By Dr David Whitehouse BBC News Online science editor

Earth-like worlds circling stars in orbital zones suitable for life may be few and far between in the cosmos, according to new research.

In the first comprehensive study of extrasolar planetary systems, astronomers have shown that in most of them it would not be possible to keep an Earth-like world in orbit around a star so that it was neither too hot nor too cold for life.

In general, other planetary systems fall into two types: those with Jupiter-like worlds circling close to their parent star, and those with more distant Jupiters in elliptical orbits.

In both systems, maintaining an Earth-like world in a temperate orbit is difficult, although not in all cases impossible.

"This work shows us just how unusual our own Solar System is when compared with the other planetary systems," Dr Kristen Menou of Princeton University, US, told BBC News Online.

Habitable zone

Eighty-five planetary systems were studied, all that were known when the research was carried out.

Dr Menou said: "They fall into two categories: large planets circling very close to their sun - the so-called 'hot Jupiters', and systems with Jupiter-like planets in distant non-circular orbits."

Dr Menou, along with Dr Serge Tabachnik, created computer dynamical models of the known exoplanetary systems to see if it was possible for Earth-like worlds to exist for long periods in the so-called habitable zone.

This work shows us just how unusual our own Solar System is when compared to the other planetary systems Dr Kristen Menou, Princeton University

This zone is the region around a star in which a planet would be able to sustain liquid water, being neither too close to the star for it all to be vaporised, nor too distant that it all freezes.

In our Solar System, the Earth is in the middle of the habitable zone. Astronomers believe such a position is essential for life to develop and thrive.

But it seems difficult for worlds to stay in the habitable zone in the majority of the extrasolar planetary systems found so far.

"We found that in the systems with the distant Jupiters, these worlds can disrupt the orbit of any Earth-like world in the habitable zone," says Dr Menou.

"Any Earth-like world in the temperate zone would either crash on to its parent star or be slung out into interstellar space," he added.

Over half of the planetary systems studied had distant Jupiters making them unlikely to contain habitable Earth-like worlds.

"We have identified some systems where distant Jupiters would pull Earth-like worlds into elliptical orbits that keep them inside the habitable zone. Such worlds would have dramatic and extreme seasons. We don't know how that would affect the development of life."

Cast asunder

The new analysis of the systems containing hot Jupiters shows that Earth-like worlds could remain orbiting in the temperate zone, seemingly an encouraging finding.

"The good news is that in about a quarter of the systems we studied, there could be habitable planets present."

But even in these systems, Earth-like worlds may have been cast asunder.

Current models of the evolution of planetary systems have hot-Jupiters reaching their tight orbits by migrating inwards from more distant ones.

This means that as they slowly travelled sunwards, they would have scattered any smaller worlds that got in their way, suggesting that there could be no Earth-like worlds in hot Jupiter systems at all.

"The way we are trying to get out of this pessimistic position," says Dr Menou, "is by seeing if Earth-like worlds could form in a planetary system after the inward migration of Jupiter worlds."

The research is to be published in a forthcoming edition of the Astrophysical Journal.