Artist's conception of the newly discovered planet MOA-2007-BLG-192Lb orbiting a brown dwarf 'star' with a mass of only six per cent of that of the Sun. (NASA's Exoplanet Exploration Program)

Planet-hunting astronomers have discovered a planet about three times the mass of Earth orbiting a dim star outside our solar system, a find that opens a new place to look for Earth-like planets.

It's the second smallest planet ever discovered outside our solar system but the first found orbiting a regular star. The smallest planet known outside our solar system orbits a pulsar, a spinning neutron star that emits high quantities of radiation.

The newly discovered planet orbits a faint star with a mass of about one 20th that of our sun and at a distance similar to that of Venus from our star, according to astronomers at the University of Notre Dame in Indiana.

"Our discovery indicates that that even the lowest mass stars can host planets," said David Bennett in a statement in advance of a presentation of his findings on Monday at the American Astronomical Society Meeting in St. Louis, Mo.

"No planets have previously been found to orbit stars with masses less than about 20 per cent that of the Sun, but this finding indicates that even the smallest stars can host planets," he said.

The finding is of particular interest for astronomers seeking Earth-like planets that lie in "the habitable zone" — a measure of whether a planet might support liquid water based on its distance from its star and the star's intensity. Finding a planet orbiting such a dim star provides astronomers with a new place to look for potentially habitable Earth-like planets.

The astronomers said the star the planet orbits could be a low-mass hydrogen-burning star called a red dwarf. Or it could be a brown dwarf, a stellar object too large to be considered a planet but too small to sustain nuclear fusion in its core as a star does.

The planet, given the unruly name MOA-2007-BLG-192Lb, lies 3,000 light years away from Earth.

It was discovered through the process of gravitational microlensing — detecting the presence of the planet based on the way a star's light bends in response to the gravitational influence of the planet.