observe atmospheres on planets that are only a few times the mass of the Earth in the habitable zones of M-type stars (red stars that are cooler than our Sun). However, the total amount of observing time needed for this far exceeds that which is needed to observe planets around white dwarfs, since the planetary signal is much weaker for the brighter M-dwarfs.
Avi Loeb from Harvard University and Dan Maoz from Tel-Aviv University in Israel decided to test what kind of information they might be able to pry from the atmosphere of a planet orbiting a white dwarf by creating a simulated JWST spectrum. Their synthetic spectrum showed that the oxygen (O2) “A-band” should be easily visible, as well as signatures of water (H2O), assuming they exist on the planet.
“Detecting any of these biomarkers in the atmosphere of an Earth-copy planet around a nearby normal star, using JWST, will be extremely challenging, if not impossible,” Maoz told Astrobiology Magazine. “The difficulty lies in the extreme faintness of the signal, which is hidden in the glare of the ‘parent’ star. The novelty of our idea is that, if the parent star is a white dwarf, that glare is greatly reduced, and one can now realistically contemplate seeing the O2 biomarker. Detecting other biomarkers will require future space telescopes that are even more ambitious than JWST.”
A strong signature of oxygen in a planetary spectrum could indicate that life is present, since oxygen needs to be produced in vast quantities in order to counteract how easily it reacts with other substances. Biological processes are the main cause for high amounts of oxygen: the 21 percent of oxygen in the Earth’s atmosphere is produced by photosynthesis in plants and algae. If life on Earth were suddenly to be extinguished, then all the oxygen would be removed in one or two million years as it combines with rocks and dissolves in the ocean. [9 Exoplanets That Could Host Alien Life]
The search for elusive planets
No planets have yet been detected orbiting a white dwarf, due to the difficulty in observing these faint stars, however there is some evidence to suggest that such planets might exist. White dwarfs should typically have a pristine spectrum of either hydrogen or helium, as any heavier elements will sink deep within the star. However, many white dwarfs show signs of pollution by heavy elements, possibly due to rocky material in circumstellar disks being perturbed inwards by unseen planets.
Some eclipsing binaries which contain a white dwarf have been observed to have unusual variations in the timing of the eclipses, which could indicate that a planet is present. Planets have also been discovered around pulsars, showing that it is possible for planets to orbit compact stellar remnants.
In order to detect Earth-like planets around white dwarfs, a survey will first need to be performed to select the brightest, nearest white dwarfs suitable for JWST observations. Many white dwarfs will need to be monitored in order to guarantee the best chance of detecting a planet. For instance, if a third of all white dwarfs host an Earth-mass planet within their habitable zones, then 500 white dwarfs would need to be monitored to discover just one transiting Earth.
“We expect to find maybe one or two Earth-like planets that transit white dwarfs, and are observable with JWST, *if* such planets at all exist around white dwarfs,” said Maoz.
A survey searching for an O2 biomarker would probably benefit from focusing on white dwarfs that are over three billion years old. It took around two billion years for life on Earth to start producing significant amounts of O2, so neglecting young white dwarfs would turn the focus more on planets where life has had time to evolve. While this is obviously biased, scientists feel it makes more sense to invest limited observing time on the most likely candidates.
Although Earth-like planets are the most interesting targets from an astrobiology perspective, it turns out that they are also the optimum targets when one is looking to detect signs of life on white dwarf planets. A planet with a diameter similar to that of the Earth is just the right size for astronomers to be able to detect a good spectrum of the planet’s atmosphere. If a planet is larger than the white dwarf, the probability of the planet’s atmosphere transiting the star will be similar to that of an Earth-size planet. However, the greater surface gravity of the bigger planet will mean that the height of the atmosphere would likely be much lower, meaning less starlight passes through it, thus making it harder to detect.
The advantage to using JWST to observe exoplanetary atmospheres lies in the fact that, since it will be a space-based telescope, it will be liberated from the Earth’s atmosphere. If an exoplanet atmosphere is very similar to the Earth’s atmosphere, then a ground-based telescope will have great difficulty disentangling it from the Earth’s atmospheric spectrum. (However, if the exoplanet atmosphere is vastly different, then it could be detected amid the Earth’s own signal, and large ground-based telescopes could then help to provide measurements.) As we move into the era of searching for biomarkers on extrasolar planets, Earth-like planets around white dwarfs may be the first alien worlds where we can detect such indications of life.
