What happens to planets when they die?
For the first time ever, astronomers have directly witnessed the shredded remains of planetary bodies lighting up the sky as they are consumed by the corpse of their host star. The milestone detection offers an unprecedented glimpse into the guts of alien worlds while also providing a preview of what our own solar system might look like after the Sun has died.
For years, scientists have seen signs that asteroids, moons, and planets are getting ripped up and devoured by white dwarfs, which are the burned out remains of stars similar in mass to the Sun. For instance, many white dwarfs are surrounded by disks of debris or sport atmospheres that are polluted by metal elements heavier than hydrogen. These findings strongly suggest that the worlds that once orbited these stars when they were alive are being broken apart and eaten by their stellar corpses.
However, capturing the exact instant when a white dwarf feeds on the accreted remains of its planets, which is marked by a blast of energetic X-ray light, requires long exposure times and the right resolution to rule out other X-ray sources in the field of view.
Now, astronomers led by Tim Cunningham, a postdoctoral research fellow at the University of Warwick, have at last snagged this elusive observation, leading to “the only direct measurement of the instantaneous accretion rate of any white dwarf accreting planetary debris,” according to a study published Wednesday in Nature. In other words: The team clocked the rate at which a white dwarf gulped down some planet dust.
“This is the first time that we've actually detected the moment that the material hits the surface” of a white dwarf, said Cunningham in a call. “It's the smoking gun of evidence that says: ‘Okay, we've got the disk. We've got the metals in the atmosphere. And now we've seen the moment that the material moves from the disk to the atmosphere of the star.’”
The team was able to capture this moment by training NASA’s sophisticated Chandra X-ray Observatory on a white dwarf named G29–38, which is about 44 light-years from Earth, for 32 hours in September 2020. During that long exposure, Chandra spotted the telltale X-ray emission produced when the dead star swallowed the remains of some ancient world that had been torn apart by the tidal forces of the hyper-dense white dwarf. This high-energy light is produced when electrons in the atoms of this lost world become excited during the impact with its dead host star.
“We know that there's iron, magnesium, calcium, and oxygen in the [white dwarf] atmosphere,” Cunningham explained. “What we're seeing is a magnesium atom that has been locked inside a planet or an asteroid for billions of years and then has been tidally disrupted and hit the surface and had its electron kick out a little X-ray which has been sent towards us.”
Scientists have already observed hundreds of polluted white dwarfs, which has led to comprehensive models of accretion rates based on indirect measurements. With this new study, Cunningham and his colleagues have provided the first direct measurement of this rate, and found that it neatly validates the consensus built by past observations.
“One of the extremely exciting things is that the accretion rate that we derived from the X-rays—which is a completely different method because the production of the emission is different physics—agrees extremely well with what's been done previously,” Cunningham said. “It provides an independent test of those white dwarf models.”
With this finding, the team has bolstered our understanding of the often messy afterlives of star systems like our own. The results open a window into the future of our solar system, in which the corpse of the Sun potentially tears up and eats its planets, including Earth. Moreover, the new study yields new insights about the contents of planetary bodies in other star systems, which can shed light on some of the most intractable questions in science, such as whether life exists elsewhere in the universe.
“When you have a white dwarf that is accrediting the ground-down constituents of a planetary system, it's like putting all of your planetary bodies into a blender,” Cunningham said. “You can use the white dwarf models, then, to really infer what the composition of that planetary system was.”
“These different elements that we have in our own solar system, which are crucial for life—white dwarfs definitely offer a tool to probe that,” he concluded. “Being able to do that accurately is super-important and this observation is a very good step towards even more accurate models.”