Astronomers using the Hubble Space Telescope believe that the discovery could constitute a new class of planets, because they hug their parent stars so closely that they complete their orbits in as little as ten hours. Stars are differentiated from planets by the nuclear reactions that take place in their core.

These “ultra-short period planets” orbit their stars more quickly than any previously known planets. The previous shortest known orbit was between 1.2 and 2.5 days.

More than 200 planets have been discovered outside our own solar system, but the new bodies identified by Hubble are 26,000 light years away, at least ten times as far from Earth as those previously discovered. This has led the scientists to conclude that there are billions of planets in our galaxy, the Milky Way.

“We all are dreamers, and part of that dream is to find life somewhere,” said Mario Liveo, head of the space programme at the Space Telescope Science Institute, Baltimore, which oversees Hubble operations. “We’re finding that the galaxy is full of planets, and the chances are, somewhere out there, we will find one with the conditions necessary to be habitable.”

All but two of the 16 planets are being considered “candidate planets”, because they are so far away it is difficult conclusively to identify them as planets. But astronomers from Sagittarius Window Eclipsing Extrasolar Planet Search (Sweeps) think most, if not all, of the candidates stand a good chance of becoming fully fledged planets eventually. The 16 planets are unnamed, because scientists have not yet even named the stars they orbit.

The hunt for extrasolar planets — those from outside our solar system — is far from straightforward. They are so distant they cannot be seen even with the strongest telescopes. Astronomers spot them because, during their orbit, they briefly block light from distant stars — an event known as a transit.

A planet must be about the size of Jupiter to block enough light to be detected by Hubble. Sweeps estimate that there are as many as six billion Jupiter-sized stars in the galaxy.

The discovery of the new “ultra-short period planets” suggests that stars smaller than the Sun can pull planets closer without burning them up, according to the findings, which were published in the science journal Nature yesterday.

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