NASA astronomers reported the first undoubtedly rocky planet outside our solar system at an astronomy meeting Monday, lifting expectations that planets with rocky cores like Earth fill many alien solar systems.
The planet identified by the Kepler space telescope, announced by mission team member Natalie Batalha of San Jose (Calif.) State University, crowns a recent series of discoveries of "exoplanets" close in size to Earth.
"Sometimes great scientific discoveries come at the end of a long and pointed search," said astronomer Geoff Marcy of the University of California, Berkeley, a pioneer in the ongoing planet-discovery boom. "This planet will go into astronomy textbooks worldwide."
Dubbed Kepler-10b, the newly announced planet orbits a sun-like star 560 light-years away (a light-year is about 5.9 trillion miles). It is 1.4 times as wide as Earth and is 4.6 times as heavy, making it about as dense as iron. Orbiting only 1.8 million miles from the surface of its star, the planet suffers blast-furnace temperatures of 2,500 degrees or more on its daytime face, "far in excess of the temperatures of molten lava," said Batalha, erasing any chances of life there. "The planet is literally glowing," she said.
Still, planetary scientists expect that planets with rocky cores are more likely to have oceans and support conditions for life, like Earth.
Astronomers have discovered about 500 planets orbiting nearby stars, as well as a few very small ones orbiting distant "pulsar" stars, in the last two decades. In recent years, better detection methods have allowed them to home in on smaller planets, such as one announced by a French team two years ago perhaps five times heavier than Earth, and one announced last fall by a University of California, Santa Cruz, team that may have been only three times heavier. Questions have continued, however, about whether those worlds have rocky cores like Earth's, unlike Kepler-10b.
"Without question, this is a rocky world," Batalha said. The $600 million Kepler spacecraft, launched in 2009 and based at NASA Ames Research Center at Moffett Field, Calif., relies on a wide-field camera capable of looking at 170,000 stars at once within 3,000 light-years of Earth. The spacecraft watches for regular drops in light from those stars, triggered by planets orbiting in front of them. Kepler-10b was spotted this way in 2009.
Combined with measures of the gravitational "wobble" the planet induces on its star, this dimming revealed the size and weight of the planet conclusively, reports the team in an upcoming Astrophysical Journal study.
"The ultimate goal is to find a habitable world," said Marcy, who is not part of the study team. He called Kepler-10b a "missing link" between past astronomical discoveries of gas giant planets and the discovery someday of a habitable planet like Earth.
Kepler has more discoveries coming. In February, the team will report a catalog of possible planets discovered from its most recent data.