An artist's impression of an Earth-like planet. Credit: Julian Baum/Copyright 2002

One of the most fascinating areas of astronomical research in recent years has been the search for other 'Earths' circling Sun-like stars far beyond our Solar System.

In recent years nearly 100 planets have been discovered in orbits around other stars, but none of these 'exoplanets' remotely resembles the Earth. However, according to the latest computer simulations by Barrie Jones and Nick Sleep (The Open University), millions of Earthlike worlds could be scattered throughout the Galaxy, just waiting for telescopes to improve sufficiently for us to find them.

"Although we do not yet have the capability to detect 'tiddlers' like the Earth, we can establish theoretically which of the exoplanetary systems are most likely to have an 'Earth'," said Professor Jones.

Jones and Sleep are using a computer model to launch 'Earths' into known exoplanetary systems, in order to find out how long the small planets last before being ejected by the enormous gravitational grip of their giant neighbours.

Professor Jones explained to the UK National Astronomy Meeting in Bristol the results of their simulations of planetary orbits within the habitable zones - popularly known as the "Goldilocks zones" - of nearby stars, where temperatures are just right to enable water to exist in liquid form on an Earth-like planet.

Any 'Earth' found in such a zone would be a potential habitat for life as we know it. In some exosystems, one or more of the giant planets is too close to the habitable zone for 'Earths' to remain in a stable orbit. But in other systems long-term survival is possible, and therefore these systems should be prime targets in searches for life beyond the Solar System.

The system most like the Solar System (so far) is that of 47 Ursae Majoris (47 UMa) - a solar-type star, a bit older than the Sun. This means that it is now slightly hotter and more luminous than the Sun, so that its habitable zone is a little further out. It extends from about 1 AU to about 1.9 AU, whereas in the Solar System today the zone extends from about 0.8 AU to 1.7 AU - roughly from the orbit of Venus to the orbit of Mars (1 AU - the Earth's average distance from the Sun - is approximately 93 million miles or 150 million km).

47 UMa is known to have two giant planets in orbit around it. The inner one has a mass at least 2.54 times that of Jupiter, whereas the outer one is rather smaller, probably a little less massive than Jupiter.

However, both giants are closer to 47 UMa than Jupiter is to our Sun. In Solar System terms, the inner giant of 47 UMa would be in our asteroid belt, while the outer one would orbit between this belt and Jupiter, so both giants are not far outside the star system's habitable zone. Nevertheless, despite their relative proximity and their larger masses, Jones and Sleep found that an Earth-like planet could survive at various orbits in the habitable zone of 47 UMa.

"It's certainly a system worth exploring for an Earthlike planet and for life," said Jones.

Overall, based on their investigations of several of the known exoplanetary systems, the OU team estimates that a "decent proportion" of them could contain habitable 'Earths', even though in all of these systems the giants are nearer to the habitable zones than Jupiter is in our system.

If this conclusion is correct, then habitable 'Earths' could be very common in the Galaxy.

"There could be at least a billion 'Earths' in the Milky Way," said Jones, "and lots more if we find systems more like ours, with their giant planets well away from the habitable zones."

All of the known exoplanets are much larger and more massive than the Earth. In composition, mass and diameter they resemble the giant planets Jupiter and Saturn - they are made predominantly of hydrogen and helium, just like the stars themselves, whereas the Earth is made of rocky materials.

The Astronomy Group at The Open University has also joined the Wide Angle Search for Planets (WASP) consortium that will search for transits of exoplanets across the face of their parent stars. Other universities in the consortium are: Belfast, Cambridge, Leicester and St. Andrews. Although the transit method is more suited for detections of large planets, the technique could soon reveal planets not much larger than the Earth.