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"If we find lots of planets like ours…we’ll know it’s likely that we aren’t alone, and that someday we might be able to join other intelligent life in the universe.”
Planets in the hundreds of billions are likely caught up in the vast whirlpool of the Milky Way galaxy. From Earth, a lonely outpost on one of its spiral arms, we’ve begun to peer across the void. We can already make out, dimly, the light from planets orbiting distant stars. We’ve even tasted a few of their atmospheres by dissecting those faint traces of light.
But the ultimate goal of NASA's exoplanet program is to find unmistakable signs of current life. How soon that can happen depends on two unknowns: the prevalence of life in the galaxy and how lucky we get as we take those first, tentative, exploratory steps.
Our early planet finding missions, such as NASA’s Kepler and its later incarnation, K2, or the upcoming James Webb Space Telescope, could yield bare bones evidence of the potentially habitable worlds. Perhaps K2’s examination of nearer, brighter stars will stumble across an Earth-sized planet in its star’s habitable zone, close enough for follow ups by other instruments to reveal oceans, blue skies and continents. Or James Webb, designed in part to investigate gas giants and super Earths, might find an outsized version of our planet. With a possible launch in the mid 2020s, WFIRST, or the Wide-Field Infrared Survey Telescope, could zero in on a distant planet’s reflected light to detect the signatures of oxygen, water vapor, or some other powerful indication of possible life.
But unless we get lucky, the search for signs of life could take decades. Discovering another blue-white marble hidden in the star field, like a sand grain on the beach, will probably require an even larger imaging telescope. Designs are already underway for that next-generation planet finder, to be sent aloft in the 2030s or 2040s.
Alien skies in a beam of light
And when we find life, how will we know? The answer has a lot to do with rainbows. As Isaac Newton recognized, white light shot through a prism (or through curtains of mist seen with the sun at your back) is exposed for what it really is: a band of color spanning violet to red, characterized by “wavelength.” Chemicals and gases in the atmospheres of planets can absorb certain slices of this band, called a spectrum, and leave behind a narrow black gap.
When we analyze light shot by a star through the atmosphere of a distant planet—a technique known as spectroscopy—the effect looks like a bar code. The slices missing from the light spectrum tell us which constituents are present in the alien atmosphere.
One pattern of black gaps might indicate methane, another, oxygen. Seeing those together could be a strong argument for the presence of life. Or we might read a bar code that shows the burning of hydrocarbons; in other words, smog. Even without listening in on their conversations, the aliens’ reasonably advanced technology would be known to us by its pollution.
Sampling more exotic recipes for life
Then there’s life as we don’t know it. While it makes sense to search first for something like ourselves, we don’t know yet if that’s really what we should expect. If alien life is organized around different combinations of molecules than life on Earth, we could pick up a robust signature and never even realize it—a smorgasbord of unrecognizable alien gases almost screaming “Life!”
An MIT physics professor, Sara Seager, has decided to tackle this problem by coming up with a roster of possible chemical combinations that could signal the presence of alien life. She and her biochemistry colleagues spent a couple of years churning out computer-generated mixtures of the six main elements associated with life on Earth: carbon, nitrogen, oxygen, phosphorous, sulfur and hydrogen. But they were combinations unseen on our planet.
“The theory ended up being, we should maybe consider all potential molecules that could be in gas form,” Seager said. “Why not consider all of them? I just combine them in any way possible, like just taking letters in the alphabet and combining them in all ways.”
It’s still a problem to pin down which ones correspond to biologically useful recipes, and which do not.
Still, it might just be a start on the trail of truly alien beings, trafficking in exotic gases and chemicals.
“We’re going to have so few planets, we have to get lucky,” Seager said. “I don’t want to miss anything. I don’t want to miss it because we weren’t smart enough to think of some molecule.”
To find out how about the advanced, space-based telescope technology being developed at NASA to search for life among the stars, read 'Inventing the Future.'