Planets with sizes between that of Earth (with radius ) and Neptune (about 4 ) are now known to be common around Sun-like stars1,2,3. 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 compositions4,5, 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. ||||| Affiliations 
 
 Observatoire Astronomique de l’Université de Genève, 51 chemin des Maillettes, 1290 Versoix, Switzerland Francesco Pepe 
 
 , Stéphane Udry 
 
 , Christophe Lovis 
 
 , Michel Mayor 
 
 , Fatemeh Motalebi 
 
 , Didier Queloz 
 
 & Damien Ségransan Scottish Universities Physics Alliance, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK Andrew Collier Cameron 
 
 , Raphaëlle D. Haywood 
 
 & Keith Horne Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA David W. Latham 
 
 , Lars A. Buchhave 
 
 , David Charbonneau 
 
 , Courtney D. Dressing 
 
 , Xavier Dumusque 
 
 , Sara Gettel 
 
 , Mercedes Lopez-Morales 
 
 , David Phillips 
 
 , Dimitar Sasselov 
 
 & Andrew Szentgyorgyi INAF - Fundación Galileo Galilei, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, Spain Emilio Molinari 
 
 , Rosario Cosentino 
 
 , Aldo F. M. Fiorenzano 
 
 & Avet Harutyunyan INAF - IASF Milano, via Bassini 15, 20133 Milano, Italy Emilio Molinari INAF - Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy Aldo S. Bonomo 
 
 & Alessandro Sozzetti Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, DK-1350 Copenhagen, Denmark Lars A. Buchhave INAF - Osservatorio Astrofisico di Catania, via Santa Sofia 78, 95125 Catania, Italy Rosario Cosentino Centro de Astrofísica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal Pedro Figueira Dipartimento di Fisica e Astronomia “Galileo Galilei”, Universita’ di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy Luca Malavolta 
 
 & Giampaolo Piotto INAF - Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy Luca Malavolta 
 
 , Valerio Nascimbeni 
 
 & Giampaolo Piotto INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90124 Palermo, Italy Giusi Micela Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK Don Pollacco Cavendish Laboratory, J. J. Thomson Avenue, Cambridge CB3 0HE, UK Didier Queloz Scottish Universities Physics Alliance, Institute for Astronomy, Royal Observatory, University of Edinburgh, Blackford Hill, Edinburgh EH93HJ, UK Ken Rice Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK Christopher A. Watson 
 
 Contributions 
 
 The underlying observation programme was conceived and organized by F.P., A.C.C., D.W.L., C.L., D. Ségransan, S.U. and E.M. Observations with HARPS-N were carried out by A.C.C., A.S.B., D.C., R.C., C.D.D., X.D., P.F., A.F.M.F., S.G., A.H., R.D.H., M.L.-M., V.N., D. Pollacco, D.Q., K.R., A. Sozzetti, A. Szentgyorgyi and C.A.W. The data-reduction pipeline was adapted and updated by C.L., who also implemented the correction for charge-transfer-efficiency errors and the automatic computation of the activity indicator log(R′ HK ). M.L.-M. and S.G. independently computed the S-index and log(R′ HK ) values. A.C.C., D. Ségransan, A.S.B. and X.D. analysed the data using the offset-correction method. A.C.C. and D. Ségransan re-analysed the data for the determination of the stellar rotational period based on the Kepler light curve. P.F. investigated for possible correlations between the radial velocities and the line bisector. L.A.B. conducted the stellar parameter classification analysis for the re-determination of the stellar parameters based on HARPS-N spectra. An independent determination of the stellar parameters was conducted by L.M. by analysis of the cross-correlation function. D. Ségransan compared many different models to fit the observed data and selected the most appropriate by using the Bayesian information criterion. D. Ségransan performed a detailed MCMC analysis for the determination of the planetary parameters, with contributions also from A.S.B. and A. Sozzetti. F.P. was the primary author of the manuscript, with important contributions by D.C., D. Ségransan, D. Sasselov, C.A.W., K.R. and C.L. All authors are members of the HARPS-N Science Team and have contributed to the interpretation of the data and the results. 
 
 Competing interests 
 
 The authors declare no competing financial interests. 
 
 Corresponding author 
 
 Correspondence to Francesco Pepe. ||||| An artist's illustration of the strange Earth-like rocky planet Kepler-78b, which orbits a sun-like star 400 light-years from Earth. The planet is 20 percent wider and 80 percent more massive than Earth. 
 
 Credit: David A. Aguilar (CfA) View full size image 
 
 A puzzling alien planet is the closest thing to an Earth twin in size and composition known beyond our solar system, though it's far too hot to support life, scientists say. 
 
 The exoplanet Kepler-78b, whose supertight orbit baffles astronomers, is just 20 percent wider and about 80 percent more massive than Earth, with a density nearly identical to that of our planet, two research teams report in separate papers published online today (Oct. 30) in the journal Nature. 
 
 "This is the planet that, in many respects, is the most like Earth that's been discovered outside our solar system," said Andrew Howard, of the University of Hawaii at Manoa's Institute for Astronomy and lead author of one of the studies. "It has approximately the same size. It has the same density, which means it's made out of the same stuff as Earth, in all likelihood." [The Strangest Alien Planets (Gallery)] 
 
 Studying a lava world 
 
 Kepler-78b, whose discovery was announced last month, orbits a sunlike star in the constellation Cygnus, about 400 light-years from Earth. 
 
