With an ever increasing number of small exoplanets being discovered, the record holder for the smallest known exoplanet gets replaced fairly quickly. Recently, NASA’s Kepler mission has found a planetary system that is home to the smallest planet yet discovered around a Sun-like star. The discovery was published in Nature in a paper titled “A sub-Mercury-sized Exoplanet”. This tiny planet is named Kepler-37b and it belongs to a planetary system consisting of two other known planets - Kepler-37c and Kepler-37d. Kepler is a space telescope which finds planets by measuring tiny dips in a star’s brightness when a planet happens to pass in front of the star. When the planet Kepler-37b transits in front of its host star, it causes the star’s brightness to dim by a mere 20 parts-per-million. This gives Kepler-37b an estimated diameter of 3850 kilometres, making it smaller than Mercury (4880 km diameter) and only barely larger than the Moon (3475 km diameter).
Figure: The line up compares artist's concepts of the planets in the Kepler-37 system to the moon and planets in the solar system. The smallest planet, Kepler-37b, is slightly larger than our moon, measuring about one-third the size of Earth. Kepler-37c, the second planet, is slightly smaller than Venus, measuring almost three-quarters the size of Earth. Kepler-37d, the third planet, is twice the size of Earth. Image credit: NASA/Ames/JPL-Caltech
|Orbital Period (days):||13.4||21.3||39.8|
|Orbital Distance (million km):||15.0||20.5||31.1|
|Planet Diameter (Earth = 1.0):||0.303||0.742||1.99|
|Planet Diameter (km):||3850||9450||25400|
Kepler-37 is the host star of the planetary system that is home to Kepler-37b and it is located at a distance of 210 light-years from Earth in the constellation Lyra. This is a star that is slightly cooler than the Sun with 0.770 times the Sun’s diameter and 0.802 times the Sun’s mass. Kepler-37b is the innermost of the three known planets around Kepler 37. At a distance of just 15 million kilometres from its host star, Kepler-37b orbits one every 13.4 days. In fact, all three known planets around Kepler 37 orbit closer than Mercury orbits the Sun, suggesting that these planets are too hot to be hospitable for life. Kepler-37b is close enough to its host star that it is very likely a scorched and airless world. Any form of volatiles would have long since vanished and its weak gravity prevents it from holding on to any appreciable atmosphere.
Thomas Barclay, et al., “A sub-Mercury-sized Exoplanet”, Nature (2013), doi:10.1038/nature11914