Scientists are a step closer to finding Earth's twin sister with their latest discovery of three small planets orbiting distant stars.
Competing teams of European and American astronomers said yesterday that the finds are major breakthroughs in the search for Earth-like planets in other solar systems which could harbour life.
The US National Aeronautics and Space Administration (Nasa) have announced that its researchers have found two Neptune-sized planets, which are smaller than any "extrasolar" planet found outside the Solar System.
In a separate announcement, astronomers from the European Southern Observatory (ESO) said they have identified an even smaller extrasolar planet, which they believe could have a rocky crust similar to Earth's.
They said the planet is orbiting a star called mu Arae, which is about 50 light years away in the southern constellation of Ara - the Altar. The star is just bright enough to be seen with the naked eye.
The two teams from the US, one based at the University of Texas and the other at the University of California, found similar-sized planets orbiting the stars Gliese 436 in the Leo constellation and rho-1 Cancri, a star the size of the Sun, which was already known to have three planets in orbit around it.
The US teams estimate that the two planets are between 15 and 20 times the mass of the Earth. Meanwhile, measurements made by the European team suggest that its new planet is about 14 times as heavy as the Earth.
This places it in same league as the relatively small, rocky planets such as Earth. Paul Butler of the University of California at Berkeley said: "We've entered a new era for planet hunting."
Geoff Marcy of the University of California at Berkeley explained that it is the first time astronomers have found a new class of extrasolar planets. Until now, all of the 135 or so extrasolar planets have been "gas giants" similar to Jupiter, which are many dozens of times bigger than Earth and are thought to be too gaseous to sustain life as we know it.
"This is a milestone. We've crossed the hurdle finally. These sorts of planets are qualitatively different from the Jupiters and the Saturns," Dr Marcy said.
"It puts us in the transition region where we're not quite seeing the Earth-like planets yet but we're seeing their big brothers," he said.
Francois Bouchy, a member of the European team from the Astrophysics Laboratory in Marseille, said that the new planet discovered by ESO is very close to mu Arae and orbits the star in just 9.5 days.
"Not only did the new measurements confirm what we previously believed to know about this star, but they also showed that an additional planet on a short orbit was present," Dr Bouchy said.
"This new planet appears to be the smallest yet discovered around a star other than the Sun. This makes mu Arae a very exciting planetary system," he said.
The discovery was made by an ESO telescope on the mountains of La Silla in Chile. It used a technique called Doppler shift to detect the "wobble" in mu Arae caused by the orbit of the nearby planet - the same technique used by the American teams.
The new planet is orbiting at a distance from mu Arae, which is less than a tenth of the distance between the Sun and the Earth. This means it is exposed to temperatures of several hundred degrees which are far too hot to sustain life.
However, Nuno Santos of the Lisbon Observatory said that finding the smallest extrasolar planet identified so far indicates that it will soon be possible to discover an extrasolar planet similar to Earth.
"We're quite confident that the new planet is 14 times the mass of the Earth. This is the first time that anyone has found a planet that could be a rocky planet like the Earth," Dr Santos said.
"It's the first step towards finding an Earth-like planet. This new planet is very hot and I wouldn't bet that there is life on it," he said.
In addition to finding a rocky planet of a similar size to Earth, scientists would also like to locate one with liquid water, believed to be an essential ingredient of life. A planet that can harbour life would therefore need to be orbiting within the "Goldilocks" zone of its star - a distance not too cold to freeze water permanently, but not too hot to boil it all away.
Scientists first detected a planet orbiting another star in 1995. Since then more than 120 extrasolar planets have been identified orbiting other stars, but all of them have been gas giants.
Planets orbiting distant stars are too dim to be seen by even the most powerful telescopes - it's like trying to see a moth flying around a London streetlamp from New York.
Scientists, though, have other means, such as measuring the slight 'wobble' of a distant star caused by the gravitational pull of an orbiting planet. About 120 planets have been discovered this way, all large enough to create the 'wobble'.
Another way is to measure changes in light from a star when an orbiting planet passes across its direct line of sight with Earth. This has identified about three or four planets.
A fleet of six powerful infra-red space-based telescopes is due to be launched in 2014 which will be able to confirm the presence of water and possibly even life on extrasolar planets.Reuse content