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53.
For the moment.
53 worlds among 3,730 confirmed exoplanets—the best spots in the galaxy for some sort of life.
But with a caveat.
Perhaps nothing is alive on any of these worlds, not even an alien microbe.
“Potentially habitable, that’s the correct phrasing,” says Abel Mendez, the planetary astrobiologist and professor at the University of Puerto Rico at Arecibo.
Mendez is the chief curator of the school’s Habitable Exoplanets Catalog, the online database perused by space scientists worldwide.
Additions to the catalog are accumulating swiftly, he says.
Newest to the list: Ross 128b, discovered last November, a rocky world eleven light years from Earth, in the constellation Virgo.
Nearly on the list: Kepler-1652b, likely rocky, more than 800 light years away, in the constellation Cygnus.
“Number 54,” Mendez says. "We'll add it soon, probably in a few weeks."
Much remains problematic; the science is young. Crucial indicators of life—like the contents of an exoplanet’s atmosphere—are virtually unknowable until new space telescopes go up.
Still, some things are understood. Size, for instance. Says Mendez: “That’s something we can measure.”
And from there, surmise: A tiny planet—like Mercury, in our solar system—won’t have much atmosphere. Something enormous, like Jupiter, might have too much.
Either way, liquid surface water won't exist; those worlds don't make the list.
Orbit is also measurable. “The planet should be within the habitable zone,” says Mendez. That’s the spot within a system where temperatures are moderate and water could flow.
“Get too close to the star, and the heat will eventually boil off the oceans,” he says. “Get too far away, and temperatures go so low that even if there’s water, it’s frozen.”
Within the catalog of 53, Mendez maintains his “Top 13,” places most likely for life.
Number one is Proxima b, near the size of Earth, orbiting in the habitable zone—and by happenstance our closest exoplanet.
Yet deadly radiation from ferocious stellar flares have bombarded the alien world for perhaps billions of years, possibly obliterating its atmosphere.
“A lot of issues,” Mendez admits. “But it’s the best candidate we have so far.”
Number two is TRAPPIST-1e, one of seven cramped worlds around a dim star more than 200 trillion miles away.
Although rocky, TRAPPIST-1e might contain more water than all the oceans of Earth combined, up to 250 times more.
But the planet, along with its six companions, is tidally-locked. Half of TRAPPIST-1e forever faces its star; the other side remains in permanent darkness.
Not a dealbreaker, says Mendez: “Tidal lock is not that bad if you have a planet with an ocean and a thick atmosphere. The heat may propagate to the night side.”
Expect the catalog’s numbers to jump over the next few years. TESS, the new planet-finding probe now in space, may discover 20,000 worlds, says NASA. A small percentage of them will surely make Mendez’s list.
Even that is only a start, he says: “Up to 50 percent of the stars you see in the sky might have a potentially habitable world orbiting it.”
Extrapolate that estimate to the entire galaxy—assume 200 billion stars—and that means 100 billion planets in the Milky Way could support life.
Yet Mendez—who created the catalog six-and-a-half years ago—is still stunned by the 53 found so far.
“This is ten times more than I expected by now,” he says.
And looking ahead ten years?
Long pause. Then: “Oh, wow. At this rate—we’ll be in the thousands.”
