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Last Updated: Wednesday, 12 May, 2004, 12:29 GMT 13:29 UK
Hubble sees 'planet' around star
By Dr David Whitehouse
BBC News Online science editor

Star with planet candidate circled, Nasa/Pennsylvania State University
The picture was taken with Hubble's infrared Nicmos camera
The historic first image of a planet circling another star may have been taken by the Hubble Space Telescope.

The "planet", 5-10 times the mass of Jupiter, is orbiting a small white dwarf star about 100 light-years away.

Astronomers are being cautious, saying they require more data to be sure it really is a planet and not a background object caught in the same field of view.

Confirmation will come if follow-up observations can show the planet and the star moving together through space.

Over the past 10 years, scientists have discovered more than 120 so-called exoplanets. However, all have been found by indirect methods - none was photographed directly.

Glowing ball

The new Hubble image was taken by John Debes, a graduate student at Pennsylvania State University, US, as part of a project to look for planets around other stars.

"The big problem in seeing such planets is not one of sensitivity but of contrast," he told BBC News Online. "The presence of a bright parent star makes the relatively dim planet difficult to see."

That is why Debes and his colleagues looked for planets around dim white dwarf stars - stars at the end of their lifetimes that have shrunk to a glowing ball the size of the Earth.

"We surveyed seven white dwarfs and around three of them we saw what might be a planet.

"In one of them, the planet could be only a few times the mass of Jupiter," he said.

The unnamed world is about the same distance from its star as Neptune is from our Sun.

Awaiting confirmation

The planet would have begun its life several billion years ago much closer to its parent star.

This star would have been more massive and brighter than our Sun. At the end of its life, it would have expanded and destroyed any nearby planets.

Only the more distant planets would have survived and, as the star lost mass, their orbits would have expanded due to the star's reduced gravity.

Debes remains cautious about the object until it has been shown to be definitely associated with the white dwarf.

"If it is a planet and not a background object that happens to be in the same direction, we should see it move across the sky with the star in a few months," he said.

Debes plans follow-up observations with the Hubble Space Telescope or possibly with ground-based telescopes.

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