But otherwise it is a lot like our planet, about 20% larger and 169% of our mass, and that makes it the smallest exoplanet to-date that has a confirmed mass and radius. The size is about the same, the density is about the same - and that's part of the mystery. How did it form so close to its star?
Kepler-78b orbits Kepler 78 and researchers made the determination by using the transit method. Kepler-78 is one of about 150,000 stars that NASA’s Kepler spacecraft has precisely measured in terms of brightness so by measuring the wobbles they discovered Kepler-78b. In doing so, two groups (see citations below) came up with similar sizes and their calculations of Kepler-78b’s density are around 5.3 and 5.57 grams per cubic centimeter - similar to that of Earth and implying a composition of iron and rock.
Kepler-78b placed on a planetary mass–radius diagram. Masses and radii represent the modes of the corresponding distributions derived from the MCMC analysis, and the error bars represent the 68.3% confidence interval (1σ). Earth and Venus are indicated for comparison. Credit and link: doi:10.1038/nature12768
In terms of mass, radius and mean density, Kepler-78b is the planet most similar to the Earth of the exoplanets for which those quantities have been determined. However, Kepler-78b does differ from the Earth, notably in its very short orbital period and correspondingly high temperature.
Andrew W. Howard, Roberto Sanchis-Ojeda, Geoffrey W. Marcy, John Asher Johnson, Joshua N. Winn, Howard Isaacson, Debra A. Fischer, Benjamin J. Fulton, Evan Sinukoff&Jonathan J. Fortney, 'A rocky composition for an Earth-sized exoplanet', Nature October 30 2013 doi:10.1038/nature12767
Francesco Pepe, Andrew Collier Cameron, David W. Latham, Emilio Molinari, Stéphane Udry, Aldo S. Bonomo, Lars A. Buchhave, David Charbonneau, Rosario Cosentino, Courtney D. Dressing, Xavier Dumusque, Pedro Figueira, Aldo F. M. Fiorenzano, Sara Gettel, Avet Harutyunyan, Raphaëlle D. Haywood, Keith Horne, Mercedes Lopez-Morales, Christophe Lovis, Luca Malavolta, Michel Mayor, Giusi Micela, Fatemeh Motalebi, Valerio Nascimbeni, David Phillips, Giampaolo Piotto, Don Pollacco, Didier Queloz, Ken Rice, Dimitar Sasselov, Damien Ségransan, Alessandro Sozzetti, Andrew Szentgyorgyi&Christopher A. Watson, 'An Earth-sized planet with an Earth-like density', Nature October 30 2013 doi:10.1038/nature12768