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Planets the Size of Earth Are Common

Kepler spacecraft

Artist’s concept of the Kepler spacecraft. Credit: NASA/Ames/JPL-Caltech.

Astronomers from the University of California, Berkeley and the IfA have found that 17 percent of all Sun-like stars have planets one to two times the diameter of Earth in close orbits. While other studies had shown that planets around stars are common in our galaxy, until this study, it remained unclear if this is true for Earth-size planets.

The team for this study consists of UC Berkeley graduate student Erik Petigura, former UC Berkeley postdoctoral fellow Andrew Howard, now at IfA, and UC Berkeley professor Geoff Marcy. Petigura will spend a year at IfA beginning in June.

To find planets, the Kepler space telescope repeatedly images 150,000 stars in a small region of the sky. It looks for a tiny dip in each star’s brightness that indicates a planet is passing in front of it, much like Venus passed between Earth and the Sun last summer.

The team’s estimate includes only planets that circle their stars within a distance of about one-quarter Earth’s orbital radius—well within the orbit of Mercury—which is the current limit of Kepler’s detection capability. Further evidence suggests that the fraction of stars having planets the size of Earth or slightly bigger orbiting within Earth-like orbits may be as high as 50 percent.

Planets one to two times the size of Earth are not necessarily habitable. Planets two or three times the diameter of Earth are typically like Uranus and Neptune, which have a rocky core surrounded by helium and hydrogen gases and perhaps water. Planets close to their stars may even be water worlds—planets with oceans hundreds of miles deep above a rocky core. Nevertheless, planets between one and two times the diameter of Earth may well be rocky and, if located within the habitable zone (where liquid water exists) could harbor life.

An analysis of three years of Kepler observations identified 119 Earth-size planets ranging from nearly six times the diameter of Earth to the diameter of Mars. While these planets were detected by Kepler, Howard and colleagues also used the Keck I telescope on Mauna Kea to characterize the host stars of the newly discovered planets and to rule out any false planet detections by Kepler.

planet size graph

The fraction of Sun-like stars having planets of different sizes, orbiting within one-fourth of the Earth–Sun distance of the host star. The graph shows that planets as small as Earth (far left) are relatively common compared to planets 8.0 times the size of Earth (similar to Jupiter). Credit: E. Petigura & G. Marcy (UC Berkeley); Andrew Howard (IfA).

“Kepler’s one goal is to answer the question, what fraction of stars like the Sun have an Earth-like planet?” Howard said. “We’re not there yet, but Kepler has found enough planets that we can make statistical estimates. Our key result is that the frequency of planets increases as you go to smaller sizes, but it doesn’t increase all the way to Earth-size planets—it stays at a constant level for planets smaller than twice the diameter of Earth.” The estimates are based on a better understanding of the percentage of big Earths that Kepler misses because of uncertainties in detection, which the team estimates to be about one in four, or 25 percent.

Howard noted that if 17 percent of all stars have big Earths within the orbit of Mercury, “where are they in our solar system? Maybe our solar system is an anomaly compared to the great variety of stars.”

UC Berkeley news release