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The search for a habitable second Earth

Scientists are torn on whether extraterrestrial life is abundant, rare or nonexistent. With 300 billion stars in the Milky Way, the late astronomer and futurist Carl Sagan figured there could be up to 10,000 advanced civilizations in our galaxy alone. Others, however, think not only are habitable planets rarer than thought, the odds of life forming are slight even on a promising world. In other words, we might be completely alone in our galaxy or even the universe.

Despite pessimism among some researchers, NASA has often implied that if a planet has the same conditions as Earth, it will automatically have life. “I see that as a marketing thing by NASA to interest people in planetary searches,” said astronomer Caleb Scharf, who co-authored a paper on the odds of life forming on a planet. “I think [the chances] could be a lot lower, personally.”

That’s reinforced by another stark reality. “We haven’t seen life everywhere we’ve looked so far,” said astrobiologist Lauri Barge. “If life really requires some unique conditions to emerge, obviously Earth had them, but how many other planets would have them?”

So alien life might be out there, but there’s a very good chance that it’s not. Researchers are excited about either outcome and believe we’re about to enter a golden age of planetary discovery. “Whether or not I’m optimistic that any life is out there, I am very optimistic that we’re going to find out [either way],” NASA exoplanet researcher Shawn Domagal-Goldman told Engadget. “And I don’t think we’ve ever had a moment like this in the history of our species.”

The hunt for habitable exoplanets

Our prospects of finding alien life will explode in the next decade. NASA is on a tear hunting “exoplanets” — planets orbiting stars outside our solar system — with the satellites Spitzer, Hubble and particularly Kepler, launched in 2009. So far, the latter has discovered 2,335 exoplanets, 21 of which are in their star’s habitable regions, and just the right size and mass for life. The most famous of those is Trappist-1, a seven-planet system discovered in February. It’s just 40 light-years from Earth, and as many as three of its planets are in the “Goldilocks zone.”

The Trappist-1 exoplanets will be one of the first targets of the $8.7 billion James Webb space telescope (JWST), set to launch in 2018. That instrument, parked a million miles from Earth, will be able to analyze the spectrum of exoplanet atmospheres to search for “biomarkers,” or signs of life.

“If I was talking to you before [Trappist-1], I’d have said don’t count on looking at the atmospheres of exoplanets, because you need darn-near-perfect targets,” Domagal-Goldman said. “The Trappist planets are darn-near-perfect targets.”

What exactly are we looking for?

Biologists don’t even know how life sprang up on Earth, let alone elsewhere. It could have come via a process called abiogenesis, where life evolves via an electrochemical process from inert organic chemicals. Or it could have formed with “panspermia,” in which simple life forms are ripped from one planet by a giant collision, survive a freezing ride through space and eventually collide into another planet, where they thrive anew. (“That’s just outsourcing the problem,” noted Scharf.)

We can’t see exoplanets with a telescope — they’re too small, too far away and too overwhelmed by the light of their stars. If they pass in front of a star, however, Kepler can detect the star’s dimming using the “transit method.” Based on how much and how often the light dips, along with any wobble in the star, astronomers can figure out the planet’s approximate size, mass and distance from its sun. Using stellar radiation models (that give the energy different stars emit) they can even estimate its surface temperature.