 The alien world circles 900,000 miles (1.5 million kilometers) or so from its parent star — just 1 percent of the distance between Earth and the sun— and completes one lap every 8.5 hours. Surface temperatures on Kepler-78b likely top 3,680 degrees Fahrenheit (2,000 degrees Celsius), Howard said. 
 
 The planet was found by NASA's prolific Kepler space telescope, which has spotted nearly 3,600 potential exoplanets since its March 2009 launch. (Kepler was hobbled in May of this year when the second of its orientation-maintaining reaction wheels failed, but scientists are still sifting through the instrument's huge databases.) 
 
 Kepler flagged alien worlds by noting the telltale brightness dips they caused when passing in front of, or transiting, their parent stars from the spacecraft's perspective. Kepler's measurements allow researchers to estimate an exoplanet's size but not its mass, meaning that other strategies are required to get a handle on a world's density and composition. [Gallery: A World of Kepler Planets] 
 
 One such method is the radial velocity technique, which measures the wobble in a host star's light induced by the gravitational pull of an orbiting planet. Both new studies employed this method to investigate the Kepler-78 system, with Howard's group using the HIRES spectrograph at Hawaii's Keck Observatory and another team, led by Francesco Pepe of the University of Geneva, relying on the new HARPS-N instrument on the Telescopio Nazionale Galileo in the Canary Islands. 
 
 The two teams came to very similar conclusions. Howard's group determined Kepler-78b's mass to be 1.69 times greater than that of Earth, while Pepe's team calculated it to be 1.86 times higher than Earth's. The results of the Pepe-led study suggest a density of 5.57 grams per cubic centimeter for Kepler-78b, while those of Howard's team imply a density of 5.3 grams per cubic cm. 
 
 These numbers agree to within the error range independently estimated by both teams, suggesting that they are quite accurate, Howard said. 
 
 "The fact that we agree to within our errors — in science, that's basically as good as you can do," Howard told SPACE.com. 
 
 Earth's density is about 5.5 grams per cubic cm, so Kepler-78b probably has an Earth-like composition, complete with a rocky interior and an iron core, both studies suggest. 
 
 A mysterious origin 
 
 The extremely tight orbit of Kepler-78b puzzles astronomers. According to prevailing theory, the alien world shouldn't exist where it does, because its host star was significantly larger when the planet was taking shape. 
 
 "It couldn't have formed in place because you can't form a planet inside a star," Dimitar Sasselov, of the Harvard-Smithsonian Center for Astrophysics and a member of the Pepe-led team, said in a statement. "It couldn't have formed further out and migrated inward, because it would have migrated all the way into the star. This planet is an enigma." 
 
 What is clear, however, is that Kepler-78b's days are numbered. The planet will continue circling lower and lower until the immense gravity of its host star tears it apart, likely within 3 billion years or so. 
 
 "Kepler-78b is going to end up in the star very soon, astronomically speaking," Sasselov said. 
 
 The search for another Earth 
 
 The hellishly hot Kepler-78b is not a good place to hunt for alien life. But the determination of its density marks a milestone in the ongoing search for a true "Earth twin" — a planet very much like Earth in size, composition and surface temperature. 
 
 "The existence of Kepler-78b shows that, at the very least, extrasolar planets of Earth-like composition are not rare," astronomer Drake Deming, of the University of Maryland, writes in an accompanying commentary article today in the same issue of Nature. 
 
 Deming points to NASA's upcoming Transiting Exoplanet Survey Satellite mission, or TESS, which is slated to launch in 2017 to hunt for transiting planets around nearby stars (as contrasted with Kepler, whose gaze was more distant). 
 
 "By focusing particularly on small stars cooler than the sun, TESS should find exo-Earths whose mass can be measured by trading the close-in orbit of Kepler-78b for more distant orbits around low-mass stars, approaching orbital zones where life is possi­ble," Deming writes. "That trade-off probably cannot be pushed to the point of measuring an Earth twin orbiting once per year around a sun twin, but it will allow future scientific teams to probe habitable planets orbiting small stars." 
 
 Follow Mike Wall on Twitter @michaeldwall and Google+. Follow us @Spacedotcom, Facebook or Google+. Originally published on SPACE.com.
– Astronomers have discovered a planet where a certain red guy with horns could make himself right at home. It's a planet much like our own Earth—about the same size, with the same mixture of rock and iron, and it orbits a star like our sun—except that Kepler 78b is an infernal ball of fire. Located less than a million miles from its host star—that's 1% of the distance between Earth and our sun—its temperature ranges between 3,500 and 5,000 degrees Fahrenheit. That's "well above the temperature where rock melts," an astronomer tells the New York Times. "This is probably one of the most hellish planets that have been discovered yet." At 400 light-years from Earth, the exoplanet was first spotted by the Kepler spacecraft but astronomers on Earth took a closer look at its light frequency to discover its orbit and mass. The findings, published in two studies in Nature, show a planet that tightly orbits its sun—which from the surface would look 80 times larger than our sun—in just eight and a half hours. Adds an expert, per LiveScience: "This planet is an enigma. It couldn't have formed in place because you can't form a planet inside a star." But while astronomers are baffled as to how the planet got there, they're fairly certain it shows friendlier Earthlike planets are still out there.