White dwarfs are the stellar remnants of stars, left behind after the outer layers are blown off. Heavier elements such as magnesium and iron should sink to their core, with only hydrogen and helium visible in their atmospheres from Earth. Why, then, have nearly half of all white dwarfs been seen with heavier elements in their atmospheres?
That mystery may now have been solved, and there is a somewhat dramatic tale to be told. Using data from the Kepler K2 mission, and follow-up observations from other telescopes, a team of scientists studied a white dwarf 570 light-years from Earth labeled WD 1145+017, and they believe a minor planet may be disintegrating around it, depositing heavier elements on the star. The study is published in the journal Nature.
“This is the first discovery of transiting debris [around a white dwarf],” lead author Andrew Vanderburg from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts told IFLScience. “It was consistent with something that’s disintegrating. So we’ve found the first direct evidence that the pollution is in fact caused by rocky objects.” This seems to confirm a previous theory on the process.
What is thought to have happened is this: A minor planet no bigger than Ceres in our own Solar System was pushed into a tight orbit around the star when it became a white dwarf, possibly by a larger Jupiter-like planet kicking it inwards. Orbiting the star in just 4.5 to 4.9 hours, the minor planet was then ripped apart by the white dwarf’s gravity into as many as six fragments, orbiting at a distance of one solar radius, although it’s unknown what its previous orbit was.
Debris from the planet obscures light from the star, as in this artist's impression. Diego Barucco/Shutterstock.
WD 1145+017 is about the size of Earth, but contains the mass of the Sun. The researchers cannot see direct evidence for the minor planets themselves, but they can see the resultant field of debris, which blocks 40% of the white dwarf’s light, to infer their existence. The star itself is believed to be 175 million years old.
Interestingly, this may be somewhat similar to what will befall our Solar System. In five billion years or so, the Sun will run out of nuclear fuel and expand into a red giant, consuming the inner planets, and leaving behind a white dwarf. “After this change in mass, all bets are off,” said Vandenburg. It could be that objects in our own system are torn apart by the Sun in a similar manner to WD 1145+017.
Eventually, in the latter system, the planet will be totally disintegrated and vaporized by the heat of the star, probably in a million years or so. “When that happens, all of the dust and gas from the planet will end up on the white dwarf,” said Vandenburg. “But on a longer time scale, it will all sink and there will be no trace.”
With as many as half of all known white dwarfs showing similar evidence for heavy element pollution, it could turn out that this process is somewhat common in the universe.