Although over 400 extra-solar planets are now known orbiting stars other than our sun, nearly all of these planets are bigger than the Earth in both mass and size. In fact, most of them are as big or bigger than Jupiter. These larger planets are easier to detect because they perturb the motions of their parent stars more noticeably. But astronomers have been searching for Earth-sized planets, both because of a natural interest in studying earth-like analogs, but also in order to investigate the range of planet sizes, study how planetary systems develop and evolve, and to determine if the earth is rare or run-of-the-mill. Planets with a mass ranging from about 1.9 to 10 Earth-masses are called super-Earths because although they are larger than the Earth they are thought to share many of the basic properties of the Earth’s composition, unlike the gas giants like Jupiter.
A team of CfA astronomers and their colleagues, led by David Charbonneau, have announced in this week’s issue of Nature the discovery of a super-Earth only about 40 light-years away. The team has been using a specially designed complex of small telescopes outfitted with sophisticated detectors and smart software to search nearby, relatively cool stars for transiting planets (a transiting planet is one whose orbit crosses the face of the star as seen from Earth, thereby dimming the star’s light in a characteristic dip). One of the CfA graduate students spotted a faint suggestion of such an event in the data, and follow-up observations have confirmed that it was caused by a super-Earth.
The new planet has an estimated mass 6.6 times that of Earth, an estimated radius 2.68 times larger than Earth, and it orbits its star at a distance almost 100 times closer than the Earth is to the sun, making its surface relatively hot -- estimated to possibly be as much as 280 Celsius. Modeling suggests that such a world could be made principally of hot water, with a small gaseous atmosphere of hydrogen and helium. The only other super-Earth known is smaller and denser than this one, suggesting it is mostly rock. The new result signals a dramatic new phase in the study of extra-solar planets, and in our ability to probe the characteristics of Earth-like planets.