Hellish exoplanet has Earth-like density and composition

An artist's depiction of Kepler 10b, another planet that is tidally locked and orbits close to its host star.

NASA

The hunt for Earth-like planets orbiting other stars is inherently challenging. Earth is relatively small in size and low in mass, and it orbits far enough from the Sun that liquid surface water is possible. All those factors together make it difficult to find similar exoplanets, so astronomers are happy if they can find worlds similar to Earth in at least one of those three properties.

Hence the excitement over the exoplanet Kepler-78b, a planet slightly larger than Earth but with a similar density—and therefore likely a similar composition. Initial observations measured the exoplanet's orbital rate and its size, but two new papers in Nature provided the first mass estimates. Kepler-78b is about 20 percent larger than Earth and 70 percent more massive, but its density nearly matches that of Earth, indicating that it is likely composed of iron and rock.

This world was discovered by the now-defunct Kepler telescope and was first described in August of this year. Kepler-78b orbits its host star once every 8.5 hours, meaning it's close enough to be tidally locked, presenting the same face to the star just as the Moon does to Earth.