This illustration presents a purely speculative view of what a hot Jupiter might look like.
Last Update: Thursday, October 5, 2006. 9:35am (AEST)

Hot new planets in speedy orbit

Astronomers have discovered a new class of planets outside the solar system that hug their parent stars so tightly they take less than a day to complete an orbit.

Using NASA's orbiting Hubble telescope, astronomers have found between eight and 16 new planets near the centre of the Milky Way that orbit their parent stars in as few as 10 hours.

At 26,000 light-years away, they are the most distant planets yet found and a further indicator others are probably scattered throughout the Milky Way.

"This allows us to say with a high degree of confidence that there are billions of planets in our galaxy," Mario Livio, an astronomer at the Space Telescope Institute in Baltimore, said.

A light year is the distance that light travels in one year- about 10 trillion kilometres.

Scientists have long conjectured that planets exist outside our solar system but they have only been able to locate them since 1993.

About 200 planets have been discovered so far, many of them gas giants similar to Jupiter locked in a close orbit to their parent stars.

Those "hot Jupiters" can be eight times closer to their parent stars than Mercury is to the sun.

The newly-discovered planets fit within that category, except they move even more quickly around their parent stars, which are smaller than the sun.

Kailash Sahu, a Space Telescope Science Institute astronomer who led the team, says surface temperatures on those "ultra-short period planets" are about 1,650 degrees Celsius.

He says their parent stars are so nearby they fill up one-third of the sky from the horizon to the zenith.

His team found the 16 possible planets by looking for stars that dimmed when planets passed in front of them.

They have been able to confirm two of those as planets by examining the slight wobble of the parent star caused by the orbiting body's gravity, but the other objects are too distant.

Dr Sahu says using other tests, the team is able to say with certainty that "at least a large fraction of our candidates must be planets".

The astronomers' findings will be published today in the scientific journal Nature.

- Reuters