Although relatively few white dwarfs have disks, Manser explained that between 25 and 50 percent of known white dwarfs show signs of having consumed planetary material in their past. And before that material sinks below the white dwarf's surface, it would circle the dead star in the form of a dusty disk. So, although this might explain why dozens of white dwarfs have dusty disks, it doesn't explain why some have gas too."We are still uncertain why these disks that host gas are so rare," Manser said, "but we hypothesize that it is the presence of a planetesimal in the disc which is producing the gas that we see." It could be that gases are evaporating off the planetesimal's exposed surface, he says, or it could be that the planetesimal is acting like a wrecking ball as it barrels through material in SDSS1228's debris disk, releasing gases as it collides with dust particles. In order to learn more about the origins and evolution of planetesimals around gas-disk white dwarfs, Manser and his team are already planning to hunt for more examples.



"There are six other known gas disks around white dwarfs," he says, "and there are a few that are bright enough to apply the method we developed here to search for additional planetesimals." So by bolstering our catalog of known planetesimals in aging planetary systems, we can begin to understand how our own planets will react when the Sun shuts down.



"Discovering more planetesimals would teach us about the extreme conditions that planetary bodies — or their fragments — can survive in," Manser says, "as well as help our overall understanding of remnant planetary systems around white dwarfs. These systems are great tools to learn about the composition of exoplanetary material."

Get psyched for Psyche

Although studying planetary remains around long-dead stars is informative, presently, there are not many locations where the methods presented here are effective. But fortunately, the solar system is likely harboring its own piece of a planetary core — 16 Psyche — which is ripe for exploration.This iron-nickel asteroid orbits between Mars and Jupiter (about where Manser and his team think their planetesimal started out) and was discovered by Italian astronomer Annibale de Gasparis in 1852, making it the 16th asteroid ever found. At some 125 miles (200 km) wide and composed of up to 95 percent metal, Psyche offers a unique natural laboratory where researchers can study an ancient planetary core that likely underwent a series of violent collisions in its past.