Analysis of spacecraft data reveals most Earth-like planet to date

SEATTLE, WASHINGTON—Scientists analyzing data from NASA’s Kepler satellite have boosted the tally of known or suspected planets beyond our solar system to more than 4000, they reported here today at a meeting of the American Astronomical Society. Most are inhospitable—too big, too hot, or too cold for any conceivable life form. But another team seeking to verify Kepler candidates announced here today that they had identified eight new potentially habitable planets, including some close to Earth in size and situation. “We’ve significantly increased the list of verified small planets in the habitable zone,” says Douglas Caldwell of the SETI Institute in Mountain View, California.

Kepler, now partially disabled, detected exoplanets by staring at a patch of sky for months at a time and monitoring the brightness of any stars that might harbor planets. If a planet moves in front of its host star, it will cause a dip in brightness. The dips can be caused by other phenomena such as binary stars orbiting each other, which is why more than 3000 of the suspected 4175 exoplanets are considered “candidates.”

Fergal Mullally of the Kepler Science Office says that the new list announced today, known as the sixth Kepler candidate catalog, contains “more Earth-like candidates than ever before.” One is “the closest analog to Earth found to date,” he says. Unpoetically named 5737.01, this candidate has an orbital period of 331 days and is 30% larger than Earth, Mullally says. That’s good news, because scientists here reported yesterday that planets more than 1.6 times the mass of Earth are unlikely to be dense rocky worlds like ours—assumed to be the only plausible habitats for life.

That candidate has not yet been validated. But Caldwell’s team has confirmed others. They developed a statistical technique dubbed BLENDER, which calculates what various false-positive objects would look like and then compares them with the brightness curves of the Kepler candidates, also incorporating any follow up data from other observations. Starting with 12 Kepler candidates believed to be small rocky worlds, the BLENDER analysis whittled them down to eight new exoplanets with radiuses smaller than 2.7 times Earth’s, all believed to be in the habitable zone.

One of those planets, known as Kepler 438b, receives a little bit more energy from its star than Earth and so may be a bit hotter, Caldwell says. But it’s likely to have a rocky surface, and its red dwarf star is likely to give it a red sky.

The whole process took 2 years; Caldwell says they are trying to develop a bulk program for speeding it up. The goal is to get a better fix on how many stars out there have Earth-like planets. Current estimates range from 10% to 50%. “We want to get the uncertainty down to 10%,” he says.

“I'm excited by the prospects of characterizing additional small, potentially habitable planets using the combination of BLENDER and ground-based follow-up observations,” says astronomer Courtney Dressing of the Harvard-Smithsonian Center for Astrophysics in Cambridge. The team, she says, “have developed a wonderful pathway for understanding planet candidates orbiting stars that are too faint for the traditional planet confirmation method.”