The galaxy is full of worlds like ours. That’s the lesson from Kepler, NASA’s prodigious exoplanet-hunting mission, which has found another 219 potential new exoplanets, bringing its total to 4034, according to a final analysis of its main 4-year search and published in a final catalog released today. Of the new candidates, 10 are near in size to Earth and sit in the habitable zone of their stars—the range of orbits in which liquid water could exist on their surfaces. Those new additions bring the total number of potentially habitable planets detected by Kepler to 49.
And that’s just in the corner of the sky that Kepler stared at. This now-complete catalog will help astronomers assess just how common Earth-like planets are in our galaxy overall. “I’m really excited to see what people will do with this catalog,” Susan Mullally, a Kepler research scientist at the SETI Institute in Mountain View, California, told a press conference today during the Kepler & K2 Science Conference at NASA’s Ames Research Center in Mountain View. One study, also presented at the conference, has discovered a clear dividing line between rocky planets larger than Earth and gassy planets smaller than Neptune.
Between 2009 and 2013, Kepler stared at the same patch of sky in the Cygnus constellation and measured the brightness of some 200,000 stars. If any of them dipped in brightness for a short period, it could be sign of a planet passing in front, blocking some of their light. Of Kepler’s list of more than 4000 likely candidates from those observations, 2335 have been verified as exoplanets with further analysis or other ground-based observations. The mission continues to occasionally discover exoplanets with more limited capabilities in other parts of the sky.
Before Kepler, astronomers only knew of giant Jupiter-sized exoplanets, some in astonishingly tight orbits around their stars. But Kepler has revealed far more variety.
For the final catalog, the team focused on teasing out Earth-sized planets around G-type stars like our sun. Such planets are harder to spot because they might have made only a few transits across their star during Kepler’s 4-year watch. The result is some of the nearest Earth-analogs found so far, including one known as KOI-7711. “It’s the closest to Earth in size and orbit,” said Mullally, “but there is still a lot we don’t know about this planet.”
One of Kepler’s other big surprises was a profusion of planets intermediate in size between Earth and Neptune. Astronomers were at a loss to explain how such planets formed and whether there was a continuum between rocky terrestrial “super-Earths” and gassy “mini-Neptunes.”
A follow-up study, described today by astronomer Benjamin Fulton of the University of Hawaii in Honolulu, fine-tuned the size measurements for some 2000 Kepler planets using the Keck telescopes in Hawaii. Far from there being a continuum of planets, the study found two distinct groups: one smaller than 1.5 times the size of Earth, the other bigger than twice Earth’s size, with very few examples in the gap in between. Fulton and his team believe that the smaller group is made up of rocky super-Earths, whereas the larger is comprised of gaseous mini-Neptunes. He likened the discovery to realizing that mammals and reptiles are on separate branches of the evolutionary tree.
“The gap in radius is really interesting and comes from meticulous follow-up of the previously discovered Kepler planets, rather than the ones announced today,” says astronomer David Kipping of Columbia University, who was not involved in the study.
Fulton said at the press conference that the team believes the division is caused by the environment a planet finds itself in at the time of its formation, along with its ability to hold onto volatile gases such as hydrogen. The gravity of a smaller super-Earth may not be strong enough to hold onto hydrogen; if it’s close to its star, the hydrogen may get blasted away. But if a planet starts out with more hydrogen, is larger, or forms further out before moving inwards, it may end up as a mini-Neptune.
In the search for life, Fulton believes it will be better to focus on super-Earths. “This has implications in the search for life,” Fulton said. “It sharpens the dividing line between potentially habitable and not habitable.”