Moon-Sized Planet Becomes Smallest Found Outside Solar System

A planet the size of our moon has become the smallest found outside the solar system, scientists from the U.S. Kepler space mission said.

The new planet, called Kepler-37b, is probably rocky with no atmosphere or water, like Mercury, the National Aeronautics and Space Administration researchers said in a paper published today in the journal Nature. It’s one of three planets orbiting a star that’s smaller, denser and cooler than the sun.

Discoveries of small planets had been handicapped by the limitations of scientists’ instruments, said Thomas Barclay, a research scientist at the NASA Ames Research Center in Moffett Field, California, and an author of today’s report. Most stars aren’t stable or bright enough for planets this small to be detected, he said.

“This was a great test case, and the fact that we found a planet this size on one of the few stars where we can find one hints that small planets like this may be extremely common,” Barclay said in a telephone interview. It’s too early to say how many such planets there might be, he said.

NASA’s Kepler mission is designed to discover Earth-size and smaller celestial bodies in regions around their stars, particularly those where liquid water may exist. The spacecraft was launched in March 2009 and has found about 2,400 planet candidates.

The planet Kepler-37b was detected through comparing the light captured when it passed in front of its star to the star’s usual light. The planet’s star is well-characterized so scientists were able to determine its size with greater certainty, Barclay said.

The previous smallest planets discovered outside the solar system were orbiting a red dwarf, the most common type of star in the Milky Way. The red dwarf was about twice the size of Jupiter. The smallest of those planets was about 0.57 the times of the radius of Earth, or about the size of Mars, according to a NASA announcement from January 2012.