That astronomers can find exoplanets at all is still a source of wonder to me. That they can find Earth-sized planets is astonishing. But a paper published in today’s issue of Nature is almost miraculous: A sub-Mercury-sized exoplanet, by Thomas Barclay and many others, describes the discovery of an exoplanet that has a radius that’s just 0.3 times that of Earth. It’s smaller than Mercury, in other words.
Kepler 37b, as it’s name implies, was found from data taken by the Kepler mission. The parent star, Kepler-37, is interesting because it’s the densest star in which solar-like oscillations have been detected. Just as a measurement of the frequencies of a musical instrument allows you to determine some of the properties of that instrument, the characteristic “ringing” of a star allows astronomers to determine some of the star’s properties with great accuracy. In this case astronomers were able to determine the radius of Kepler-37 with great precision, and this in turn allowed them to determine the radius of its planets with precision. Transit signals suggest that Kepler-37 has three planets. Kepler 37d has a radius about 1.99 times that of Earth’s; Kepler 37c has a radius about 0.74 times that of Earth’s; and Kepler 37b has a radius just 0.3 times that of Earth’s. It’s not much bigger than our Moon – and Kepler detected it!
For every 200 stars that Kepler studies you’d expect to see the transit signal in the data of perhaps one star. So the fact that the astronomers were able to identify this sub-Mercury-sized object does rather tend to suggest that small planets are extremely common.