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Thursday, 10 January, 2002, 14:31 GMT
Planet found circling dying star
Planet, JPL/Nasa
Artist's impression of the planet circling the star
By BBC News Online science editor Dr David Whitehouse

Astronomers have made the first discovery of a planet orbiting a giant star.


Observing the fate of this companion to a dying star is a reminder of the ultimate fate of our own Earth

Debra Fischer, University of San Francisco
Unlike our Sun, this giant, called iota Draconis, is an old star that has already burned its hydrogen fuel at its core.

Such stars grow much bigger towards the end of their lives and this one has reached a radius 13 times that of the Sun.

What is interesting, however, is that iota Draconis has not devoured the planet during its expansion - a fate that may befall the Earth when our star dies in a few billion years' time.

"Until now, it was not known if planets existed around giant stars," says Sabine Frink of the University of California, US.

"This provides the first evidence that planets at Earth-like distances can survive the evolution of their host star into a giant."

Earth's future

Iota Draconis, also known as Edasich, is located at a distance of 100 light-years from Earth in the constellation of Draco. It is currently visible with the unaided eye in the morning sky, just east of the Plough.

Like all of the extrasolar planets that have been discovered orbiting Sun-like stars, the one around iota Draconis was detected using the Doppler technique - where the gravitational pull of the planet causes a wobble in the measured velocity of the parent star.

The planet completes one orbit every 1.5 years and the shape of its orbit is elliptical rather than circular. Its mass is 8.7 times the mass of Jupiter.

Because the Doppler technique determines the minimum mass, the astronomers say, it is possible that the true mass of this companion is a brown dwarf - a "failed star" that lacks enough mass to start nuclear fusion.

However, even if this companion is a brown dwarf, the researchers say its detection around an evolved star represents a first.

Scorched Earth

Astronomers say it is more difficult to detect the signature of a planet orbiting a giant star because they often pulsate, producing wobbling effects that could give the illusion they had planetary companions.

It is believed that our Sun will eventually undergo a similar fate to iota Draconis. Several billion years from now, when the Sun evolves into a giant star, the Earth's temperature will rise to several hundred degrees Celsius.

"The oceans will evaporate, and the water vapour will escape the Earth's atmosphere because of the high temperature," says Andreas Quirrenbach of the University of California.

"Observing the fate of this companion to a dying star is a reminder of the ultimate fate of our own Earth," says Debra Fischer of the University of San Francisco.

See also:

09 Jan 02 | Sci/Tech
Earth granted reprieve
27 Nov 01 | Sci/Tech
Planet circling another star probed
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