Forecasting when stars will die in giant explosions may one day be possible by looking for the warning outbursts they release beforehand, researchers say.
Supernovas are the most powerful stellar explosions in the universe, visible all the way to the edge of the cosmos. These stars detonate for two known reasons: either from gorging on too much mass stolen from a companion star or by running out of fuel and abruptly collapsing.
Astronomers have suggested that stars can give off smaller explosions just before they go supernova. To find out more about supernovas, researchers used three telescopes — the Palomar Observatory, the Very Large Array and NASA’s Swift mission — to investigate a star 500 million light-years away. The star, which had about 50 times the mass of the sun, ultimately detonated as a supernova named SN 2010mc.
The researchers’ data suggest that 40 days before the final explosion, the dying star produced a giant outburst, releasing as much matter as 1 percent the mass of the sun — about 3,330 times the mass of Earth — at about 4.5 million mph (7.2 million km/h). [Photos of Great Supernova Explosions]
“What is surprising is the short time between the precursor eruption and the eventual supernova explosion; one month is an extremely tiny fraction of the 10-million-year lifespan of a star,” said one of the study authors, Mansi Kasliwal at the Carnegie Institution for Science in Pasadena, Calif.
This explosion radiated “about a million times more than the energy output of the sun in an entire year,” author Mark Sullivan of the University of Southampton in England told SPACE.com. But this precursor “is still about 5,000 times less than the energy output of the subsequent supernova.”
The close timing between the outburst and the ensuing supernova suggest they are related, lead author Eran Ofek of the Weizmann Institute of Science in Israel said in an email interview. Probability models revealed there was only a 0.1 percent chance that the outburst was a random event.
“Our discovery of SN 2010mc shows that we can mark the imminent death of a massive star. By predicting the explosion, we can catch it in the act,” Kasliwal said.
Comparing their data with three models proposed for how the preceding explosion might have occurred, the researchers found that gravity waves helped drive mass to the star’s atmosphere. Gravity waves are fluctuations caused by matter rising due to buoyancy and sinking due to gravity.
“For a star like our sun, the energy it is emitting from the fusion of hydrogen into helium deep in the core exerts an outward pressure on the star, usually counteracted by an inward pressure from gravity. However, if the star’s luminosity increases above a certain amount — the so-called Eddington luminosity — the outward pressure from the resulting radiation is strong enough to overcome the gravity, which can then power an outflow of material,” Sullivan explained. “Gravity waves can act as a conduit to translate this large, super-Eddington luminosity in the core into an ejection of material from the outer envelope of the star, just like we observed.”
Mars is farther away than any near-Earth asteroid that NASA would target, but this disadvantage may be outweighed by the greater knowledge scientists have gained of the Red Planet thanks to the many Mars missions that have launched over the years, experts say.
Further, mapping out an asteroid mission is nearly impossible at this point, since NASA does not yet know where it’s going.
“There are still no good asteroid targets for such a mission, a necessary prerequisite for determining mission length and details such as the astronauts’ exposure to radiation and the consumables required,” states a December 2012 report from the U.S. National Research Council (NRC). [How NASA Will Explore Asteroids (Gallery)]
The road to Mars
Landing astronauts on Mars has been the long-term goal of NASA’s human spaceflight program for decades, but the agency’s vision of how to get there was shaken up recently.
NASA had viewed the moon as a stepping stone, working to get humans to Earth’s natural satellite by 2020 under a program called Constellation, which was initiated during the presidency of George W. Bush. But President Barack Obama cancelled Constellation in 2010, after an independent review panel found it to be significantly under-funded and behind schedule.
instead directed NASA to send astronauts to a near-Earth asteroid by 2025, then on to the vicinity of Mars by the mid-2030s. The agency is developing a new crewed capsule called Orion and a huge rocket called the Space Launch System to make it all happen.
The new “asteroid-next” plan has not been enthusiastically embraced by NASA or the broader space community, the NRC report concluded.
“Despite isolated pockets of support for a human asteroid mission, the committee did not detect broad support for an asteroid mission inside NASA, in the nation as a whole or from the international community,” write the authors of the report, which is called “NASA’s Strategic Direction and the Need for a National Consensus.”
A tough proposition
The NRC report was based on research, interviews, site visits and analysis conducted by a 12-member independent committee over the course of about five months in 2012.
