This artist's conception shows the newly discovered planet, which orbits a red dwarf star 40 light years from Earth. (David A. Aguilar/Harvard-Smithsonian Center for Astrophysics) Astronomers using small, off-the-shelf telescopes in Arizona have found a planet orbiting a red dwarf star 40 light years away.

The planet is about 6.5 times heavier than Earth and nearly three times its diameter, making it a so-called super-Earth.

The newly discovered planet orbits a red dwarf star, called GJ1214, once every 38 hours at a distance of two million kilometres.

By comparison, the closest planet to our sun, Mercury, orbits at an average distance of 58 million kilometres and completes a circuit of the sun once every 88 days.

Although the planet is so close to its star, its surface temperature is no hotter than a preheated oven because its star is so small and relatively cool.

"Being so close to its host star, the planet must have a surface temperature of about 200 C," said David Charbonneau, lead author of the paper reporting the discovery.

'This appears to be a waterworld'

That's too hot for liquid water but, paradoxically, the planet is probably mostly composed of water ice and other ices, astronomers say.

Researchers believe that 75 per cent of the planet's core could be composed of ice, some of it in the form of Ice VII, an exotic form of water ice that exists only under high pressure. The rest of the planet is probably silicon and iron, they said.

"Despite its hot temperature, this appears to be a waterworld," said Zachory Berta, a graduate student at the Harvard-Smithsonian Center for Astrophysics (CfA). "It is much smaller, cooler, and more Earth-like than any other known exoplanet."

Astronomers found the planet by watching a list of 2,000 nearby red dwarf stars for changes in their brightness resulting from planets partially eclipsing our view of the star.

The smallest planet detected outside our solar system, called Corot-7b, was also found using this method. The two planets are the only super-Earths for which scientists know both the mass and diameter.

But while Corot-7b is an extremely hot, rocky planet, this newly found planet could have an atmosphere, the astronomers said.

Bigger than expected

When the researchers measured the radius of the planet by watching it pass in front of its star and compared it to theoretical models of planet formation, they found that it is bigger than expected.

The researchers said this could be the result of an atmosphere 200 kilometres thick partially blocking light from the star.

These eight 16-inch telescopes at the F. L. Whipple Observatory on Mt. Hopkins in Arizona were used to find the exoplanet. (Dan Brocious/Harvard-Smithsonian Center for Astrophysics) "This atmosphere is much thicker than that of the Earth, so the high pressure and absence of light would rule out life as we know it, but these conditions are still very interesting, as they could allow for some complex chemistry to take place," said Charbonneau.

The planet was found by astronomers at an Arizona-based observatory called MEarth, which uses an array of eight telescopes, each one a 41-centimetre-diameter model that costs about as much as a new luxury car.

All the telescopes are attached to a single charge-coupled device, or CCD, similar to those found in digital cameras.

"Since we found the super-Earth using a small ground-based telescope, this means that anyone else with a similar telescope and a good CCD camera can detect it, too. Students around the world can now study this super-Earth," said Charbonneau, head of the MEarth project.