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EDITIONS
 Monday, 27 January, 2003, 11:20 GMT
Doomed planet orbits giant star
Star, Eso
HD 47536: Far more massive than our Sun

Astronomers have discovered a planet which they think will soon be consumed by the giant star it orbits.

The world that circles HD 47536 - the biggest star found to date which has a planetary companion - gives us a glimpse of the death that may also await the Earth.

In a few tens of millions of years from now - a short period on the cosmic timescale - HD 47536 will incinerate and then engulf its nearby planet.

Many astronomers expect the Sun to do the same thing with Earth - but in a few billion years' time.

Bigger still

The planet orbiting HD 47536 was found by a European-Brazilian team of astronomers using an advanced spectrograph - a device that splits incoming light into its component colours - installed on the 1.52-metre telescope at the La Silla Observatory in Chile.

Planet, BBC
The star will expand to consume the planet
Changes in the spectrum of the star revealed the presence of the planetary companion.

Until its discovery, planets had only been detected around three other stars larger than our Sun: HD 27442, iota Draconis and gamma Cephei.

With a diameter of 33 million kilometres (23 times the diameter of our Sun), HD 47536 is bigger even than these three.

Expanding girth

"We are very excited about this discovery," said Luca Pasquini of the European Southern Observatory organisation, which operates the La Silla facility, "because it now widens the search for exoplanets towards more massive stars."

At a distance of 396 light-years, the new exoplanet is the second most remote one found to date.

Its orbital period is 712 days and the planet's mass is 5-10 times that of Jupiter.

The distance of the planet from HD 47536 is about 300 million km - a safe margin now, but astronomers say this will not always be the case.

The star is slowly expanding and is expected to eventually consume its companion.

Lithium puzzle

First, the star's radiation will cause the planet's temperature to rise. Then, in a few tens of millions of years from now, the unlucky planet will lose its gaseous layers entirely as its surface becomes burning hot.

The discovery has other implications. Astronomers have been studying certain giant stars that contain the element lithium. The metal's presence is a puzzle since lithium is rapidly consumed in stars and should not really be there.

Licio da Silva, from the Observatório Nacional in Rio de Janeiro, said: "One obvious possibility is that those stars have obtained their lithium by recently swallowing a planet.

"Until a short time ago, this hypothesis was considered rather exotic, because of the lack of evidence of planets in danger. But the discovery of a giant planet near a giant star makes the explanation plausible."

See also:

07 Jan 03 | Science/Nature
05 Dec 02 | Science/Nature
24 Oct 02 | Science/Nature
09 Jan 02 | Science/Nature
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