We often focus on the search for Earth-like planets (whatever that means) when we talk about exoplanets: planets orbiting other stars. However, another important goal is to categorize as many planetary systems as possible, determining what kind of planets orbit what sort of stars. That catalog is gradually revealing the way planets form, and how the detailed history of each system leads to what we observe today. For example, Mercury is the smallest planet in the Solar System, but it’s far from being the smallest spherical body—and nobody expects it to be the smallest planet of any star system. Now a new observation has found an exoplanet noticeably smaller than Mercury, which will help us fill in the catalog a little more.

Now researchers may have found the smallest exoplanet yet, a world with a diameter about 80 percent of Mercury’s. This planet candidate, named Kepler-37b, orbits very close to its star: its orbital radius is about 1/4 the size of Mercury’s, so it takes only about 13 days to zip around. Thomas Barclay and collaborators also identified two other planets in the same system—labeled Kepler-37c and Kepler-37d—one of which is slightly larger than Mars, and the other which has twice Earth’s diameter. [Read more…]