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 consists of UC Berkeley graduate student Erik Petigura, former UC Berkeley postdoctoral fellow Andrew Howard, now on the faculty of the UH Manoa Institute for Astronomy, and UC Berkeley professor Geoff Marcy.

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.

We took a census of the planets detected by the Kepler Space Telescope,” said Howard. “Erik Petigura wrote a new pipeline to detect the shallow dimmings of Earth-size planets in Kepler photometry. With his efficient and well-calibrated pipeline we could confidently report the size distribution of close-in planets down to Earth-size. The result is that Earth-size planets are just a common as planets twice Earth size. Remarkable.”  

The Keck Observatory played a crucial role in this project, he said. While these planets were detected by the space-borne Kepler Telescope, the mighty Keck I Telescope fitted with HIRES (High Resolution Echelle Spectrograph) and a newly upgraded guider system were used to characterize the host stars of the newly discovered planets and to rule out any false planet detections from Kepler. “We took HIRES spectra of most of the planet host stars to characterize the Kepler-discovered systems and to search for double lines (which would indicate a possible false planet detection),” he said.The Observatory’s HIRES was improved in 2012 with Keck’s custom Multi-object Acquisition, Guiding and Image Quality, or MAGIQ, system to dramatically improve the instrument’s performance. MAGIQ was made possible by contributions from the Observatory’s growing base of private supporters.

Broadly speaking, new the MAGIQ guide camera is helpful for several reasons: “The wider field of view, finer pixel scale, and improved noise characteristics make the experience of observing more efficient – and enjoyable,” Howard said. “Those characteristics also yield better guiding on faint stars, which improves our efficiency and crucially improves the stability of the HIRES spectra.”

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.