Journal Article

An abundance of small exoplanets around stars with a wide range of metallicities

Ford, Eric B., Borucki, William J., Gilliland, Ronald L., Latham, David W., Laird, John B., Marcy, Geoffrey W., Morse, Jon A., Ciardi, David R., Endl, Michael, Batalha, Natalie M., Bizzarro, Martin, Esquerdo, Gilbert A., Caldwell, Caroline, Brugamyer, Erik, Robertson, Paul, Rowe, Jason F., Still, Martin, Cochran, William D., Shporer, Avi, Geary, John C., Quinn, Samuel N., Hansen, Terese, Isaacson, Howard, Bryson, Stephen T., Johansen, Anders, Lucas, Philip W., Buchhave, Lars A., Stefanik, Robert P., Torres, Guillermo

2012

Overview

Whereas large planets, such as gas giants, are more likely to form around high-metallicity stars, terrestrial-sized planets are found to form around stars with a wide range of metallicities, indicating that they may be widespread in the disk of the Galaxy. Exoplanets around metal-poor stars A key discovery of the past decade in the field of exoplanet research was the realization that stars of high metallicity are those most likely to harbour giant exoplanets, supporting the model in which planets form by the accumulation of dust and ice particles. Whether the planet–metallicity correlation holds for terrestrial planets remained unclear, but the Kepler mission's discovery last year of hundreds of small exoplanet candidates provided an opportunity to find out. The spectroscopic metallicities of the host stars of 226 small exoplanet candidates have now been determined. The smaller ones, of less than four Earth radii, were found around stars with a wide range of metallicities, on average close to that of the Sun. Larger planets were more common around stars of high metallicity. These findings suggest that terrestrial planets may be widespread in the disk of the Galaxy, with no special requirement of enhanced metallicity for their formation. The abundance of heavy elements (metallicity) in the photospheres of stars similar to the Sun provides a ‘fossil’ record of the chemical composition of the initial protoplanetary disk. Metal-rich stars are much more likely to harbour gas giant planets 1 , 2 , 3 , 4 , supporting the model that planets form by accumulation of dust and ice particles 5 . Recent ground-based surveys suggest that this correlation is weakened for Neptunian-sized planets 4 , 6 , 7 , 8 , 9 . However, how the relationship between size and metallicity extends into the regime of terrestrial-sized exoplanets is unknown. Here we report spectroscopic metallicities of the host stars of 226 small exoplanet candidates discovered by NASA’s Kepler mission 10 , including objects that are comparable in size to the terrestrial planets in the Solar System. We find that planets with radii less than four Earth radii form around host stars with a wide range of metallicities (but on average a metallicity close to that of the Sun), whereas large planets preferentially form around stars with higher metallicities. This observation suggests that terrestrial planets may be widespread in the disk of the Galaxy, with no special requirement of enhanced metallicity for their formation.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 639/33/34/124 / Astronomi, astrofysik och kosmologi / Astronomy / Astronomy, Astrophysics and Cosmology / Chemical elements / Earth / Earth, ocean, space / Exact sciences and technology / Extrasolar planetary systems / Extrasolar planets / Fysik / Heavy elements (Cosmochemistry) / Humanities and Social Sciences / letter / Migration / multidisciplinary / Natural history / Natural Sciences / Naturvetenskap / Noise / Physical Sciences / Planets / Science / Science (multidisciplinary) / Stars / Stars & galaxies / Statistics / Substellar companions ; planets