Distant exoplanet: Most similar to Earth of any yet discovered

U.S. and Swiss astronomers say a planet around a distant star has been found to be the most similar to the Earth in terms of mass and density. In two studies published in the journal Nature, the astronomers report measurements of the Earth-sized exoplanet Kepler-78b suggest it has an iron core and a rocky interior. While measuring the radius of an exoplanet is relatively straightforward, the astronomers noted, measuring the planet's mass -- and therefore its density, which can yield clues to its makeup -- is more difficult. Two independent groups -- at the University of Hawaii and the University of Geneva in Switzerland -- were able to determine the mass of Kepler-78b by measuring "wobbles" in the light from its host star as the exoplanet orbits around it. They found the mass of Kepler-78B is about 1.75 that of Earth, suggesting a density of about 340 pounds per cubic foot. That density is close enough to that of Earth to imply a composition of iron and rock, they said. While similar in mass and density to the Earth, Kepler-78b does differ from our planet in its very short orbital period and correspondingly high temperature, the astronomers said.