Ten more planets found and billions more to come

What was once science fiction is now science fact. Over the past few years, astronomers have amassed convincing evidence that the universe is awash with planets, many of which may be suitable for life.

What was once science fiction is now science fact. Over the past few years, astronomers have amassed convincing evidence that the universe is awash with planets, many of which may be suitable for life.

Only five years ago the only known planets in the universe were the nine - including Earth - that orbit our own Sun. Now astronomers have monitored the movements of at least 50 extra-solar planets, 10 of which are new to science and will be announced today at a meeting of the International Astronomical Union in Manchester.

Included in the latest recruits to the burgeoning army of "exoplanets" is one orbiting a star - Epsilon Eridani - that is close enough to be seen with the naked eye. It is just 10.5 light years away, the astronomical equivalent to our own backyard.

Also to be announced in Manchester is further evidence of stars that have captured more than one planet in orbit. The existence of solar systems other than our own significantly raises the stakes in terms of the possible evolution of complex alien lifeforms.

Geoff Marcy, an astronomer at the University of California, Berkeley who has led the race to find exoplanets, said: "With 10 new planets announced, we are suddenly drenched in a planetary shower. Our Milky Way galaxy certainly contains billions of planetary systems, many being quite different from our solar system.

"Surely there must be many lukewarm worlds that can host complex organic chemistry, leading to replicating molecules that compete with one another for energy and resources. The seeds of life are sown in such environments, and there is no reason to doubt that increased biological complexity follows naturally."

The breakthrough in the search for exoplanets began in 1995 when Michel Mayor and Didier Queloz of the Geneva Observatory in Switzerland discovered indirect evidence of a planet orbiting at a distance of some 4.6 million miles from a star known as 51 Pegasi. This distance is about a twentieth of the distance between the Earth and the Sun.

Astronomers do not rely on the direct observation of planets because the light of their mother suns outshines any light that the planets themselves reflect. Instead they look for the gravitational "wobbles" of a star caused by the interference of an orbiting planet.

The technique, refined over the years with experience and better instruments, has proved remarkably successful but it has obviously been biased towards finding only the largest planets, those that are about as big as Jupiter. It is also easier with this technique to identify those planets that have highly elliptical orbits.

To find a planet suitable for life, astronomers are more interested in finding those with more circular orbits, as this will mean that their temperatures remain relatively constant through the solar year - much as the Earth does. They also want to find planets orbiting within a certain distance from a star so that temperatures fall within the range of 0C to 100C, making it possible for life to "germinate" in the presence of liquid water.

Last year astronomers found more than one Jupiter-sized planet orbiting the star Upsilon Andromedae, some 44 light years (259 trillion miles) from Earth. The implication of this discovery is that other solar systems are common and that smaller planets, capable of harbouring life, might be protected against meteorite bombardment by Jupiter-like giants. Astronomers believe life on Earth was to some extent nurtured by the effect of Jupiter's gravity acting as a shield against cosmic collision.

The most exciting prospect in years to come, however, is the potential for seeing exoplanets directly. So far, only one planet has been directly observed, as it crossed in front of its star, causing the light to diminish by a small but discernible fraction.

William Cochran, of the University of Texas who was part of the team that found the exoplanet orbiting Epsilon Eridani, said: "I would be astonished if there were not Earth-like planets around other stars. At the moment we just don't have the technology to detect them. This particular planet's orbit is about 3.5 times the distance from the Earth to the Sun. This means there is room in the inner system for a rocky, Earth-like planet where the temperatures might be suitable for liquid water and even life."

Already, astronomers have been able to find planets smaller than Jupiter.

The Swiss team from Geneva will tell the meeting today of two planets orbiting a star called HD 83443. One of the planets, at about half the size of Saturn, is the smallest extrasolar planet yet found.

Finding a precise parallel to Earth in size and orbit may, however, take far longer, and may even not be possible with existing technology. However, astronomers hope that a new space-based telescope, called the Terrestrial Planet Finder, will do the job. Nasa hopes to launch the next generation of planet hunter in 2005.