Star is caught devouring planet

Artist's impression of a planet's destruction The team thinks the planet was destroyed as its ageing star expanded in size, as in this artist's impression

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Astronomers have found evidence for a planet being devoured by its star, yielding insights into the fate that will befall Earth in billions of years.

The team uncovered the signature of a planet that had been "eaten" by looking at the chemistry of the host star.

They also think a surviving planet around this star may have been kicked into its unusual orbit by the destruction of a neighbouring world.

Details of the work have been published in Astrophysical Journal Letters.

The US-Polish-Spanish team made the discovery when they were studying the star BD+48 740 - which is one of a stellar class known as red giants. Their observations were made with the Hobby Eberly telescope, based at the McDonald Observatory in Texas.

Rising temperatures near the cores of red giants cause these elderly stars to expand in size, a process which will cause any nearby planets to be destroyed.

"A similar fate may await the inner planets in our solar system, when the Sun becomes a red giant and expands all the way out to Earth's orbit some five billion years from now," said co-author Prof Alexander Wolszczan from Pennsylvania State University in the US.

Lithium boost

The first piece of evidence for the missing planet comes from the star's peculiar chemical composition.

Spectroscopic analysis of BD+48 740 revealed that it contained an abnormally high amount of lithium, a rare element created primarily during the Big Bang 14 billion years ago.

Hobby-Eberly telescope The discovery was made using the Hobby-Eberly telescope in Texas

Lithium is easily destroyed in stars, so its high abundance in this ageing star is very unusual.

"Theorists have identified only a few, very specific circumstances, other than the Big Bang, under which lithium can be created in stars," Prof Wolszczan explained.

"In the case of BD+48 740, it is probable that the lithium production was triggered by a mass the size of a planet that spiralled into the star and heated it up while the star was digesting it."

The second piece of evidence discovered by the astronomers is the highly elliptical orbit of a newly discovered planet around the red giant star. The previously undetected world is at least 1.6 times as massive as Jupiter.

Co-author Andrzej Niedzielski of Nicolaus Copernicus University in Torun, Poland, said that orbits as eccentric as this one are uncommon in planetary systems around evolved stars.

"In fact, the BD+48 740 planet's orbit is the most elliptical one detected so far," he added.

Because gravitational interactions between planets are often responsible for such peculiar orbits, the astronomers suspect that the dive of the missing planet toward its host star before it became a giant could have given the surviving massive planet a burst of energy.

This boost would have propelled it into its present unusual orbit.

Team member Eva Villaver of the Universidad Autonoma de Madrid in Spain commented: "Catching a planet in the act of being devoured by a star is an almost improbable feat to accomplish because of the comparative swiftness of the process, but the occurrence of such a collision can be deduced from the way it affects the stellar chemistry.

"The highly elongated orbit of the massive planet we discovered around this lithium-polluted red giant star is exactly the kind of evidence that would point to the star's recent destruction of its now-missing planet."

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