Astronomers find more Earth-like planets

The search for Earth-like planets around other stars took a substantial step Monday with a pair of announcements involving Mauna Kea astronomy.

Not only are rocky, Earth-sized planets abundant, scientists said, but they also form one of two common “species” of planets.

At a news conference Monday at NASA’s Ames Research Center in Mountain View, Calif., the Kepler space telescope team released a catalog of 219 new planet candidates, including 10 that are near-Earth size and orbiting in their star’s “Goldilocks zone” — neither too hot nor too cold — where liquid water could exist.

The Kepler mission looks for so-called exoplanets in a small swath of the constellation Cygnus.

So far, Kepler has identified 4,034 suspected or “candidate” planets, and 2,335 of them have been verified. Of 50 Earth-size planets in the habitable zone, more than 30 have been confirmed.

Confirming the findings falls to ground-based astronomy, most notably the Keck Observatory on Mauna Kea.

“The Kepler data set is unique, as it is the only one containing a population of these near-Earth analogs — planets with roughly the same size and orbit as Earth,” said Mario Perez, Kepler program scientist in the Astrophysics Division of NASA’s Science Mission Directorate, in a statement. “Understanding their frequency in the galaxy will help inform the design of future NASA missions to directly image another Earth.

“Are we alone? Maybe Kepler today has told us indirectly, although we need confirmation, that we are probably not alone,” Perez said.

The Kepler telescope finds exoplanets by detecting a slight dip in a star’s brightness as the planet passes in front of it. So it can’t see planets that orbit like the circles of a bull’s-eye target.

Given that limitation, and the telescope’s narrow field of view, the large number of Earth-like planets found so far suggests that they are quite plentiful in the galaxy.

“Earthlike planets are very common among nearby stars,” said Guenther Hasinger, director of the University of Hawaii Institute for Astronomy, by email Monday from Mauna Kea, where he was preparing an observing run on the Subaru Telescope. Hasinger was not involved in the Kepler research, but pointed to earlier work by Andrew Howard, a former UH astronomer now with the California Institute of Technology.

In a second announcement, Howard and colleagues reported that the galaxy appears to have a preference for two distinct types of planets — either rocky planets up to 75 percent larger than Earth, or gaseous “mini-Neptunes.”

The Milky Way rarely makes planets in between those sizes, Howard and colleagues found.

“Astronomers like to put things in buckets,” said UH doctoral candidate B.J. Fulton, the lead author of the study to be published in the Astrophysical Journal. “In this case, we have found two very distinct buckets for the majority of the Kepler planets.”

In a statement released by the Keck Observatory, Howard added, “This is a major new division in the family tree of planets, analogous to discovering that mammals and lizards are distinct branches on the tree of life.”

The Caltech-led study used a high-resolution spectrometer at the Keck Telescope to determine accurate sizes for some 2,000 Kepler planets around 1,300 stars.

“Before, sorting the planets by size was like trying to sort grains of sand with your naked eye,” Fulton said. “Getting the spectra from Keck Observatory is like going out and grabbing a magnifying glass. We could see details that we couldn’t see before.”

Hasinger said astronomers got a “big surprise” when they discovered that mini-Neptunes, or “super-Earths,” are very common in the galaxy. Our solar system is “atypical,” he said, in that it contains no planets between the size of Earth and Neptune.

The Associated Press contributed to this report.