Rocky planets may harbour life, latest research shows

Our galaxy is teeming with Earth-like planets. Up to half of the 100 billion stars in the Milky Way could have rocky planets, and this increases the probability that at least some might harbour life, according to the latest research.

The search for planets outside our solar system has so far identified 55 giant planets similar to Jupiter or Saturn. A star with more than one giant planet has also been identified, but an Earth-sized planet has yet to be found.

New research presented to the American Association for the Advancement of Science annual meeting in San Francisco indicates, however, that our type of planet is plentiful, despite the fact that we cannot yet see them. And if there are rocky planets there could also be life.

"If there are terrestrial bodies around these stars then the likelihood of there being life as we understand it is more likely than before these discoveries," said Dr Norman Murray of the University of Toronto, assistant director of the Canadian Institute for Theoretical Astrophysics.

All of the 55 planets found so far are gas giants, said Prof John Percy of the University of Toronto, Mississauga. "There is also evidence of small planets, but direct detection of these planets is presently beyond our abilities."

The search was teaching us a great deal about how planets form and how unusual our solar system actually is, he said.

Some stars have giant planets that lie so close that the planet can complete an orbit in as little as three days. It takes Jupiter almost 12 years to complete a trip around the sun.

Dr Murray's research looks for signs of iron mixed into a star's outer layers. The theory holds that if there is iron-rich rocky material orbiting a star some of it will eventually plunge into the star and mix into its surface.

If the iron was just dust, little of it would reach the star, but if it was lumped into large chunks then the pieces could collide, knocking some out of orbit and into the sun, he explained.

Dr Murray analysed the spectral "signature" of almost 500 stars and found the iron he was looking for. Nine of these were known to have gas giant planets, he added. "There is terrestrial-type material orbiting these stars."

Statistical analysis of the data so far suggests that between 30 per cent and 100 per cent of stars in the Milky Way are likely to have this distinct iron signature, he said.

If there were rocks there could be rocky planets like Earth, he said. All models of planet formation predict that if chunks of terrestrial material form then they will accumulate into a planet-like body.

This did not mean, however, that they would have the water necessary to support life. Nor would it guarantee that the planet would be near enough to its star to remain warm but not so close that it would bake.

If the theory proves true then millions of the stars visible in the night sky will have rocky planets like Earth. At least some of them should be capable of harbouring life.