Astronomers have discovered a planet about as massive as three Earths, orbiting an object smaller than our Sun.

Even smaller exoplanets have been found previously around stellar corpses called neutron stars. But this is the lightest planet ever found orbiting a star in the prime of its life.

In fact, the host star itself is very lightweight, and is thought to be a brown dwarf weighing between 6 and 8% as much as the Sun. Brown dwarfs are more massive than planets but not massive enough to sustain nuclear reactions in their cores, as normal stars do.

"Our discovery indicates that even the lowest-mass stars can host planets," David Bennett of the University of Notre Dame, who led an international team of astronomers to the discovery, said on Monday at the American Astronomical Society meeting in St Louis, Missouri, US.

The planet, dubbed MOA-2007-BLG-192Lb, is around 3000 light years from Earth. Planet formation theory suggests it is made mostly of rock and ice.

It orbits its star at about the same distance that Venus orbits the Sun, although it is likely to be much colder than Pluto because its host star is a brown dwarf.

Small host

The host's small size implies that NASA's James Webb Space Telescope, due to launch in 2013, could explore any such planetary systems that are found relatively close by, bringing the search for alien life much closer to home.

"If someone can find planets like these, we're certainly hoping to be able to find out about them – and the smaller the host star's mass, the better for us," John Mather of NASA's Goddard Space Flight Center, Nobel Laureate and senior project scientist for the JWST, told New Scientist.

A smaller star makes the transit of a planet easier to see, giving a better chance to measure the chemical composition of the atmosphere – a key part of finding the signs of life.

Bent light

Bennett's team made their discovery through a phenomenon called gravitational microlensing. This relies on analysing the way the host star bends light coming from another, more distant star. The presence of a planet can further distort the light.

Performing such observations is notoriously difficult, but made possible in this instance thanks to the sensitivity of the Microlensing Observations in Astrophysics (MOA) II telescope in New Zealand.

It has been independently confirmed by observations at the Very Large Telescope (VLT) in Chile. Bennett is confident that the microlensing technique is just beginning to come into its own, and could be the first method that detects an Earth-sized planet.

Journal reference Astrophysical Journal (in press)

Astrobiology - Learn more in our out-of-this-world special report.

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