Nearly one in four stars like the Sun could have Earth-size planets, according to a University of California, Berkeley, study of nearby solar-mass stars.
UC Berkeley astronomers Andrew Howard and Geoffrey Marcy chose 166 G and K stars within 80 light-years of Earth and observed them with the powerful Keck telescope for five years in order to determine the number, mass and orbital distance of any of the stars’ planets. The Sun is the best known of the G stars, which are yellow, while K-type dwarfs are slightly smaller, orange-red stars.
The researchers found increasing numbers of smaller planets, down to the smallest size detectable today—planets called super-Earths, about three times the mass of Earth.
“Of about 100 typical Sun-like stars, one or two have planets the size of Jupiter, roughly six have a planet the size of Neptune, and about 12 have super-Earths between three and 10 Earth masses,” said Howard, a research astronomer in UC Berkeley’s Department of Astronomy and at the Space Sciences Laboratory. “If we extrapolate down to Earth-size planets—between one-half and two times the mass of Earth—we predict that you’d find about 23 for every 100 stars.”
“This is the first estimate based on actual measurements of the fraction of stars that have Earth-size planets,” said Marcy, UC Berkeley professor of astronomy. Previous studies have estimated the proportion of Jupiter and Saturn-size exoplanets, but never down to Neptunes and super-Earths, enabling an extrapolation to Earth-size planets.
“What this means,” Howard added, “is that, as NASA develops new techniques over the next decade to find truly Earth-size planets, it won’t have to look too far.”
Because the researchers detected only close-in planets, there could be even more Earth-size planets at greater distances, including within the habitable zone located at about the same distance as the Earth is from our Sun. The habitable, or “Goldilocks,” zone is the distance from a star neither two hot nor too cold to allow the presence of liquid water.
The researchers’ results conflict with current models of planet formation and migration, Marcy noted. After their birth in a protoplanetary disk, planets had been thought to spiral inward because of interactions with the gas in the disk. Such models predict a “planet desert” in the inner region of solar systems.
“Just where we see the most planets, models predict we would find no cacti at all,” Marcy said. “These results will transform astronomers’ views of how planets form.”
Howard and Marcy reported their results in the Oct. 29 issue of the journal Science.
“One of astronomy’s goals is to find ‘eta-Earth,’ the fraction of Sun-like stars that have an Earth,” Howard said. “This is a first estimate, and the real number could be one in eight instead of one in four. But it’s not one in 100, which is glorious news.”
Twelve possible planets also were detected, but they need further confirmation, Marcy said. If these candidate planets are included in the count, the team detected a total of 45 planets around 32 stars.