The UK's SuperWASP telescope is scanning the skies for new planets

The search for planets orbiting other stars, otherwise known as "exoplanets", is unearthing new discoveries at an ever increasing rate.

In fact, this is one of the fastest-developing aspects of humanity's investigation of the Universe.

This and other themes are explored in the Open University's new series Cosmos: A Beginner's Guide.

At the last count there were at least 244 known exoplanets. More than 40 of these celestial bodies have been discovered in the first seven months of 2007 alone.

The tally is even more impressive when you consider than the first definite discovery was as recently as 1995, by Swiss astronomers Michel Mayor and Didier Queloz.

"It was a completely crazy time, with calls from papers, from television, from radio, from all the world," the pair recalled.

Searching for shadows

Using current technology, it is scarcely possible to "see" an exoplanet, the most fruitful way to find one is by measuring the slight wobble in its star's speed as the planet orbits around it.

It can only be a matter of time before scientists find another planet like ours Adam Hart-Davis

The effect is most marked in the case of very large planets orbiting very close to their stars, so most known exoplanets fall into the "hot Jupiter" class.

However, automated telescopes, such as those used in the UK-led SuperWASP survey, which scan the sky to spot the infinitesimal dip in a star's brightness when one of its planets happens to hide part of its disc are beginning to pay off.

Before too long, scientists expect to have located a considerable number of Earth-like "habitable" worlds.

"Many of us think that almost every star has some planets around it, and some of us even believe that there are Earth-like planets around almost every star," says Dr Wesley Traub, chief scientist on Nasa's planet finding programme.

David Rothery, an academic consultant for the Cosmos series and chairman of the Open University's planetary science courses, said the rate at which new exoplanets were being discovered made it hard to keep courses up to date.

"It's not so much the number of currently known exoplanets that bothers us," explained Dr Rothery. "The real issue is that, so far, few solar systems seem to be built like ours, and ideas keep changing about the long-term stability of planets' orbits.

"The discovery of so many 'hot Jupiters' seems to show that they formed far from their stars and migrated inwards.

"It would be hard for an Earth-like planet to survive this episode."

However, at least one "habitable" planet - rocky and at the right temperature for liquid water, but five times as massive as the Earth - has now been found.

Adam Hart-Davis, the main presenter of the series, concludes: "It can only be a matter of time before scientists find another planet like ours. The hunt for another Earth is on."