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Kepler-37b is an extrasolar planet (exoplanet) orbiting Kepler-37 in the constellation Lyra. As of February 2013 it is the smallest extrasolar planet ever discovered around a main sequence star, with a mass and radius slightly greater than that of the Earth's Moon.

Kepler-37b, along with two other planets, Kepler-37c and Kepler-37d, were discovered by the Kepler space telescope, which observes stellar transits. After observing transits of the planet, astronomers had to compare it with the size of the parent star. The size of the star was obtained using asteroseismology;[clarification needed] Kepler-37 is currently the smallest star to be studied using this process. This allowed the size of the planet to be determined "with extreme accuracy". To date, the planet is the smallest discovered around a main sequence star outside the solar system. The discovery of a planet like Kepler-37b has led Jack Lissauer, a scientist at NASA's Ames Research Center, to conjecture that "such little planets are common".

The planet, about 210 light-years away from Earth, is slightly larger than the Earth's moon, with a diameter of about 3,900 kilometres (2,400 mi). NASA states that the planet probably has no atmosphere and cannot support life. Furthermore, the planet is most likely composed of rocky materials. Because it is so close to its star, the planet's mean temperature is estimated to be around 425 °C (800 °F).

The innermost known planet in its system, Kepler-37b, orbits its parent star at a distance of about 15 million kilometers (9.3 million miles), with a period of roughly 13 days. The outer two planets in the system have orbital periods within one percent of 8:5 and 3:1 commensurabilities (integer ratios) to Kepler-37b's period.