This Is How We'll Find the Next Earth

It took hundreds of millions of years for natural selection to turn a bacterium's light-sensing photoreceptors into a sophisticated human eye with color, depth and focus. Which is why, when it comes to detecting exoplanet—the alien worlds beyond our solar system that might harbor life—it'd be fair to say our vision is improving at evolutionary lightspeed.

It's an exciting time to be alive if you're curious about alien worlds. Two decades ago, we knew of no planets outside our solar system; in the past five years, NASA's Kepler mission has verified over a thousand. This week, astronomers announced the discovery of the two most Earth-like planetsyet. And all this is just the tip of the iceberg, because within the next decade, direct-imaging mi​ssions will allow scientists, for the very first time, to capture the glow of distant exoplanets and decode their atmospheric compositions. What sorts of worlds will we discover, once we set our new eyes to the night sky?

NASA is so excited to find out, it's commissioned a serie​s of reports that hash out the expected scientific outputs from the first direct imaging missions. While early missions will focus on Jupiter-sized planets, we'll probably get a peek at a handful of super-Earths, as well: rocky balls whose atmospheres may contain biosignatures like methane, carbon dioxide, and oxygen.

Until now, detecting exoplanets has been a matter of studying starlight. Transit surveys like NASA's Kepler mission measure tiny dips in a star's luminosity as a planet crosses its path. The James Webb Space Telescope, slated to launch from a spaceport in Fre​nch Guiana in 2018, will manipulate starlight more cleverly, measuring slight imprints in the radiation emitted from a star as it passes through a planet's atmosphere. This will give us our first glimpse into the skies of distant worlds, albeit an indirect one. And it's only the beginning.

Following closely on the James Webb Telescope's heels will be the Wide Field Infrared Survey Tele​scope (WFIRST), a retrofitted spy scope that'll allow us to measure the light emitted from exoplanets themselves. To do so, it will make use of a starlight-blocking instrument called a coro​nograph. Future, more ambitious exoplanet exploration missions will use larger aperture telescopes, equipped with either a coronograph or a giant, starlight-blocking parasol known a​s a starshade, to seek out and study Earth-like worlds. The wicked challenge facing all of these missions is the overwhelming brightness of distant stars compared with the planets that orbit them.

"If we were looking at a system with a star like the sun and planet like the Earth, reflected light from that planet would be something like 10 billion times less than the star," Doug Hudgins, program scientist for NASA's Exopla​net Exploration Program, told me. "So, we need a new type of instrument, something capable of canceling out or blocking the light from a star, without blocking the reflected light of a planet that lies right next to it."

Once such an instrument is ready, astronomers will, for the first time, be able to capture the light reflected from exoplanets themselves. The glows of distant alien worlds, measured at visible and near-infrared wavelengths, will clue scientists in to what sorts of compounds are present in their atmospheres, including, we hope, the chemical fingerprints of life.

First gen direct imaging missions will focus on planets much larger than Earth—mostly, blustery balls of gas. We're expecting to find all sorts of weird and hellish places, from Jupiter-sized blobs filled with asphyxiating clouds of ammonia, to highly exotic worlds with glassy, silica-rich skies. By studying these planets, which'll have tight orbits compared with our solar system's gas giants, scientists will gain insight into how such worlds form and evolve.

"Our understanding of gas giants is limited, because we only have four in our solar system. But in the future we'll have a much larger sample to study these planets in a comparative way," said NASA Jet Pro​pulsion Laboratory's Reynu Hu, who authored a recent​ paper on the first sorts of planets we can hope to find via direct imaging.

We're also hoping to get a better look at a smaller number of rocky worlds. Recent missions have shown that super-Earths are commo​n in our interstellar neighborhood. While the first super-Earths we check out will probably be too cold to support life, some may have atmospheres rich in life's precursors.

"I would view the new missions as an indispensable stepping stone toward identifying Earth-like exoplanets that could harbor life," Hu told me.

"Of course, holy grail of this program is a mission to separate light from an Earth-sized planet in the habitable zone of a sun-like star and figure out if it has signs of life," said Hudgins.

Snapping the first truly Earth-like atmosphere will be the charge of whatever advanced telescope comes after WFIRST, one that's capable of taking higher contrast images. Nailing down the specs on such a telescope has been a hot topic at the American Astronomical Society meeting this ​week, says Hudgins.

A promising step in that direction is Exo-S, another planne​d direct imaging mission that'll use a giant starshade to block the background light of distant stars. Astronomers are hopeful this mission will allow us to capture the spectra of a few rocky, Earth-sized worlds in the habitable zone.

"This is such an exciting field to be in right now," Hudgins said. "One of the fundamental questions facing humankind is whether we're alone. It's amazing to say that we might have an answer within our lifetime."

Yep, it sure is.