Astronomers have spotted a star with a dark metallic “scar” on its surface, believed to be the imprint of a doomed planetary fragment that got too close to its host.
The white dwarf star, called WD 0816-310, is a dense, Earth-sized remnant of a star about 63 light-years away that would have been similar to our Sun in its lifetime. Observations have revealed a concentrated patch of metals on its surface, which appears to be the remains of a captured piece of planet or an asteroid.
“It is well known that some white dwarfs – slowly cooling embers of stars like our Sun – are cannibalizing chunks of their planetary systems,” said Stefano Bagnulo, an astronomer at the Armagh Observatory and Planetarium in Northern Ireland, UK , and lead author of the study.
Previously, it was assumed that these fragments would sink into the star and be spread evenly across the surface. However, the latest findings show that planetary material appears to be trapped by the star’s magnetic field, resulting in a shadowy surface structure. The metallic patch covers a larger part of the pole than the equivalent of Antarctica on Earth.
A white dwarf is the stellar core left behind after a dying star has exhausted its nuclear fuel and expelled its outer layers to form a glowing cloud known as a planetary nebula. At about the size of Earth, they are so dense that their gravity can pull apart planets or asteroids that come into their vicinity.
Jay Farihi, a professor of astrophysics at University College London and co-author of the study, said: “When a planet or asteroid gets close, it gets tidally shredded. It’s the final death spiral [of one of these fragments]. We have never seen this before. We now know the last moments before it is eaten.”
Calculations suggest that the metals detected on the star’s surface came from a planetary fragment as large as or possibly larger than Vesta, which is about 500 km across and the second largest asteroid in the solar system.
The team noticed that the strength of the metal detection changed as the star rotated, suggesting that the metals were concentrated in a specific area on the white dwarf’s surface, rather than spread evenly across it.
They also found that these changes are synchronized with changes in the white dwarf’s magnetic field, indicating that this metal scar is located at one of its magnetic poles. Taken together, these clues indicate that the magnetic field pulled metals toward the star and created the scar.
Prof John Landstreet, from the University of Western Ontario, Canada, and the Armagh Observatory and Planetarium, said: “This scar is a concentrated patch of planetary material, held together by the same magnetic field that guided the infalling fragments. Nothing like this has been seen before.”
Astronomers observe so-called polluted white dwarfs because measuring the metals and elements present can provide unprecedented insights into the bulk composition of exoplanets.
The findings are published in the Astrophysical Journal Letters.
