Earth-like planets could litter the galaxy

Lots of places to escape to

Would-be galactic colonists got a boost this week when a paper in the journal Science suggested that Earth-like planets might be more common than previously thought.

Despite the fact that almost all of the extra-solar planets we have detected have been gas giants - often much bigger than Jupiter - research from the US suggests many of these extra-solar solar systems could also harbour smaller, rocky worlds like Earth.

The key lies in how the so-called "hot Jupiter" systems form. These are planetary systems with a very large gas giant orbiting a star in an extremely tight orbit - often even more tightly than Mercury orbits our sun.

Rocky worlds have been found orbiting stars inside the orbit of the huge gas giants, but there "hot Earths" would be utterly hostile to human life.

It is thought the gas giants start to form much further away from their suns, but gradually move inwards over time. As they move closer to their stars, they move through the protoplanetary disk - the debris that goes into building planets.

For years, researchers thought the gas giants would clean up all this material as they swept inwards, either sucking it all in like giant vacuum cleaners, or by scattering it out of the system entirely.

However, the US team, led by Sean Raymond of the University of Colorado at Boulder, has developed computer simulations showing that enough material can remain to form planets in the so-called habitable zone, the distance from the sun where water on a planet will remain liquid.

The researchers explain that although some material is scattered, the protoplanetary discs are often dense enough to damp down the scattering effect.

The debris is often flung into highly eccentric orbits, but eventually settles down into circular orbits where it can coalesce to form planets in the normal way. These planets would circle their stars outside the gas giants' orbit.

The team found that roughly one in three systems will behave this way. ®

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