Seismic waves on a star have helped astronomers pin down the smallest known exoplanet. The newly announced world, called Kepler-37b, is not much larger than Earth's moon, and becomes the first known planet orbiting a sunlike star that is smaller than any in our solar system.
Kepler-37b is one of three new worlds found around Kepler 37, a star slightly cooler than our sun near the constellation Cygnus. Another planet is smaller than Earth, while the third is twice Earth's size. All three are probably rocky and extremely hot, with no atmosphere or chance of supporting life.
"This shows us the diversity of exoplanetary systems," says Thomas Barclay at the NASA Ames Research Center in Moffett Field, California. "We knew there were very large systems – now we know there are very small ones as well."
The three planets were found with NASA's Kepler space telescope, which searches for dips in starlight when a planet passes in front of its star, as seen from Earth. The greater the dip, the larger the planet.
Kepler-37b beats out the previous smallest world, Kepler-42d, which was discovered last year and is about half as big as Earth. But two challengers could rewrite the record books yet again. An evaporating exoplanet announced last week is thought to be the size of the moon, while one of the first known exoplanets, found orbiting a dead star called a pulsar, has a mass only 2 per cent that of Earth.
The evaporating planet was also found by Kepler, but its size is an estimate based on modelling. The pulsar planet was discovered via changes in the star's radio pulses caused by the orbiting world's gravitational tugs, a method that gives mass but not size. That means neither planet's radius has been confirmed, leaving their claims to the tiniest exoplanet title in doubt.
Barclay's team is much more certain about the sizes of the Kepler-37 planets because they made measurements on their star used a technique called asteroseismology. This involves looking at waves on the star's surface to gather information about its interior, in much the same way as geologists use seismic waves to study structure deep inside Earth. This allowed the researchers to better calculate the star's mass, size and density.
"One of our big limitations in studying planets is actually understanding the star," says Barclay. Knowing the star's size more accurately, the team could better gauge the sizes of its planets. "We've got an accurate measure of just how small they are."