NASA’s Kepler space probe has discovered the smallest planet yet orbiting a Sun-like star. Dubbed Kepler-37b, the exoplanet orbits the star Kepler-37 about 210 light-years from Earth in the constellation Lyra. It’s only one-third the size of Earth and smaller than Mercury, which makes it not only the smallest planet yet found outside the Solar System, but the smallest planet ever discovered.
Kepler-37b is a rocky planet slightly larger than the Moon. It has no atmosphere and orbits its star at less than one-third the distance of Mercury to the Sun. This gives it a year of only 13 days and an estimated surface temperature is more than 800º F (700º K, 425º C), which would melt the zinc in a penny. Not surprisingly, Kepler-37b is regarded as unable to support life.
It’s one of three planets orbiting Kepler-37. Kepler-37c, the closer neighboring planet, is slightly smaller than Venus and almost three-quarters the size of Earth, and has a year 21 days long. The farther planet, Kepler-37d, is twice the size of Earth and orbits its star every 40 days. Kepler-37 itself is the same class as the Sun, but cooler and smaller. All three planets orbit the star at a distance closer than Mercury is to the Sun. This makes them all very hot and inhospitable.
Kepler-37b was found by the Kepler spacecraft as part of its ongoing mission to find Earth-like planets orbiting other stars in what is called the “habitable zone” – the band of space around a star where the temperature is right to sustain liquid water and therefore possibly life.
The Kepler spacecraft simultaneously and continuously measures the brightness of more than 150,000 stars every 30 minutes, watching to see if any planets pass in front of them. If one does, it causes the light from the star to dip momentarily. By studying how long and how large the dip is, scientists can determine how large the planet is and how far it orbits from its star. This technique first discovered planets larger than Jupiter, but smaller ones have also been detected as the technology matures.
To learn more about Kepler-37, scientists took a page from geology. When geologists want to learn about the interior of the Earth they rely on seismology, where sound is used either in the form of earthquakes or by setting off explosive charges These send waves echoing through the planet and how subterranean structures change these waves provides a wealth of information. When astronomers use this technique, it’s called asteroseismology.
Kepler-37 provides its own earthquakes by the constant convection of hot gases in the star, which causes high-frequency oscillations that are reflected in changes in Kepler-37’s brightness. Kepler is able to measure these changes and analysis has determined with an accuracy of three percent that the star is three-quarters the size of the Sun. This makes it the smallest star ever to have been measured this way.
The discovery of Kepler-37b suggests that these small exoplanets may be very common – especially because the Kepler probe required an exceptionally bright star to detect it.
"Even Kepler can only detect such a tiny world around the brightest stars it observes," said Jack Lissauer, a planetary scientist at NASA's Ames Research Center in Moffett Field, California. "The fact we've discovered tiny Kepler-37b suggests such little planets are common, and more planetary wonders await as we continue to gather and analyze additional data."