One of the people the study team met with was Bill Gerstenmaier, NASA’s associate administrator for human exploration and operations.
Gerstenmaier “talked about how NASA had discovered, in the two years that had elapsed by the time he was speaking to us, just how hard [a manned asteroid mission] was,” committee member and space policy expert Marcia Smith said during a presentation with NASA’s Future In-Space Operations working group on Jan. 30.
“He said in many respects, it’s easier to go to Mars, because we know a lot about Mars,” Smith added. “We know where it is, and we’ve done all these reconnaissance missions already, so we have a knowledge base from which to work in terms of sending humans, whereas no particular asteroid has been selected yet.”
While sending astronauts to an asteroid has never been done before, unmanned probes have successfully rendezvoused with the objects in deep space multiple times.
For example, NASA’s Dawn spacecraft orbited the protoplanet Vesta — the second-largest body in the main asteroid belt between Mars and Jupiter — for more than a year before departing to head to the belt’s largest denizen, Ceres, last September. And in 2005, Japan’s Hayabusa probe plucked some pieces off the near-Earth asteroid Itokawa, sending them back to Earth for analysis.
NASA plans to launch its own asteroid-sampling mission, called Osiris-Rex, in 2016. And two private companies — Planetary Resources and Deep Space Industries — intend to loft reconnaissance spacecraft over the next few years, kicking off an ambitious efforts to mine water, metals and other resources from asteroids.
Herschel’s claim was contested almost immediately, with some saying he had seen only a fairly common star in 1842.
Now the question of whether Herschel actually saw a recurrent supernova or a common star has finally been answered, clearing up a point about stars that “go off” periodically.
To solve the 150-year-old mystery, Bradley Schaefer of Louisiana State University dug through the records of the Royal Society in Britain, to which Herschel donated his papers. Schaefer was unable to find the astronomer’s original chart, but he found the second best thing: a copy made by Herschel and sent to another astronomer a few short weeks after the 1866 explosion.
The document revealed that what Herschel observed was not the recurrent nova T Coronae Borealis (T CrB) but another star, BD+25°3020.
Blown away — again
Rather than dying in a single blaze of glory, recurrent novas cycle through explosions on a steady basis. White dwarf stars pull in material from companion stars, and they flare up when enough material has fallen onto their surfaces. Understanding just how often an individual nova, such as T Coronae Borealis (T CrB), explodes is crucial to understanding objects that could eventually evolve into Type 1a supernovas.
But in 1866, novas were not well understood.
“When T CrB went off, the world of astronomy became ablaze,” Schaefer said during a presentation in January at the 221st meeting of the American Astronomical Society. [Supernova Photos of Star Explosions]
Back in 1866, John Herschel, son of astronomer Sir William Herschel, dug through his records to find a map of the night sky he had made nearly 24 years before. But the published chart seemed to place what Herschel claimed was the explosion near the spot of another star, and generated an almost immediate response from the astronomical community.
“We had a couple of people coming up to Herschel saying, ‘Hey, are you sure this isn’t just the BD star?’” Schaefer said.
The recurrent nova white dwarf exploded again in 1946, which would give it a time scale of 80 years between flares. But if Herschel saw it explode in 1842, that would change the time scale of the star, and call into question astronomers’ understanding of these repeating explosions.
Solving the mystery
For Schaefer, who studies recurrent novas, solving the mystery wasn’t as simple as determining exactly where Herschel’s mystery object sat in the sky. The BD star is too faint to be observed at sea level with the naked eye, according to Schaefer, so Herschel could not have seen it without assistance. If Herschel was relying on his own eyes to map the night sky, he must have seen T CrB — or so the argument goes.
Digging through letters, Schaefer found a notation that all of Hershel’s observations weren’t made unaided. On occasion, the British scientist used an opera glass, which would have allowed him to see the BD star.
Still, this wasn’t definitive enough. Schaefer kept digging, searching for the original sky map.
Instead, he found a letter from Herschel to another astronomer with a diligently replicated chart.
According to Schaefer, Herschel placed a heavy piece of paper under the original chart and used pins to precisely map the location of each star in the sky. He sent the duplicate chart to the fellow astronomer.
“We have him guaranteeing it’s a fair copy,” Schaefer told SPACE.com.
The chart revealed that the object Hershel observed sat in the same position as the BD star, and not where T CrB lit up the sky.
“T CrB did not go off in 1842,” Schaefer said, closing the door on 150-year-old mystery.