Super-Earth spotted in life-friendly zone

Latest exoplanet entry creeps closer to long-sought goal of habitability elsewhere

Inching ever closer toward the goal of finding another Earth, scientists have announced the most promising extraterrestrial incubator so far: a planet of at least 4.5 Earth masses, orbiting its star in the region where liquid water can stably exist.

SUPER-EARTH Though GJ 667Cc (second from left) is the latest addition to a list of life-friendly exoplanets, it’s also the most promising. This planetary lineup depicts the four best bets for exo-incubators so far, compared to Earth and Mars. Numbers beneath the planets rate their score on the Earth Similiarity Index, a metric that considers characteristics such as planetary density and temperature. Habitable Exoplanets Catalog/UPR Arecibo

The super-Earth is called GJ 667Cc, and it circles one member of a triple-star system shining 22 light-years from Earth near the curving tail of the constellation Scorpius.

Unlike other recent tantalizing discoveries, the planet is both well within its star’s life-friendly zone, and it’s just about the right size to host life as we know it. But that doesn’t mean GJ 667Cc is habitable, scientists caution, as crucial pieces of information are missing.

“It’s definitely the best candidate,” says astronomer Abel Méndez of the University of Puerto Rico in Arecibo, who is not a member of the planet-finding team. “If it’s rocky, then it will be more compelling evidence that this is a true habitable planet.”

An international team of scientists pulled the newly announced planet from data gathered by the High Accuracy Radial Velocity Planetary Search, or HARPS, a Swiss-led project that monitors stars for the gentle gravitational tugs produced by orbiting planets. A star’s wobble reveals the mass of a planet circling it. But unless scientists can use other methods to find a planet’s radius, they don’t know how dense it is or what it might be made of.

Astronomers debate who really discovered GJ 667Cc, but this announcement comes from a team that developed a new method for processing HARPS data. As described February 2 in the Astrophysical Journal Letters, the scientists sniffed out the signatures of as many as three planets circling the star GJ 667C, says study coauthor and astrophysicist Guillem Anglada-Escudé of the University of Gottingen, in Germany.

The closest-in planet, which is also a super-Earth, zooms around the star once every 7.2 days, making it “a well-lit charcoal briquette,” says coauthor Steven Vogt, an astronomer at the University of California, Santa Cruz. The second planet — the one of interest — is a relative dawdler, taking 28.15 days to complete an orbit. The third signature could be a planet on a 75-day orbit, but it could also just be an artifact of the star’s activity.

The star is a red dwarf, dimmer and cooler than the sun, and with a habitable zone that’s snuggled in correspondingly closer, creating cozy conditions for GJ 667Cc.

“We’re nestled well within the habitable zone, just in the place Earth would be,” says Vogt. “There’s every indication that it’s a pretty benign place to be.”

Even though GJ 667Cc is in the right place for life doesn’t mean it’s habitable, say scientists including astrophysicist Sara Seager of the Massachusetts Institute of Technology. “The truth is, we may never know,” she says. It’s difficult to determine habitability without knowing the planet’s composition, or anything about its atmosphere and how much of the star’s energy is absorbed. “But overall I’m excited,” she says. “Whatever is going on, this is the start of the next era in exoplanets. We’re going to start seeing super-Earths in habitable zones all the time.”

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