Marking a further step in the hunt for worlds orbiting other stars, astronomers on Wednesday said they had found a cool planet the size of Jupiter that encircles a sun at searing proximity.

The work is a technical exploit in the field of exoplanets, as planets outside our Solar System are called, they said.

"This is the first (exoplanet) whose properties we can study in depth," said Claire Moutou, one of 60 astronomers who took part in the discovery. "It is bound to become a Rosetta Stone in exoplanet research."

More than 400 exoplanets have been spotted since the first came to light in 1995.

To the disappointment of those dreaming of a home from home, none has yet proved to be a small, rocky, watery world like our own.

Instead, most are "hot Jupiters," or huge gassy balls that are so close to their stars that their surfaces can be scorched to a thousand degrees Celsius (1,800 degrees Fahrenheit) or more.

The new find, named CoRoT-9b after the French orbital telescope that originally spotted it in 2008, takes a little over 95 days to orbit its host star, CoRoT-9, located 1,500 light years away in the constellation of Serpens, the Snake.

By comparison, our own Mercury takes 88 days to orbit the Sun.

CoRoT-9b, though, is a gas giant with a mass about 80 percent that of Jupiter and -- compared with other such exoplanets -- is relatively temperate, with a surface temperature of between 160 and -20 C (320 and -4 F), according to the research published by the journal Nature.

The big range in estimates stems mainly from uncertainty about the reflectivity of clouds in the planet's upper atmosphere.

More information about CoRoT-9b is likely to flow, for it is one of only 70 exoplanets that have been captured because they happen to transit directly between the star and the telescope.

This alignment means that the star's light passes through the planet's atmosphere, yielding key data about the planet's size and chemical composition.

In the case of CoRoT-9b, the transit takes about eight hours, which gives an extraordinary opportunity for astronomers.

"It's the first extrasolar planet where we are quite sure it is fairly similar to one in our own Solar System, and it's the first extrasolar planet where we can test models that we have developed for Solar System planets," lead researcher Hans Deeg of the Institute of Astrophysics, on Spain's Atlantic archipelago of the Canary Islands, told AFP.

In the early stages of exoplanet hunting, the planets that showed up were very hot or orbited their stars in wildly eccentric orbits.

But as their skills and tools improve, astronomers are more and more able to spot planets with characteristics that look similar to those in our own backyard, said Deeg.

One discovery is that there is "quite a large variation" in the types of planets that orbit close to their star, said Deeg.

"For instance, Venus was probably apt for life in its early phases before a greenhouse effect set in and elevated temperatures by several hundred degrees," he said.