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Last Updated: Tuesday, 5 April, 2005, 09:48 GMT 10:48 UK
Plenty of Earths await discovery
By Jonathan Amos
BBC News science reporter, in Birmingham

Exoplanet visualisation, Nasa
The Universe could host billions of Earths
British researchers are more confident than ever that there are "Earths" out there waiting to be discovered.

The scientists say perhaps a half of all the known planetary systems today could be harbouring habitable worlds.

It must be said most of these systems are strange places where supergiant planets orbit close in to their stars.

But Barrie Jones and colleagues say their modelling work suggests that even with this oddness, there should be room for small rocky planets.

The Open University team presented its ideas here at the UK National Astronomy Meeting on Tuesday.

They extend recent and previously published theoretical work, "putting it on a firmer modelling basis," Professor Jones told the BBC News website.

The research calculates the likely number of Earths out there, based on what we know about how planets form and the conditions needed for life - in particular, the requirement to sit in the part of a solar system that is neither too hot for liquid water, nor too cold.

'Disaster' area

"The conclusions haven't changed, I'm pleased to say. Roughly half the systems out there could have Earths in their habitable zones today and have been there long enough for life to develop," Jones added.

The limitations of current telescope technology make it extremely difficult to view so-called extrasolar planets directly.

Astronomers have therefore made most of their detections indirectly - by finding stars that appear to "wobble" under the gravitational tug of what must be nearby, very large planets.

The technique has the bias of only showing up apparently bizarre systems - where planets that are sometimes many times the mass of our own Jupiter circle their stars in orbits that are smaller than Mercury's.

And this presents a problem because current thinking holds that these huge Jupiters probably formed some way out from their stars before migrating inwards. And if they did that, the chances are they would have destroyed all in their path, including any fledging Earths.

"We've now got some simple rules for establishing how far these disaster zones extend," explained Professor Jones.

Moving zones

Encouragingly, his team finds there is plenty room and time for Earths to evolve.

"At the stage these great giants sweep through, the Earths are not formed - they are still smallish planetary embryos. They get scattered but the simulations show enough material remains that Earths can form after the migration of the great giants has taken place."

The team found about half of the known exoplanetary systems offer a safe haven for a period extending from the present into the past that is at least long enough for life to have developed on any such planets.

The situation is complicated slightly by the fact that the habitable zone migrates outwards as the star ages, and in some cases this changes the potential for life to evolve.

Thus, in some cases a safe haven might have been available only in the past, while in other cases it might exist only in the future.

These scenarios of past extinction and future birth increase to about two-thirds the proportion of the known exoplanetary systems that are potentially habitable at some time during the main-sequence lifetime of their central star.

The research by Barrie Jones, Nick Sleep, and David Underwood has been published in Astrophysical Journal.

The Royal Astronomical Society's National Astronomy Meeting is being held at the University of Birmingham.

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01 Jul 02 |  Science/Nature

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