Our galaxy is teeming with planets. Over the last 25 years, astronomers have cataloged about 2,000 worlds in 1,300 systems scattered around our stellar neighborhood. While most of these exoplanets look nothing like Earth (and in some cases, like nothing that orbits our sun), the bonanza of alien worlds implies a tantalizing possibility: There is a lot of real estate out there suitable for life.
We haven’t explored every corner of our solar system. Life might be lurking beneath the surface of some icy satellites or in the soil of Mars. For such locales, we could conceivably visit and look for anything wriggling or replicating. But we can’t travel (yet) to worlds orbiting remote suns dozens of light-years away. An advanced alien civilization might transmit detectable radio signals, but primitive life would not be able to announce its presence to the cosmos.
On Earth, life alters the atmosphere. If plants and critters weren’t around to keep churning out oxygen and methane, those gases would quickly vanish. Water, carbon dioxide, methane, oxygen and ozone are examples of “biosignatures,” key markers of a planet crawling with life as we know it. Setting aside questions about how recognizable alien life might be, detecting biosignatures in the atmosphere of an exoplanet would give astronomers the first strong clue that we are not alone.
Biosignatures aren’t proof of thriving ecosystems. Ultraviolet light from a planet’s sun can zap water molecules and create a stockpile of oxygen; seawater filtering through rocks can produce methane. “We’ll never be able to say 100 percent that a planet has life,” says Sarah Rugheimer, an astrophysicist at the University of St. Andrews in Scotland. But astronomers hope that, given enough information about an exoplanet and the star it orbits, they can build a case for a world where sunlight and geology aren’t enough to explain its chemistry — one where life is a viable possibility. Finding a planet similar to Earth is probably still decades away, but thanks to a couple of upcoming telescopes, astronomers might be on the verge of spying on habitable worlds around nearby stars.
NASA’s Transiting Exoplanet Survey Satellite, or TESS, will launch in 2017 on a quest to detect many of the exoplanets that orbit the stars closest to us. One year later, the James Webb Space Telescope will launch and peek inside some of these newfound atmospheres. With their powers combined, TESS and James Webb could identify nearby planets that are good candidates for life. These worlds will probably be quite different from Earth — they’ll be a bit larger and orbit faint, red suns — but some researchers hope that a few will offer hints of alien biology.
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