Pepe, FrancescoCameron, Andrew CollierLatham, DavidMolinari, EmilioUdry, StéphaneBonomo, Aldo S.Buchhave, Lars A.Charbonneau, DavidCosentino, RosarioDressing, Courtney DanielleDumusque, XavierFigueira, PedroFiorenzano, Aldo F. M.Gettel, SaraHarutyunyan, AvetHaywood, RaphaelleHorne, KeithLopez-Morales, MercedesLovis, ChristopheMalavolta, LucaMayor, MichelMicela, GiusiMotalebi, FatemehNascimbeni, ValerioPhillips, DavidPiotto, GiampaoloPollacco, DonQueloz, DidierRice, KenSasselov, DimitarSégransan, DamienSozzetti, AlessandroSzentgyorgyi, AndrewWatson, Christopher A.2017-01-172013Pepe, Francesco, Andrew Collier Cameron, David W. Latham, Emilio Molinari, Stéphane Udry, Aldo S. Bonomo, Lars A. Buchhave, et al. 2013. An Earth-Sized Planet with an Earth-Like Density. Nature 503 (7476) (October 30): 377–380. doi:10.1038/nature12768.0028-0836http://nrs.harvard.edu/urn-3:HUL.InstRepos:29990221Recent analyses1–4 of data from the NASA Kepler spacecraft5 have established that planets with radii within 25 per cent of Earth’s (R⊕) are commonplace throughout the Galaxy, orbiting at least 16.5 per cent of Sun-like stars1. Because these studies were sensitive to the sizes of the planets but not their masses, the question remains whether these Earth-sized planets are indeed similar to the Earth in bulk composition. The smallest planets for which masses have been accurately determined6,7 are Kepler-10b (1.42R⊕) and Kepler-36b (1.49R⊕), which are both significantly larger than the Earth. Recently, the planet Kepler-78b was discovered8 and found to have a radius of only 1.16R⊕. Here we report that the mass of this planet is 1.86 Earth masses. The resulting mean density of the planet is 5.57 g cm−3, which is similar to that of the Earth and implies a composition of iron and rock.en-USJournal Article2017-01-1710.1038/nature12768