Hungry Red Giant

An aging star polluted with lithium. A massive planet with a highly elliptical orbit. To an international team of astronomers, these add up to the first clear evidence of a planet’s destruction by its star—and maybe a glimpse of Earth’s far-off fate.

The team, including Alexander Wolszczan, Evan Pugh Professor of Astronomy and Astrophysics at Penn State, was using the Hobby-Eberly Telescope in Fort Davis, Texas, to study the star named BD+48 740, which is older than the Sun with a radius about 11 times bigger. They were also looking for planets that might orbit the star.

In the course of their study, they found something unexpected, says team member Monika Adamow. “Our detailed spectroscopic analysis reveals that BD+48 740 contains an abnormally high amount of lithium, a rare element created primarily during the Big Bang 14 billion years ago.” Lithium is easily destroyed in stars, which is why its abundance in this older star is so unusual.

“Theorists have identified only a few, very specific circumstances other than the Big Bang under which lithium can be created in stars,” Wolszczan adds. “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 spiraled into the star and heated it up while the star was digesting it.”

The astronomers also discovered a massive planet revolving around the same star in a surprisingly elliptical orbit. They suspect that the dive of the missing planet toward the star as the star began expanding into a red giant could have given the surviving planet a burst of energy, throwing it into an eccentric orbit like a boomerang.

“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,” acknowledges Eva Villaver, another member of the team, “but the occurrence of such a collision can be deduced from the way it affects the stellar chemistry.”

“The highly elongated orbit of the planet we discovered,” she adds, “is exactly the kind of evidence that would point to the star’s recent destruction of its now-missing planet.”

“A similar fate may await the inner planets in our solar system,“ suggests Wolszczan, ”when the Sun becomes a red giant and expands all the way out to Earth’s orbit some 5 billion years from now.”

Alexander Wolszczan, Ph.D., is Evan Pugh Professor of Astronomy and Astrophysics. The other members of the research team were Monika Adamow, Grzegorz Nowak and Andrzej Niedzielski of Nicolaus Copernicus University in Torun, Poland; and Eva Villaver of the Universidad Autonoma de Madrid in Spain. The paper describing this discovery was posted in an online edition of the Astrophysical Journal Letters.

This research received financial support from the U. S. National Aeronautics and Space Administration, the Polish Ministry of Science and Higher Education, the Spanish Ministry of Science and Innovation, and the Marie Curie Seventh Framework Programme. The Hobby-Eberly Telescope is a joint project of the University of Texas at Austin, Penn State, Ludwig-Maximilians-Universitat Munchen, and Georg-August-Universitat Gottingen.

Last Updated October 17, 2012