When Will Humans Find Another Earth?

It may be much sooner than you think.

Discovering another planet like our own—with warm, blue oceans, an oxygen-rich atmosphere, perhaps even signs of life—once sounded like pure fantasy. Not anymore, according to the astronomers, planetary scientists, and biologists leading the search. We know what we have to do to find another Earth, and we're building the tools that'll get us there.

"We are standing on the shoulders of giants to be able to make this leap," said astronomer Lisa Kaltenegger, the founder of Cornell's new Carl Sagan Institute. "Yes, there is a huge distance between stars, and the next star system is far away, but we are glimpsing these worlds on the horizon, and we are reaching."

The CSI, whose name pays homage both to the famous Cornell astronomer and the detective-like nature of the researchers' work, is one of the first institutes wholly devoted to the search for life in the universe. (SETI is the best known of current efforts.) The CSI corrals researchers from disciplines as varied as astrophysics and microbiology to study the incredible diversity of potentially habitable worlds we're just beginning to see, both within and beyond our solar system.

To CSI scientists, the hunt for another Earth—possibly dozens or hundreds of Earths—is already on.

Artist's concept of the inner four planets orbiting Gliese 581, a red dwarf star just 20 light-years from Earth. In the foreground is Gleise 581-g, a "Goldilocks" world within the star's habitable zone. Image: Lynette Cook

"[This] is something we can accomplish," Natalie Batalha, an astronomer at NASA's Ames Research center, said in her remarks at the CSI's inauguration. "Maybe not within my lifetime, maybe within my daughter's life. Humans should be able to find evidence of life around an exoplanet."

Batalha's belief that we're roughly a generation away from being able to robustly search other star systems for evidence of life was echoed by most of the scientists I spoke with at the institute's inauguration. But, if we choose our targets right, we may just find a second Earth much sooner.

"If I could pick any time in history to live, I'd pick exactly this time."

To understand where these predictions are coming from, a little background on the technology-driven field of exoplanet discovery is needed. Compared with today, the galaxy we knew a generation ago was a lackluster place. We had no idea if there were any planets at all outside our solar system, because our telescopes simply weren't powerful enough to detect them.

"The problem you have when you want to find a planet around another star is that the planet is almost nothing compared to the star," said astronomer Didier Queloz of the University of Cambridge during his talk at the CSI's opening ceremony. "The mass of a planet is a thousand [times] smaller, and it produces almost no light, so you really have to use tricks."

In the 90s and early 2000s, astronomers like Queloz labored to identify the very first extrasolar planets, using the radial velocity method, wherein a planet is detected via its gravitational tug on its star. Early on, astronomers also used transit photometry—essentially, measuring a faint dip in star's light as an orbiting planet crosses its path—and other direct imaging methods, with instruments that were just barely powerful enough to do the job.

Artist's impression of the Kepler space telescope observing planets transiting a distant star. Image: NASA Ames/W Stenzel

But in 2009, Kepler changed everything. Since NASA's space-based dedicated exoplanet-hunting scope was launched into orbit around our sun, we've used transit to identify more than 4,600 exoplanet candidates—including roughly 800 Earth-sized worlds and 1200 larger but still potentially habitable "super-Earths." And by using statistics to extrapolate from Kepler's small galactic census, astronomers have come to extraordinary conclusions.

"On average, every star in our galaxy has at least one planet," said Bill Borucki, the architect behind NASA's Kepler mission, in his remarks at the CSI's inauguration. "That's amazing—a hundred billion stars in our galaxy, a hundred billion planets. So, the mission has accomplished one of its major goals: Are Earths rare or common? They are very common."

"Literally, our veil has been lifted," Batalha said. "Now the universe looks completely different. And this is what happens every time you build a new piece of technology— you see the universe in a different way."

Among Kepler's most exciting finds are a few dozen so-called "Goldilocks" worlds—rocky planets that are in the not-too-hot, not-too-cold "habitable zone" of a star. It's these worlds that interest the CSI's Earth-hunters the most.Butif we want to learn whether any of these Goldilocks candidates is a true Earth, we need to know more. We need to stop chasing shadows and start looking at the light emitted from these worlds directly.

"All of what we see when we look at Earth—continents, oceans, the biology that breathes—leaves a spectral fingerprint," Kaltenegger said. "Even if the planet we're looking at is one tiny pixel, one spot of light, we can read that pixel. From light years away, we can explore other worlds."

The ingredients present in planet's atmosphere influence the light that world projects across the cosmos. And certain combinations of molecules, such as oxygen and methane together, are what Earth-hunters call a biosignature, meaning they offer powerful evidence of life's metabolism. Our current telescopes aren't yet powerful enough to study the glow of distant worlds and pick out these biosignatures. But future missions will be.

"Now we know Earth-like planets are common," Batalha told me. "NASA has a roadmap for how this is going to play out, it's just a matter of money and resources."

That roadmap includes the James Webb Space Telescope, which launches in 2018. JWST isn't designed to study Earth-sized planets, but it will help us decode the atmospheres of gas giants, and perhaps a handful of rocky super-Earths. After JWST, NASA will funnel resources into the Wide Field Infrared Survey Telescope (WFIRST), which will pick up where Kepler left off, using more powerful optics to observe the transit of exoplanets at farther-than-Earth orbits.

Artist's impression of the transiting exoplanet Corot-9b. Image: ESO/L. Calçada

The future "life finder" mission that'll allow us to study the atmospheres of rocky, Earth-sized worlds in the habitable zone isn't going to be built until after the WFIRST launches in the mid 2020s.

"Between now and the life finder, we're not going to find nearby Earths in great numbers," Batalha told me. "But as long as we're well positioned by 2025 to start putting money into a life finder, then I think we'll have a hope of really making headway in three decades."

Still, Kaltenegger says, if we get very lucky, and habitable worlds turn out to be common, it may be a different story. While Kepler focuses on star systems 500 to 3,000 light years away, the Transit Exoplanet Survey Telescope (TESS), which launches in 2017, will scan the entire sky, searching our very closest neighbors for planets. If we find rocky worlds in the habitable zones of dim red dwarf stars within a few light years, these will be prime candidates for the JWST to set its sights on.

"If you're asking whether we're going to have definitive proof of life in 15 years, I don't think any scientist from the exoplanet community will sign on to that," Kaltenegger told me. "But are we going to have our first steps? Yes."

Whether or not habitable worlds are common, no matter how long it takes us to find one, the sentiment expressed at the Carl Sagan Institute's opening ceremony was one of extraordinary optimism.

"This is an exploration," Queloz told me. "It's not like between now and 20 years from now nothing is going to happen, we're not just waiting around for someone to bring the good news. Exploration means it's a process. The reason I'm optimistic is, if a lot of people are now getting enthusiastic about this, we're going to see a string of discoveries and breakthroughs that are going to bring us there."

Kaltenegger agrees. To her and the other CSI researchers, we've only just begun to catch our first fleeting glimpses of the wonders that await us out there.

"This whole idea that we live in the first time in history, where we can take a first step at the question of whether we are alone, or just what other worlds look like, is amazing," Kaltenegger said. "I think if I could pick any time in history to live, I'd pick exactly this time."

Top image: NASA engineer Ernie Wright looks on as the first six flight ready James Webb Space Telescope's primary mirror segments are prepped to begin final cryogenic testing at NASA's Marshall Space Flight Center. Image: NASA/MSFC/David Higginbotham