  Since the discovery of the first exoplanet we have known that other planetary
systems can look quite unlike our own. However, until recently we have only
been able to probe the upper range of the planet size distribution. The high
precision of the Kepler space telescope has allowed us to detect planets that
are the size of Earth and somewhat smaller, but no previous planets have been
found that are smaller than those we see in our own Solar System. Here we
report the discovery of a planet significantly smaller than Mercury. This tiny
planet is the innermost of three planets that orbit the Sun-like host star,
which we have designated Kepler-37. Owing to its extremely small size, similar
to that of Earth's Moon, and highly irradiated surface, Kepler-37b is probably
a rocky planet with no atmosphere or water, similar to Mercury.
