Two groups of researchers have for the first time succeeded in measuring the mass of an Earth-sized exoplanet — in this case, Kepler-78b, which is only slightly wider than Earth and orbits a star in the constellation Cygnus, 123 parsecs (400 light years) away.
Astronomers are getting better at finding the many small, Earth-sized planets that scatter the Universe outside our Solar System, but finding clues to what they are made of is much harder.
The figures for Kepler-78b are published today in Nature1, 2 and show that the planet’s density is similar to that of Earth, suggesting that it also has a similar composition of rock and iron.
That, however, is where the similarities end. Kepler-78b travels in orbit about 40 times closer to its star than Mercury is to the Sun; its year lasts just 8.5 hours; and its surface is thought to reach a scorching 2,300–3,100 Kelvin.
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