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.