• Extend search for life beyond 'alien Earths,' astronomer urges

    NASA / Ames / JPL-Caltech

    This artist's concept depicts Kepler-62e, a super-Earth planet in the habitable zone of a star smaller and cooler than the sun, located about 1,200 light-years from Earth in the constellation Lyra.

    By Mike Wall
    Space.com

    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 Thursday 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."

    Extending the habitable zone
    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.

    Looking for life
    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.

    Follow Mike Wall on Twitter @michaeldwall and Google+. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.

    Copyright 2013 Space.com, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

  • Estimate suggests that our galaxy contains 17 billion sizzling-hot Earths

    NASA / JPL-Caltech / T. Pyle

    An artist's rendering shows a typical close-in Earth-size planet, Kepler-20e, which is about 0.87 times as wide as our planet but orbits its parent star more closely than Mercury orbits our sun.

    By Alan Boyle, Science Editor, NBC News


    A simulation based on data from NASA's planet-hunting Kepler mission has determined that about one out of every six stars has an Earth-sized planet, which would translate to at least 17 billion such worlds in our Milky Way galaxy. And that's not even counting the alien Earths we'd want to live on.

    These 17 billion planets would be circling their parent stars more closely than Mercury orbits our own sun — which means that, in many cases, the planets would be too hot for liquid water to exist. A few such worlds already have been found, including a "lava planet" known as Alpha Centauri Bb that's just 4.3 light-years away from us.

    Someday, the type of simulation that astronomers used to estimate the number of hot Earths can be used to estimate how many habitable Earths could provide a home for life as we know it in the Milky Way. But not just yet.

    "For an estimate of Earth-sized planets in the habitable zone, it's simply too early to call," said Francois Fressin of the Harvard-Smithsonian Center for Astrophysics, or CfA.

    Fressin and his colleagues lay out their estimates for Earth-sized planets, as well as bigger worlds, in a paper that's been accepted for publication in the Astrophysical Journal. Their research is being discussed today at the American Astronomical Society's winter meeting in Long Beach, Calif.

    The estimates are based on a list of 2,400 planet candidates that have been detected by the Kepler probe since its launch in 2009. Kepler looks for planets in a patch of sky overlapping the constellations Cygnus and Lyra, by checking for the faint dimming of a star as an alien world passes across its disk. One of the challenges is to make sure the dimming is really caused by a planet, rather than some other phenomenon such as an eclipsing binary star. Another challenge is that Kepler is sure to miss some planets, because those planets are not in a position to block the light of its parent star, as seen from Earth.

    Now that the Kepler mission has been churning out detections for more than three years, there's enough of a database to arrive at some statistical conclusions about the total number of planets in the Milky Way — at least 100 billion. There's also enough data to determine what the breakdown of detections should be, and even how many of those detections will be wrong.

    "We have a knowledge of false positives that's good enough that we can do a study from scratch," Fressin said.

    The simulation suggests that the false-positive rate should vary depending on the size of the planet candidates, from a low of 6.7 percent for small Neptune-scale planets to a high of 17.7 percent for Jupiter-type giants. The false-positive rate for close-in planets between 0.8 and 1.25 times as wide as Earth is 12.3 percent. When all these factors were added to the calculations, the astronomers arrived at a breakdown for five types of planets currently detectable by Kepler:

    • 17 percent for Earths with orbital periods up to 85 days.
    • 26 percent for super-Earths (1.25 to 2 times as wide as Earth) with orbits up to 145 days.
    • 26 percent for small Neptunes (2 to 4 times Earth's width) with orbits up to 245 days.
    • 3 percent for large Neptunes (4 to 6 times Earth's width) with orbits up to 418 days.
    • 5 percent for giants (6 to 22 times Earth's width) with orbits up to 418 days.

    The results indicate that for every size of planet except for gas giants, the type of star doesn't matter. Earth-sized planets should be just as likely to form around red dwarfs as around sunlike stars. That runs counter to what was previously thought.

    "Earths and super-Earths aren't picky. We're finding them in all kinds of neighborhoods," the CfA's Guillermo Torres, a co-author of the study, said in a news release.

    The researchers emphasized that these are just minimum estimates — and that as Kepler provides more planet candidates at smaller scales and wider orbits, the numbers could increase. Eventually, such simulations could spit out a long-sought number: the tally of Earth-sized planets in the Milky Way expected to have conditions capable of supporting life.

    "This result is a significant step towards the determination of eta-earth, the occurrence of Earthlike planets in the habitable zone of their parent stars," they wrote in their research paper.

    More about the planet search:

    In addition to Fressin and Torres, the authors of "The False Positive Rate of Kepler nd the Occurrence of Planets" include David Charbonneau, Stephen Bryson, Jessie Christiansen, Courtney Dressing, Jon Jenkins, Lucianne Walkowicz and Natalie Batalha.

    Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.