Inching ahead on their quest for what they call Earth 2.0, astronomers from NASA’s Kepler planet-hunting spacecraft announced Thursday that they had found what might be one of the closest analogues to our world yet.
It is a planet a little more than 1½ times as big in radius as Earth. Known as Kepler 452b, it circles a sunlike star in an orbit that takes 385 days, just slightly longer than our own year, putting it firmly in the “Goldilocks” habitable zone where the temperatures are lukewarm and suitable for liquid water on the surface — if it has a surface.
The new planet’s size is right on the hairy edge between being rocky like Earth and being a fluffy gas ball like Neptune, according to studies of other such exoplanets. In an email, Jon Jenkins, an astronomer at NASA’s Ames Research Center, home of the Kepler project, and lead author of a paper being published in The Astronomical Journal, said the planet had between a 50 percent chance and a 62 percent chance of being rocky, depending on uncertainties in the size of its home star. That would mean its mass is about five times that of Earth.
To determine whether Kepler 452b deserves a place on the honor roll of possible home worlds, astronomers have to measure its mass, which requires being close enough to observe the wobbling of the star as it is tugged around by the planet’s gravity. For now that is impossible, as Kepler 452 is 1,400 light-years away.
The planet is the first to be confirmed in a new list of candidates unveiled by Kepler astronomers at a news conference Thursday. It brings the list of possible planets discovered by Kepler to 4,675.
The spacecraft, launched in 2009, spent four years staring at a patch of the Milky Way on the border between the constellations Cygnus and Lyra, looking for the dips in starlight caused by planets passing in front of their stars. Its pointing system failed in 2013, but astronomers are still analyzing the data Kepler collected. Every time they sift through the data, new planets pop out.
In the meantime, Kepler has switched to a different mode of observing in a new mission they call K2.
The NASA news conference coincided with a major anniversary: It was only 20 years ago this fall that Michel Mayor and Didier Queloz, of the University of Geneva, discovered a planet circling the star 51 Pegasi, about 50 light-years from here. It was the first planet known to belong to a sunlike star outside our solar system, and its discovery ignited an astronomical revolution that is still growing.
Astronomers say they now know from Kepler that about 10 percent of the 200 billion stars in the Milky Way have potentially habitable Earth-size planets, Kepler 452b probably among them. This means that of the 600 stars within 30 light-years of Earth, there are roughly 60 E.T.-class abodes, planets that could be inspected by a future generation of telescopes.