NASA’s prolific exoplanets-hunting satellite Kepler has found its strongest candidate yet for an Earth-like planet in a life-friendly orbit around a sunlike star. Known as Kepler 452b, the world is estimated to be a bit on the hefty side, at five times the mass of Earth, but it is receiving just 10% more heat and light than we do from its G-type star, just like our sun but 1.5 billion years older. “It would feel a lot like home in terms of the sunshine you would experience,” says Jon Jenkins, who leads Kepler data analysis at NASA's Ames Research Center in Mountain View, California. “This is the closest we have … to another place someone might call home.”
The discovery of Kepler 452b was announced today along with the latest edition of Kepler’s catalog of exoplanet candidates, adding 500 new possible planets for a total of 4175. Kepler detects exoplanets by staring continually at 150,000 stars and recording their brightness for long periods. If the brightness of a star dips slightly for a while and then recovers, that could be the sign of an orbiting planet passing in front of it.
Other things can also cause brightness fluctuations, so all Kepler candidates must be confirmed either by other sorts of observations or more detailed statistical analysis of the Kepler data. “Confirmation is very time consuming,” says Jeff Coughlin, Kepler research scientist at SETI Institute in Mountain View, California. As a result, the Kepler team has automated the confirmation process for this latest catalog. The results are beginning to show that small rocky planets are the most common of all planet types in the catalog, making up as much as 25%. “The numbers are increasing exponentially,” Coughlin says. Kepler operated from 2009 to 2013 when it was hobbled by the failure of stabilizing reaction wheels. It is continuing to make observations at reduced capacity.
Among the new additions to the catalog are several small, probably rocky planets that reside in the habitable zone—at a distance from their star that allows liquid water to exist on their surface. These newcomers bring the total number of small habitable-zone planets discovered by Kepler to 12. What makes Kepler 452b different is the star it is orbiting. Four of the 12 orbit around M-stars, which are considerably smaller and dimmer than our sun. Seven orbit K-stars, which are a bit more like the sun. But Kepler 452b is the first found around a G-star, the same sort as our sun. “We’ve made amazing progress at finding the right-sized planets, in the right-sized orbits, around the right-sized stars,” Jenkins says.
Kepler 452b is 1400 light-years from Earth, orbits its star every 385 days, and is 1.6 times the diameter of Earth. Transit measurements don’t give any information about mass, but, judging from similar exoplanets for which masses are known, the Kepler team estimates it is five times as heavy as Earth, so any visitor would feel twice the gravity we are used to at home. The team consulted planetary geologists about what conditions there may be like, and they predicted that it would likely have experienced active volcanos for some time. Its higher mass may give it a thicker atmosphere and more cloud cover than Earth has. The greater age of the star means it will be heating up, so Kepler 452b may be experiencing a runaway greenhouse effect similar to the one that is currently toasting Venus. However, because of its size and distance, it is unlikely that we will find out more about Kepler 452b for a considerable time, if ever.
This is “a fascinating new step forward,” says Didier Queloz of the University of Cambridge in the United Kingdom, who, along with Michel Mayor, made the first observations of an exoplanet around a normal star. And it comes in the year of the 20th anniversary of that first discovery. “This is a great time we are living in. We now know that there are planets around almost all stars,” he says. “If we keep working as well [as we are now], we can be sure the issue of finding life on another planet will be solved.”