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 new incarnation, K2, or the soon-to-be-launched 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.
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