'Cannibal' Star Left With Metal Scar After Swallowing Its Planet

A strange star deep in space has been left with a metallic scar across its surface after eating one of its own planets.

The star—named WD 0816-310—is a white dwarf star situated around 63 light years away and was spotted with a bizarre scar stretching across its surface by the European Southern Observatory's Very Large Telescope (VLT), according to a new paper in The Astrophysical Journal Letters.

This scar is thought to have formed after the star cannibalized one of the planets orbiting it, the researchers claim, based on clues found in the star's magnetic field.

"It is well known that some white dwarfs—slowly cooling embers of stars like our Sun—are cannibalizing pieces of their planetary systems. Now we have discovered that the star's magnetic field plays a key role in this process, resulting in a scar on the white dwarf's surface," study lead author Stefano Bagnulo, an astronomer at Armagh Observatory and Planetarium in Northern Ireland, said in a statement.

White dwarfs are the remains of low to medium-mass stars that have exhausted their fuel and collapsed in on themselves. They are incredibly dense objects, typically about the size of Earth but with masses comparable to that of the Sun. They are the hot, dense cores of stars like the sun that are left behind after a dying star swells into a red giant and then expels its outer layers.

White dwarfs are one of the endpoints of stellar evolution for stars with initial masses less than about 8 times that of the Sun. If the star is more massive, it may undergo further collapse to become a neutron star or black hole after the white dwarf stage. White dwarfs will slowly fade into black dwarfs over billions of years of cooling.

This white dwarf, WD 0816-310, is thought to have consumed a small planet or planetary fragment around the same size as one of the largest asteroids in our solar system. This theory arose from observations that the metal levels of the star changed as it rotated, indicating that they were concentrated in a certain area of its surface.

"We have demonstrated that these metals originate from a planetary fragment as large as or possibly larger than Vesta, which is about 500 kilometres [310 miles] across and the second-largest asteroid in the Solar System," study co-author Jay Farihi, a professor at University College London, said in the statement.

The researchers also found that these changes in the level of metals came at the same time as changes in the magnetic field of the white dwarf, indicating that the magnetic field may have funneled the metals onto the star's surface in a similar manner to how charged particles are funneled towards the Earth's poles to form the aurora borealis.

"Surprisingly, the material was not evenly mixed over the surface of the star, as predicted by theory. Instead, this scar is a concentrated patch of planetary material, held in place by the same magnetic field that has guided the infalling fragments," co-author John Landstreet, a professor at Western University, Canada, who is also affiliated with the Armagh Observatory and Planetarium, said in the statement. "Nothing like this has been seen before."

The researchers were able to detect the metal scar and the magnetic field signals using a special instrument on the VLT called FORS2.

"ESO has the unique combination of capabilities needed to observe faint objects such as white dwarfs, and sensitively measure stellar magnetic fields," Bagnulo said.

The researchers hope to use these observations to find out more about the compositions of exoplanets, and how active stars and solar systems can remain even long after the star itself has died.

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