NASA's chief scientist thinks we'll find alien life by 2025. Here's how we'd do it.

"I believe we are going to have strong indications of life beyond Earth in the next decade and definitive evidence in the next 10 to 20 years," NASA's chief scientist, Ellen Stofan, said as part of a panel on Tuesday. "We know where to look, we know how to look, and in most cases we have the technology."

Her comments have made waves. Stofan clarified that we're most likely to find extraterrestrial microbes, not little green men. Still, to many people, it sounds like a brash, far-fetched prediction.

she's not alone in her prediction

But Stofan isn't alone. Scientists studying other worlds, both in our solar system and orbiting distant stars, have also predicted that we're likely to find out a lot about the existence of extraterrestrial life in the coming decades.

"With new telescopes coming online within the next five or 10 years, we'll really have a chance to figure out whether we're alone in the universe," Lisa Kaltenegger, an astronomer and director of Cornell University's search for distant habitable planets, told me for an article last year. "For the first time in human history, we might have the capability to do this."

Read more: 40 maps that explain outer space

Aliens might live on moons in our solar system

(NASA/JPL-Caltech/SETI Institute)

Jupiter's moon Europa, which has an icy surface crisscrossed with cracks and ridges.  (NASA/JPL-Caltech/SETI Institute)

For years, scientists assumed all star systems had what's called a "habitable zone": an area that's close enough to the star that water doesn't freeze, but not so close that it boils away, enabling liquid water, thought to be necessary for life. In our solar system, the only other planet in the habitable zone is Mars, a place where we haven't found any life so far. So the odds of alien life in our solar system didn't seem very high.

But recently, scientists have found evidence of liquid water oceans in at least three different places way out past the habitable zone: Jupiter's moons Europa and Ganymede, as well as Saturn's moon Enceladus.

(Kevin Hand (JPL/Caltech), Jack Cook (Woods Hole Oceanographic Institution), Howard Perlman (USGS))

An illustration showing the calculated volume of water on Europa, compared with Earth. (Kevin Hand (JPL/Caltech), Jack Cook (Woods Hole Oceanographic Institution), Howard Perlman (USGS))

All three moons, scientists believe, have vast liquid water oceans covered by sheets of ice. Different mechanisms (such as tidal heating) warm up the moons' insides, allowing for liquid water even though they're extremely far away from the sun.

What's more, data collected by both space probes and telescopes on Earth suggest that these moons could have other key elements for life, as well. "We think Europa has the ingredients for life," Robert Pappalardo, a scientist working on NASA's upcoming Europa mission, told me in February. "Not just liquid water, but probably the right elements and chemical energy that might permit life, too."

A rendering of the hot springs believed to exist on Saturn's moon Enceladus. (NASA/JPL-Caltech)

The energy needed to fuel life could come from hydrothermal vents — the same sort that exist on Earth's ocean floor and have given rise to entire ecosystems that live off the chemicals dissolved in the hot water. In fact, last month, scientists found the first evidence of these sorts of vents on Enceladus.

To actually find direct evidence of life, we'll need to send probes to these places. Right now, planning is underway to send a probe to Europa in 2025, but it'll be an orbiter, not a lander — so even though it'll help us learn more about the moon, the only way it could find life directly is by passing through a plume of water shooting out of the ice. There are no current plans to explore Enceladus or Ganymede, partly because NASA's shrinking planetary science budget has led to cutbacks on the number of missions it can launch.

Aliens probably live on distant planets orbiting other stars

An artist's depiction of Kepler-186f, the first Earth-size planet ever found in a habitable zone. (NASA Ames/SETI Institute/JPL-CalTech).

For most of modern history, there was no concrete evidence that planets even existed in other solar systems — but as technology has improved, we've learned they're remarkably plentiful. We've now spotted more than 1,800 distant planets (called exoplanets) with telescopes, and scientists now believe most stars in the Milky Way have at least one.

Even forgetting about the possibility of distant ice-covered moons like Europa, some of the exoplanets we've spotted are in their stars' habitable zones. If the process of evolution elsewhere occurs anything like it has on Earth, it's a fair bet that among the billions of planets that likely exist in the Milky Way, there are some other life forms out there.

A chart of the huge number of new planets discovered last year. (Imgur.com)

Finding evidence of it will require us to spot potentially habitable exoplanets and analyze their atmospheres for biosignatures — gases, such as dimethyl sulfide, that are produced only by life forms, at least on Earth.

So far, most the planets we've found so far are too big, too gaseous, or too hot to be capable of supporting life (because these ones are easier to spot), but we're finding more of the smaller, rockier ones that are like Earth all the time, and the Transiting Exoplanet Survey Satellite — a telescope to be launched in 2017 — will allow us to find many more. Another pair of telescopes (the James Webb Space Telescope, to be launched in 2018, and the European Extremely Large Telescope, a ground-based telescope to be built in Chile in 2024) will allow us to survey these planets' atmospheres for biosignatures.

(NASA)

A rendering of the James Webb Space Telescope. (NASA)

Even if we do find a biosignature, it'll be open to interpretation, and won't be nearly as definitive as if, say, we found actual microbes shooting out of a plume on Europa. But if we keep surveying the galaxy, the sheer number of stars and planets means the math is on our side. With enough time and investment in telescopes and probes, Stofan's prediction will look more and more likely — and we'll eventually see evidence of extraterrestrial life.

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