SAN FRANCISCO — It must be crowded out there in the Milky Way.

Thousands if not billions more Earth-sized planets than scientists have ever thought of are orbiting sun-like stars — and some comets are zipping around the stars, too — UC Berkeley astronomers report.

At least one in seven “exoplanets” are flying close-in orbits around their suns much like Earth’s orbit around its sun, the scientists say. And the group’s leader says the number is probably more like half — which means there may be a staggering number of them within their stars’ so-called “habitable zones,” where temperatures would be just right for water to exist.

To come up with their estimate, Erik Petigura, a UC Berkeley graduate student, and Andrew Howard, formerly a Berkeley postdoctoral fellow now at the University of Hawaii, analyzed three years of data from the planet-hunting Kepler spacecraft.

Their analysis included only planets roughly the size of Earth, and with orbits around their stars that are closer than the orbit of Mercury to our sun. But by factoring in the exoplanets discovered by Kepler to date, a team of scientists headed by Francois Fressin of the Harvard-Smithsonian Center for Astrophysics say there could be at least
17 billion exoplanets
in the Milky Way.

“This is an exciting moment in science history,” said Geoffrey Marcy, the pioneer discoverer of planets beyond our solar system who mentored the Berkeley group. “We are measuring the occurrence of Earth-sized planets around sun-like stars for the first time.”

Another Berkeley astronomer, Barry Welsh of UC’s Space Sciences Laboratory, and his colleague, Sharon L. Montgomery of Clarion University in Pennsylvania, meanwhile, reported discovering six new comets flashing around distant stars — bringing the number found so far to 10 in the galaxy that orbit stars.

Unlike the comets of our own night sky, like Haley’s, that fly around the sun and past the planets of our solar system, Welsh’s objects are flying past stars where planets haven’t formed yet, although the ingredients for planets do exist in clouds of gas and dust.

Like the comets in Earth’s night sky, those objects that astronomers now call “exomets,” are icy remnants of a time — billions of years ago — when all planets must have formed from clouds of frozen rocky rubble.

“Interstellar dust under the influence of gravity becomes blobs, and the blobs grow into rocks, the rocks coalesce and become bigger things — planetesimals and comets — and finally, you get planets,” said Welsh at a press conference during a meeting of the American Astronomical Society in Long Beach.

The new discovery of “exomets” suggests that they are just as common as “exoplanets” in other distant solar systems, Welsh said.

And that might mean a “staggering” number, according to planetary astronomer John Johnson of Caltech whose team analyzed the probable formation of five planets discovered around one distant star.

From their analysis, “there’s at least 100 billion planets in the galaxy,” Johnson said.

His team’s report has been accepted for publication in The Astrophysical Journal, according to a Caltech announcement.