Experts from St Andrews University have been involved in weighing an Earth-sized planet orbiting another star for the first time. They concluded that Kepler-78b had a diameter of 9200 miles, and weighed almost twice as much as Earth. Measuring the radius of exoplanets is straightforward, but checking the planet's mass and therefore its density, which provides a clue to its composition, is more difficult.
Research published in the journal Nature confirms Kepler-78b as the first known Earth-sized exoplanet with an Earth-like density.
The results are baffling astronomers because Kepler-78b is a planet that shouldn't exist. It is a scorching world that circles its star every eight-and-a-half hours at a distance of less than one million miles in one of the tightest known orbits. According to current theories of planet formation, it couldn't have formed so close to its star, nor could it have moved there.
It has a density similar to Earth's, which suggests a similar composition of iron and rock.