Super-Earths 'in the billions'

Liquid water is considered essential if a planet is to support life

There could be many billions of planets not much bigger than Earth circling faint stars in our galaxy, says an international team of astronomers.

The estimate for the number of "super-Earths" is based on detections already made and then extrapolated to include the Milky Way's population of so-called red dwarf stars.

The team works with the high-precision Harps instrument.

This is fitted to the 3.6m telescope at the Silla Observatory in Chile.

Harps employs an indirect method of detection that infers the existence of orbiting planets from the way their gravity makes a parent star appear to twitch in its motion across the sky.

"Our new observations with Harps mean that about 40% of all red dwarf stars have a super-Earth orbiting in the habitable zone where liquid water can exist on the surface of the planet," said team leader Xavier Bonfils from the Observatoire des Sciences de l'Univers de Grenoble, France.

"Because red dwarfs are so common - there are about 160 billion of them in the Milky Way - this leads us to the astonishing result that there are tens of billions of these planets in our galaxy alone."

The Harps team came up with its numbers after surveying 102 carefully chosen red dwarfs, which are dimmer and cooler than our Sun.

The group found a total of nine super-Earths (which are defined as planets with one to 10 times the mass of the Earth), with two judged to be orbiting inside their stars' habitable zones.

Putting all its data together, including observations of stars that did not have planets, the team was able to produce an estimate for how common different sorts of planets are around red dwarfs.

This assessment suggests super-Earths in the habitable zone occur in 41% of cases, with a range from 28% to 95%.

Given how many red dwarf stars there are in close proximity to the Sun, it means there could be perhaps 100 super-Earth planets in the habitable zones of stars that are less than about 30 light-years distant.

It raises the obvious question as to whether any of these planets are not just habitable but do indeed host life.

Because red dwarfs are relatively dim and cool, their habitable zones are closer in to the star than the Earth finds itself to the Sun.

But red dwarfs are known to be prone to stellar eruptions, or flares, which could bathe a nearby planet in X-rays or ultraviolet radiation, and that might make life there less likely.

"We already have ideas to find traces of life on these planets," commented co-researcher Stephane Udry from Geneva Observatory.

"If we are lucky to have an eclipse of the star by the planet - it's called a transit - then the light of the star will be going through the atmosphere of the planet before coming to Earth.

"This light will carry information about the chemical composition of the atmosphere.

"If we can see traces of elements related to life such as oxygen in that light, then we could get some clues that there is life on that planet.

"But we will need some big telescopes and probably in space," he told the BBC's Science In Action programme.