Tiny exoplanet is smaller than Mercury (and probably hotter, too)

Exoplanets in the Kepler-37 system, compared with Solar System bodies. The smallest of the planets, Kepler-37b, is only slightly larger than the Moon, and noticeably smaller than Mercury.

NASA/Ames/JPL-Caltech

What's the smallest exoplanet we can detect from Earth? Smaller bodies are inherently harder to observe than larger ones, but the orbiting Kepler telescope has enabled astronomers to identify a number of small planet candidates, including a few smaller than Earth, by measuring the fluctuations in the host star's light as the planets pass in front of it.

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. While these worlds are all in very small orbits, their existence contributes to our understanding of the variety of planets that can form.

The Kepler telescope works by collecting light from a huge number of stars in a single patch of the sky. By searching for periodic fluctuations in the light from an individual star, astronomers can find exoplanets as they transit, crossing in front of the disk of the star and blocking a tiny amount of its light. The rapidity of the decrease in light and the fraction of the star's light that is lost to the eclipse reveal the size of the planet, while time between transits indicates the duration of the orbit. Since these effects are small, researchers use sophisticated computer algorithms to pick out exoplanet signals from other fluctuations, such as starspots (sunspots on other stars) .