Galaxy may have 11 billion Earth-like planets

This artist's rendition shows Kepler-69c, an Earth-like planet in the habitable zone of a star like our sun, located about 2,700 light-years from Earth. Photo: Associated Press

Our Milky Way galaxy is crowded with far more habitable Earth-like planets than previously thought - at least 11 billion of them in orbit around distant stars, a team of planet hunters led by UC Berkeley astronomers said Monday.

Erik Petigura, a Berkeley graduate student, analyzed data from the Kepler spacecraft and calculated that at least 50 billion stars much like Earth's sun are blazing throughout the galaxy. Kepler itself has been crippled since last summer by damage to its steering gear and is no longer providing new information to earthbound scientists.

But based on current data provided by Kepler and its telescope over the past four years, Petigura estimates that 11 billion planets roughly the size of Earth are flying in orbits around those suns - at distances that make temperatures on the planet neither too hot nor too cold for liquid water to exist.

Astronomers call that kind of orbit the "Goldilocks zone," and it's where astronomers will focus their search of life, they say.

It's possible that even more Earth-like planets exist than Petigura estimates, for his analysis takes into account only one class of hot stars that are known to be very much like our sun. There are other stars called red dwarfs that are about the size of our sun only cooler, and many so-called "exoplanets" may be circling them too, he said.

When those red dwarf stars are included, there may be as many as 40 billion Earth-size planets in habitable zones of the Milky Way with mild temperatures that are similar to climates on Earth, Petigura said.

Petigura's colleagues are Andrew Howard, a former Berkeley postdoctoral fellow now at the University of Hawaii, and Geoffrey Marcy, the Berkeley professor and pioneer planet hunter who has been leading the search for "exoplanets" since the first was discovered 18 years ago.

Their report is published this week in the Proceedings of the National Academy of Sciences, and they discussed it Monday during a news conference.

Marcy called the Kepler spacecraft, with its remarkable telescope, "the best planet-hunting machine ever."

Its flood of data, he said, is answering the question countless others around the world are asking: "whether our planet Earth is some kind of cosmic freak, or instead is a common occurrence within our Milky Way galaxy."

The astronomers are joining 400 other scientists reporting new results this week at the second international Kepler Science Conference at NASA's Ames Research Center in Mountain View.

November 4, 2013

David Perlman is The San Francisco Chronicle science editor.