SEATTLE — Astronomers announced today the discovery of eight new planets in their stars' habitable zones, the region where liquid water can exist on a planetary surface. Three of these planets are similar in size to Earth, nearly doubling the number of possible Earth twins found so far.
“We’re now closer than we’ve ever been for finding a twin for Earth,” said astronomer Fergal Mullally of the Kepler Science Office at a press conference here today at the meeting of the American Astronomical Society.
The newly confirmed planets were discovered by the Kepler space telescope, bringing the mission's total to over 1,000. Previously, only five confirmed planets are similar to Earth in size and reside in the habitable zone. Of course, being in the habitable zone only means that it's possible for liquid water to exist, that it's not too hot or too cold. Just because these planets are in the zone doesn't mean they're covered with balmy seas.
And even if a planet does have water, considered a prerequisite for life based on what we now about life in Earth, the planet still has to have the right chemical compositions, atmospheres, geology, and a host of other factors for life to exist. Two of the three new Earth-size planets, dubbed Kepler-438b and Kepler-442b, have a good chance of clearing one of those hurdles: having a solid surface. The third, Kepler 440b, is less likely to be a rocky planet.
One of the new Earth-sized planets, Kepler-438b, is about 470 light-years away, has a diameter just 12 percent bigger than Earth's, gets 40 percent more light than our planet, and has a 70 percent chance of being in the habitable zone. Astronomers place the likelihood that this planet's surface is rocky at 70 percent.
The second planet, Kepler-442b is 1,100 light-years away from Earth, is a third larger than our planet and receives a third less light. Its chance of being in the habitable zone is 97 percent, and the likelihood of being rocky is 60 percent.
Both planets orbit M dwarfs, stars that are smaller and cooler than the sun. A year on those planets is quick: a year on Kepler-438b is only 35 days while a year on Kepler-442b is 112 days.
Meanwhile, astronomers have sifted through more data from the planet-hunting Kepler space telescope and found 554 more objects that could be planets. Kepler has now found a total of more than 4,000 planet candidates, with more than 2,000 of them having radii less than twice that of Earth's.
Among the new planet candidates are yet another eight that may be in their stars' habitable zones. And two of those may be even more similar to Earth than Kepler-438b and Kepler-442b, as they orbit stars more similar to our sun. One object (labeled 5737.01) circles its star every 376 days—not too different from a year on Earth—and has a radius of about 30 percent bigger than Earth’s. The other (2194.03) is the third planet from its star, orbits every 445 days, and has a radius about 40 percent larger than Earth’s.
But these are only planet candidates. These objects have to go through a vigorous series of follow-up observations and analyses before they can be deemed confirmed planets. As astronomers learn more about these systems, the estimated radii and distance from their stars could change significantly, says astronomer Doug Caldwell of the SETI Institute. Or, they may turn out not to be planets at all. Still, the vast majority of the 554 new planet candidates are expected to be bona fide worlds, he says.
