A mini-world the size of the Moon is the smallest planet yet discovered beyond our Solar System (exoplanet).
The hot, barren planet orbits a Sun-like star called Kepler-37 and is thought to be a smaller version of Mercury.
Such small rocky worlds offer the best chance of finding life among the stars. The new planet is believed to be far too inhospitable for life, but considered special because of its small size.
The new planet, Kepler-37b, is the innermost planet and about 80% the size of Mercury. It is the first exoplanet found to be smaller than any planet orbiting the Sun.