When earthlings finally discover their alien neighbors, it probably won't be aboard a warp-speed spaceship or via intergalactic Morse code.
The more likely—and decidedly less sexy—scenario, scientists said at a Monday NASA panel, is a giant space-based telescope finding a planet with gases in the atmosphere that indicate life.
"We are looking for biological signatures, gases that are produced by life," said Sara Seager, a planetary scientist at the Massachusetts Institute of Technology. "We're looking for whatever's out there that will be generating gases in the atmosphere."
And since our search at the moment extends to about 200 light-years away, it may be a long, long time before we find out what that life looks like.
So where does our search start?
According to NASA, we're well on our way. The agency's plan involves a series of telescopes, each bigger and better than the last. After one narrows down the search, the next will follow up with more detailed findings—until eventually one of them confirms that we're not alone.
That all started with the Hubble Space Telescope, launched in 1990. It's been followed by the Spitzer and Kepler telescopes. Their findings have helped us learn more about galaxies, stars, and solar systems—and where we might find "Earth 2.0."
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."
But it's not just as simple as scaling up. Bigger satellites need bigger rockets, and even then the telescopes are nowhere close to fitting in a designated payload area. The 6.5-meter James Webb, once deployed, will have to go through a complex unfolding process before it reaches its full size. Solar panels splay out from the sides, cables pull out a five-layer sunshield into place, and the folded-back mirrors come together. It's essentially like building a ship in a bottle, then removing it in space. "This is what it takes to get a big telescope into a not-big-enough rocket," said John Mather, Webb's senior project scientist.
Bigger rockets, like the giant Space Launch System, will help future payloads get to space. And the next planned mission after the James Webb Telescope, an infrared telescope in 2024, will measure only 2.4 meters. Because the brightness of stars tends to make their planets invisible, these missions will only be able to observe planets much larger than Earth orbiting stars much smaller than the sun. "Somehow we need to block out that starlight," Seager said.
To have a real shot, scientists say we'll eventually need a telescope that measures 16 or 20 meters. Mountain showed a graphic displaying the number of nearby, potentially habitable planets the James Webb Telescope is capable of viewing. The screen showed only a handful. "How lucky do we feel?" he asked. "With a 20-meter telescope, we can see hundreds of Earth-like planets around other stars. That's what it takes to find life."
The large telescope is part of the New Worlds Mission. This plan involves a separate satellite with foldout petals tens of meters in diameter, much like the shape of a sunflower. That shape is specifically designed to diffract stars' rays and let only planets' light through. That satellite, known as a starshade, would separate from the telescope and fly tens of thousands of kilometers away. Then the telescope would point at it, using the starshade as a filter to better capture images from planets.
The starshade/giant satellite plan is still in the works, and for now, scientists are excited about the results they plan to get from the James Webb Telescope. And whether it's Webb or a successor that first finds an inhabited planet, they're confident that moment will come. "It's highly improbable in the limitless vastness of the universe that we humans stand alone," said NASA Administrator Charles Bolden. "I find it hard to believe that we're in this world alone."
Mountain put it in more romantic terms. "Just imagine the moment when we find potential signatures of life," he said. "Imagine the moment when the world wakes up and the human race realizes that its long loneliness in time and space may be over—the possibility we're no longer alone in the universe."