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New monster planet 'could float on water'

Barbara Miller and staff reporters

Astronomers have found what they say is the largest planet ever discovered in the universe, so big that it is baffling scientists and so light that it could theoretically float on water.

The new planet, TrES-4, is located 1,435 light years away from Earth in the constellation of Hercules.

The planet is a gas giant, like Jupiter, but it is much hotter and lighter, and about 70 per cent larger.

Being only 7 million kilometres from its parent star, its temperature is about 1,327 degrees Celsius.

TrES-4 was discovered using three telescopes, two in Arizona, in the US, and one in the Canary Islands.

Georgi Mandushev from the Lowell Observatory in Arizona is the lead author of the paper announcing the discovery in the Astrophysics Journal. He says TrES-4 appears to be almost puffed up.

"It's a very enigmatic planet, I would say, because it's so big and has such a low density," he said. "At this point, theoretical models of planets cannot explain this one."

Dr Mandushev says the planet is unexpectedly light.

"This planet is almost twice the size of Jupiter but only about three-quarters of its mass, so that really makes it very unusual," he said.

He says its density of 0.2 grams per cubic centimetre is somewhere between a wine cork and balsa wood.


Dr Mandushev says it is possible that still larger planets are yet to be discovered.

"At this point, this is the largest planet that we can actually measure the size of," he said. "There are many planets that we just don't know how big they are because they're not transiting planets. That means they do not cross the disc of their home star and therefore, we cannot measure the size."

He says the next step is to try to explain why the planet is so big and yet so light.

"[We will] plan a few more follow-up observations to try and measure the temperature of the planet, to try and study the composition of its atmosphere, to try to see if it really has this huge hydrogen envelope that we speculate it does," he said.

"We have a lot of interesting studies in front of us."

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