@article{b7f561d834f64fce8ff7f82b202dc234,
title = "A rocky composition for an Earth-sized exoplanet",
abstract = "Planets with sizes between that of Earth (with radius) and Neptune (about 4) are now known to be common around Sun-like stars. Most such planets have been discovered through the transit technique, by which the planet's size can be determined from the fraction of starlight blocked by the planet as it passes in front of its star. Measuring the planet's mass - and hence its density, which is a clue to its composition - is more difficult. Planets of size 2-4 have proved to have a wide range of densities, implying a diversity of compositions, but these measurements did not extend to planets as small as Earth. Here we report Doppler spectroscopic measurements of the mass of the Earth-sized planet Kepler-78b, which orbits its host star every 8.5 hours (ref. 6). Given a radius of 1.20 ± 0.09 and a mass of 1.69 ± 0.41, the planet's mean density of 5.3 ± 1.8 g cm -3 is similar to Earth's, suggesting a composition of rock and iron.",
author = "Howard, {Andrew W.} and Roberto Sanchis-Ojeda and Marcy, {Geoffrey W.} and Johnson, {John Asher} and Winn, {Joshua N.} and Howard Isaacson and Fischer, {Debra A.} and Fulton, {Benjamin J.} and Evan Sinukoff and Fortney, {Jonathan J.}",
note = "Funding Information: Acknowledgements This Letter and another10 were submitted simultaneously and are the result of coordinated, independent radial-velocity observations and analyses of Kepler-78. We thank the HARPS-N team for their collegiality. We also thank E. Chiang, I. Crossfield, R. Kolbl, E. Petigura, and D. Huber for discussions, S. Howard for support, C. Dressing for a convenient packaging of stellar models, and A. Hatzes for a thorough review. This work was based on observations at the W. M. Keck Observatory granted by the University of Hawaii, the University of California, and the California Institute of Technology. We thank the observers who contributed to the measurements reported here and acknowledge the efforts of the Keck Observatory staff. We thank those of Hawaiian ancestry on whose sacred mountain of Mauna Kea we are guests. Kepler was competitively selected asthe tenth Discovery mission with funding providedbyNASA{\textquoteright}s Science Mission Directorate. J.N.W. and R.S.-O. acknowledge support from the Kepler Participating Scientist programme. A.W.H. acknowledges funding from NASA grant NNX12AJ23G.",
year = "2013",
doi = "10.1038/nature12767",
language = "English (US)",
volume = "503",
pages = "381--384",
journal = "Nature",
issn = "0028-0836",
publisher = "Nature Research",
number = "7476",
}