Thanks to Kepler, we know that our Milky Way galaxy has something like 100 billion planets — knowledge we didn't have five years ago. Another important discovery? "We actually know now what life might look like," said Matt Mountain, a NASA telescope scientist. "We know life can imprint itself on the atmosphere of planets going around other stars. This is what a living planet looks like."
Essentially, Kepler has taken a census of nearby stars, giving the next telescope a catalog from which to search. NASA believes 10-20 percent of those stars have an Earth-like planet in the habitable zone. That's the area that's "not too hot, not too cold, just right for life," Seager said, though there's some dispute over where the boundaries of that zone start and end.
So what's the next telescope in the queue? That would be TESS, the Transiting Exoplanet Survey Satellite, scheduled to launch in 2017. Its job is to watch about half a million nearby stars, looking for temporary drops in brightness that could indicate an orbiting planet is passing in front. TESS will find about 1,000 stars that will serve as a search list for the next telescope in line.
A year later, our best and brightest hope so far will continue the search. The James Webb Space Telescope comes in at 6.5 meters, more than twice the size of Hubble. Its 18 hexagonal mirrors will give scientists a much broader view of light wavelengths as they search the sky. "The James Webb Telescope will transform our view of the universe," said John Grunsfeld, a former astronaut and current member of NASA's science team.
NASA's team will use the telescope to look for signs of life — blue sky, oxygen, carbon dioxide, sulfur dioxide, plant life, liquid water — as the light of a star behind a planet filters through its atmosphere and exposes its characteristics. This is known as transit spectroscopy.
It's still a long shot. "We have to get really lucky," Seager said. "With the James Webb we have our first chance, our first capability, of finding life on another planet. Now nature just has to provide for us."
That's where things get tricky. It's really hard to get a good look at planets, which are around 10 billion times fainter than the stars they circle. And even though our galaxy has an abundance of Earth-like planets (the "holy grail" of our alien search, as Grunsfeld put it, though not the only candidates for supporting life), we don't know how difficult it is for life to get a foothold. In other words, we could find many Earth twins before we discover one with anybody home.
And even the James Webb can only see so much. It will probably take another generation or two of satellites before our odds really start to go up. The issue is size. "You need bigger telescopes, you need more collecting area," said Dave Gallagher, who works in NASA's Jet Propulsion Laboratory. "You've gotta get bigger."