NASA Ames/SETI Institute/JPL-Caltech
The alien planet Kepler 186f is the first extrasolar world ever found to be about the size of Earth and in the habitable zone of its parent star. But if scientists ever hope to try to find life on such a planet, giant new space telescopes are needed.
Cowboys & Aliens are Coming!
July 29, 2011 --
If aliens are going out of their way to kick up dust in the Wild West, as they do in the upcoming movie "Cowboys & Aliens," they must be coming from somewhere. Life could take root on a moon or a meteorite. But to nurture the kind of life that could destroy our saloons and harass our livestock, a planet might be the most suitable. So far, Kepler, a NASA orbiting telescope that searches for planets beyond our solar system, has detected over 1,200 exoplanets. Surely there must be a few candidates among this group that could meet some of the most basic requirements to host life? Explore some far-out worlds that could support aliens, be they cattle-rustling characters or a more peaceful people.
First, let's lay out some basic criteria. Kepler hasn't identified many rocky worlds and a solid surface is essential for life to take root. Size matters: The mass of the planet helps astrophysicists infer what it's made of. Some planets are Earth-sized. Others are several times the size of our planet. And then there are gas giants, which can range from "Neptune sized" to "super-Jupiters." Orbit: To support life, a planet must be in a stable orbit around its star -- no planets with wonky orbits that will eventually dump them into their star for a fiery death. Goldilocks Zone: This is a region not too hot or too cold that gives the planet enough distance from its parent star to have liquid water, key for life. Loner Stars: Single stars make better parents. In 2010, a pair of closely orbiting binary stars was spotted surrounded by what could be the debris of former planets. Unknowns: Some factors for life can't be confirmed one way or the other from the data available about extrasolar planets. These include: water, chemical compounds such as ammonia; a nitrogen-rich atmosphere; a magnetic field to repel solar and cosmic radiation; and more. BUT, some planets do have a head-start, beginning with Gliese 581D.
Located a mere 20 light-years away, practically our backyard in cosmic terms, Gliese 581d is situated on the "outer fringes" of the Goldilocks zone, orbiting a red dwarf star. The planet may be warm enough and wet enough to support life in much the same manner as Earth. It might also contain a thick carbon atmosphere. If we ever need a new Earth and have the means to get there, Gliese 581d may be our best bet for now.
When it was first detected and reported last year in Astrophysical Journal, Gliese 581g appeared to be the perfect candidate for a true "Earth-like" planet. Located in the same star system as Gliese 581d (and detected earlier), Gliese 581g seemed to be the right size and located within a habitable zone away from its parent star. Gliese 581g was said to have three times the mass of Earth, making it possible for the planet to hold an atmosphere. However, since its discovery, follow-up studies have alleged that Gliese 581g might have been a false alarm. In other words, the planet might not exist at all.
Dubbed a "waterworld" and located a mere 42 light-years from Earth, GJ 1214b orbits near a red dwarf star about one-fifth the size of our sun. What makes this planet unique is that it appears to be primarily composed of water, although GJ 1214b is 6.5 times the mass of Earth and 2.7 times wider, which classifies it as a "super-Earth." This planet also has a steamy atmosphere composed of thick, dense clouds of hydrogen, which, although it might not the case with this planet, could incubate life.
Situated 150 light-years from Earth, HD 209458b is a planet that holds traces of water vapor in its atmosphere, and also contains basic organic compounds that, on Earth, foster the development of life. But there are two factors working against HD 209458b as a suitable habitat. The planet is very hot due to its close proximity to its parents star, and it's a gas giant, so no solid surfaces.
If Kepler-10b were located further from its parent star, it might have had a chance of hosting life. Kepler-10b was the first "iron-clad proof of a rocky planet beyond our solar system" back in 2001. It was even dubbed the "missing link" of extrasolar planetary research. When it comes to the search for life, though, Kepler 10-b is missing a lot of other ingredients -- just minor things like water or an atmosphere.
When venturing to a new star system to explore the possibility of extraterrestrial life, trying a star that has already shown itself to nurture planets -- even if they're not the kind you're looking for -- could be a promising strategy. Project Icarus, an ambitious five-year study into launching an unmanned spacecraft to an interstellar destination, has identified two stars located within 15 light-years that might fit the bill: "epsilon Eridani, a single K star 10.5 light-years away, and the red dwarf GJ 674, 14.8 light-years away." Indirect evidence has also shown that epsilon Eridani may already hold smaller worlds scientists simply haven't detected yet. Also, red dwarf star systems generally may be a safe haven for life.
Are We Alone?
Taking into account the number of exoplanets that have been detected, as well as the vastly greater number that are estimated to be out there, some astrophysicists are convinced that extraterrestrial life is inevitable. After all, the Milky Way may be loaded with as many as 50 billion alien worlds. Some even think we'll find alien life by 2020. Others, however, say it may not exist at all. Recently, astrophysicists David Spiegel of Princeton University and Edwin Turner from the University of Tokyo suggested we might be alone in the universe, based on their interpretation of the Drake equation, a formula meant to determine loosely the probability of the existence of life beyond Earth. According to their analysis, just because life on Earth took shape early, endured and prospered doesn't mean the same process would naturally and inevitably occur elsewhere in the universe. Discovering life elsewhere, however, would be the only means of settling this debate. Unless the aliens find us first, of course.
