The Kepler mission has discovered an exoplanet smaller than Mercury orbiting a roughly Sun-like star.An international team of astronomers analyzing data from the Kepler satellite may have just stumbled upon the tip of the tiny planet iceberg. Thomas Barclay (NASA/Ames Research Center) and his colleagues announced their results in the February 20th issue of Nature, describing their detection of Kepler-37b, the smallest planet ever discovered around a normal (main-sequence) star.
The newly discovered runt of the planetary litter orbits Kepler-37, a star with about 80% the mass of the Sun and not quite the Sun's temperature. Data containing the telltale signals of planetary transits were collected over 978 days, showing three planets in the system: Kepler-37b, Kepler-37c, and Kepler-37d. Kepler-37b is the record-breaker, even smaller than Mercury.
Measuring the size of transiting planets is possible because the star is almost as massive as the Sun, so astronomers can observe oscillations similar to those seen on the Sun to determine a precise radius and mass for the host star. Once the size of Kepler-37 is known, the size of any transiting exoplanet is straightforward to calculate, and they are all relatively small: 0.3, 0.7 and 2 times the size of Earth, respectively, for Kepler-37b, Kepler-37c, and Kepler-37d.
With a radius about a third of Earth's, Kepler-37b is about the size of the Moon and smaller than Mercury. It’s pretty close to its host star, with an orbital period of just 13 days, so it's much closer than Mercury, which takes 88 days to complete an orbit. Even though the host star is a little cooler than the Sun, Kepler-37b is likely too hot to support liquid water, and thus life.
"Owing to its extremely small size, similar to that of Earth’s Moon, and its highly irradiated surface, Kepler-37b is very likely a rocky planet with no atmosphere or water, similar to Mercury,” Barclay and his team report.
Kepler has been a gold mine for transiting exoplanets, due to its exquisite sensitivity and precision. With Kepler’s most recent data release, astronomers have shown that Earth-size worlds are probably common.
As smaller planets are discovered, those conclusions may be extended to even smaller planets: "While a sample of only one planet is too small to use for determination of occurrence rates,” Barclay and his colleagues add, “it does lend weight to the belief that planet occurrence increases exponentially with decreasing planet size.”