Kepler shows small exoplanets are either super-Earths or mini-Neptunes

Small worlds come in two flavors. The complete dataset from the original mission of the planet-hunting Kepler space telescope reveals a split in the exoplanet family tree, setting super-Earths apart from mini-Neptunes.

Kepler's final exoplanet catalog, released in a news conference June 19, now consists of 4,034 exoplanet candidates. Of those, 49  are rocky worlds in their stars' habitable zones, including 10 newly discovered ones. So far, 2,335 candidates have been confirmed as planets and they include about 30 temperate, terrestrial worlds.

Careful measurements of the candidates’ stars revealed a surprising gap between planets about 1.5 and two times the size of Earth, Benjamin Fulton of the University of Hawaii at Manoa and Caltech and his colleagues found. There are a few planets in the gap, but most straddle it.

That splits the population of small planets into those that are rocky like Earth — 1.5 Earth radii or less — and those that are gassy like Neptune, between 2 and 3.5 Earth radii.

“This is a major new division in the family tree of exoplanets, somewhat analogous to the discovery that mammals and lizards are separate branches on the tree of life,” Fulton said.

The Kepler space telescope launched in 2009 and stared at a single patch of sky in the constellation Cygnus for four years. (Its stabilizing reaction wheels later broke and it began a new mission called K2 (SN Online: 5/15/13).) Kepler watched sunlike stars for telltale dips in brightness that would reveal a passing planet. Its ultimate goal was to come up with a single number: The fraction of stars like the sun that host planets like Earth.

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The Kepler team has still not calculated that number, but astronomers are confident that they have enough data to do so, said Susan Thompson of the SETI Institute in Mountain View, Calif. She presented the results during the Kepler/K2 Science Conference IV being held at NASA’s Ames Research Center in Moffett Field, Calif.

Thompson and her colleagues ran the Kepler dataset through “Robovetter” software, which acted like a sieve to catch all the potential planets it contained. Running fake planet data through the software pinpointed how likely it was to confuse other signals for a planet or miss true planets.

“This is the first time we have a population that’s really well-characterized so we can do a statistical study and understand Earth analogs out there,” Thompson said.

Astronomers’ knowledge of these planets is only as good as their knowledge of their stars. So Fulton and his colleagues used the Keck telescope in Hawaii to precisely measure the sizes of 1,300 planet-hosting stars in the Kepler field of view. Those sizes in turn helped pin down the sizes of the planets with four times more precision than before.

The split in planet types they found could come from small differences in the planets’ sizes, compositions and distances from their stars. Young stars blow powerful winds of charged particles, which can blowtorch a growing planet’s atmosphere away. If a planet was too close to its star or too small to have a thick atmosphere — less than 75 percent larger than Earth — it would lose its atmosphere and end up in the smaller group. The planets that look more like Neptune today either had more gas to begin with or grew up in a gentler environment, Fulton said.

That divergence could have implications for the abundance of life in the galaxy. The surfaces of mini-Neptunes — if they exist — would suffer under the crushing pressure of such a thick atmosphere.

“These would not be nice places to live,” Fulton said. “Our result sharpens up the dividing line between potentially habitable planets and those that are inhospitable.”

Upcoming missions, like the Transiting Exoplanet Survey Satellite due to launch in 2018, will fill in the details of the exoplanet landscape with more observations of planets around bright stars. Later, telescopes like the James Webb Space Telescope, also scheduled to launch in 2018, will be able to check the atmospheres of those planets for signs of life.

“We can now really ask the question, ‘Is our planetary system unique in the galaxy?’” exoplanet astronomer Courtney Dressing of Caltech says. “My guess is the answer’s no. We’re not that special.”

Editor’s note: This story was updated June 20, 2017, to correct the number of confirmed exoplanets and the description of the 10 newly discovered rocky exoplanets. Those 10 are planet candidates, not confirmed planets.