Humanity will probably have to wait a few decades to find out if life is common beyond our own solar system.
While NASA's James Webb Space Telescope (JWST) — which is scheduled to launch in 2018 — will be capable of finding signs of life on nearby exoplanets, a broad and bona fide hunt for life beyond Earth's neighborhood will require bigger spacecraft that aren't even on the agency's books yet, experts say.
"To find evidence of actual life is going to take another generation of telescopes," JWST telescope scientist Matt Mountain, director of the Space Telescope Science Institute in Baltimore, said during a NASA briefing Monday (July 14). "And to do that, we're going to need new rockets, new approaches to getting into space, new approaches to large telescopes — highly advanced optical systems." [10 Exoplanets That Could Host Alien Life]
A Chance to Find Signs of Life
The $8.8 billion JWST features 18 hexagonal mirror segments that will work together to form one 21-foot-wide (6.5 meters) mirror — larger than any other mirror that's ever flown in space, NASA officials said. (For comparison, the agency's iconic Hubble Space Telescope sports an 8-foot, or 2.4 m, primary mirror.)
JWST is optimized to view in infrared light. The telescope should be able to do lots of different things during its operational life, researchers say, including scanning the atmospheres of alien planets for oxygen and other gases that could be produced by living organisms. (Such delicate work is best performed by space telescopes, which don't have to look through Earth's atmosphere.)
JWST will work in concert with another NASA space mission in this regard, performing follow-up observations on promising nearby worlds found by the agency's Transiting Exoplanet Survey Satellite (TESS), which is scheduled to blast off in 2017.
"With the James Webb, we have our first chance — our first capability of finding signs of life on another planet," MIT astrophysicist Sara Seager said during Monday's NASA briefing. "Now nature just has to provide for us." [5 Bold Claims of Alien Life]
A Numbers Game
But nature may not be so willing, at least during the JWST mission, Seager and other experts stress. And it all comes down to numbers.
There is no shortage of planets in the Milky Way. Our galaxy teems with at least 100 billion planets, 10 to 20 percent of which, Mountain said, likely circle in their host star's "habitable zone" — that just-right range of distances that could allow liquid water to exist on a world's surface. If there's nothing terribly special about Earth, then life should be common throughout the cosmos, many scientists think.
The SETI Institute's Allen Telescope Array (ATA) is hunting for radio signals from hypothetical intelligent alien life in our galaxy.SETI Institute
But most exoplanets are very far away, and all of them are faint. JWST, while large by current standards, won't have enough light-collecting area to investigate more than a handful of potentially habitable planets, researchers say.
A spacecraft with a 33-foot (10 m) mirror would give researchers a much better chance of finding biosignatures in alien atmospheres, but Mountain would like something even bigger.
"With a 20-meter telescope, we can see hundreds of Earth-like planets around other stars," he said. "That's what it takes to find life."
Laying the foundation
There are no concrete plans to build and launch such a large space telescope, whose size would pose a number of logistical and engineering challenges. However, JWST is a potentially big step along the way to this goal.
For example, the JWST team figured out how to make mirror segments with incredible precision — a skill that could come in handy down the road.
"They're basically perfect," said JWST senior project scientist John Mather of NASA's Goddard Space Flight Center in Greenbelt, Maryland, who won a Nobel Prize in 2006 for his work with the agency's Cosmic Background Explorer satellite.
"If we were to expand the mirror to the size of the continental United States, the mirror would be accurate to within 3 inches," Mather said. "This is completely amazing technology we have now mastered and are using."
The hunt for life on distant worlds will be a multi-generational effort that goes from TESS and JWST to other, larger space telescopes, Seager said. And overcoming the various challenges involved will almost certainly require the cooperation of a number of different countries and organizations.
"Putting together the partnership that can find Earth 2.0 is a challenge worthy of a great generation," Mountain said.
Closer to home
All of this does not necessarily mean, however, that alien life won't be detected until humanity launches an enormous space telescope. Indeed, confirmation that Earthlings aren't alone in the universe may come from worlds much closer to home.
For example, NASA's next Red Planet rover, which is due to launch in 2020, will hunt for signs of past Mars life. And both NASA and the European Space Agency have plans to mount a mission to Jupiter's ocean-harboring moon Europa, which many experts regard as the solar system's best best to host alien life.
Europe's JUpiter ICy moons Explorer (JUICE) mission is currently scheduled to blast off in 2022 to study the Jovian satellites Callisto and Ganymede in addition to Europa. NASA officials have said they hope to launch a Europa mission sometime in the mid-2020s.
More from SPACE.com:
Ten Alien Encounters Debunked
Alien Planet Quiz: Are You an Exoplanet Expert?
New Earth-Size Planet Could Have Water | Video
Copyright 2014 SPACE.com, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.