Astronomers find smallest known planet — smaller than Mercury

A team of scientists including two Yale University astronomers has discovered the smallest planet yet detected.

''With this discovery we now have an inventory of planets that are both much larger than Earth and also much smaller than Earth,'' says Sarbani Basu, professor of astronomy at Yale and a member of the team. ''The detection of Earth-sized, and hopefully Earth-like, planets cannot be far behind.''

In research published online 20 February in the journal Nature, the scientists reported finding a planetary system, Kepler-37, with three planets. Two of them are smaller than Earth, and one of these is smaller than Mercury, the smallest of the eight planets in Earth's solar system.

Until now, astronomers had never found a planet smaller than Mercury. The discovery of one establishes that stars indeed host planets smaller than any in Earth's solar system, researchers said. They already knew that stars host planets much bigger than Earth.

The smallest planet in the Kepler-37 system is slightly larger than Earth's Moon and it is likely that it has no atmosphere or water, like Mercury itself, researchers said.

The Moon has a radius of about 1080 miles, slightly more than one-quarter of Earth's.