Sky-watchers have found the smallest planet yet outside our solar system orbiting a normal star 3,000 light years away. OK, it’s still three times bigger than Earth, but it represents an impressively accurate piece of astronomising nonetheless.

The planet has been given the catchy name MOA-2007-BLG-192L and is in orbit around a low mass star. National Geographic thinks this is a ‘shot in the arm’ for those looking for ET.

“This discovery is very exciting because it implies Earth-mass planets can form around low-mass stars, which are very common,” said Michael Briley, a National Science Foundation astronomer (press release). “It is another important step in the search for terrestrial planets in the habitable zones of other stars, and it would not have been possible without the international collaboration of professional and amateur astronomers devoted to measuring these signals.”

Exactly how ‘low mass’ the star is is unclear. Space.com:

The host star’s mass is uncertain, with one estimate putting it at roughly 6% of the sun’s mass, or just below the mass needed to sustain nuclear reactions in its core, thus making it a brown dwarf. Another estimate shows a host mass slightly above 8% of a solar mass, which would make MOA-2007-BLG-192L a very low-mass hydrogen-burning star.

The BBC notes a smaller planet has been seen going round a pulsar.

The discovery relies on something called gravitational microlensing, where a star passes in front of the star you’re looking at, acting as a lens for light from your target star. This means it looks a bit brighter. If a planet passes in front as well it gets a tiny bit brighter still. [Video on microlensing from the NSF.]

Image: NASA’s Exoplanet Exploration Program