Astronomers have finally observed the debris left over by disintegrating planets entering the atmosphere of a white dwarf, the exposed degenerate core of an ancient star. For the first time, researchers have seen fragments of these bygone worlds being heated up to millions of degrees as they are consumed within the surface of the star.
What the astronomers saw were emissions of x-rays. These were created as the rocky and gaseous material surrounding white dwarf G29—38 got heated to incredible temperatures, once past the point of no return. For decades, astronomers have found indirect evidence of this process and now, as reported in Nature, there are also direct observations.
“We have finally seen material actually entering the star’s atmosphere. It is the first time we’ve been able to derive an accretion rate that doesn’t depend on detailed models of the white dwarf atmosphere. What’s quite remarkable is that it agrees extremely well with what’s been done before,” lead author Dr Tim Cunningham of the University of Warwick, said in a statement.
In previous studies, observations suggested the presence of heavy elements around and on the surface of these white dwarfs. Between a quarter and a half of all known white dwarfs were “polluted” by iron, calcium, magnesium. This suggested they had been snacking on planetary material from the worlds that once orbited them.
“Previously, measurements of accretion rates have used spectroscopy and have been dependent on white dwarf models. These are numerical models that calculate how quickly an element sinks out of the atmosphere into the star, and that tells you how much is falling into the atmosphere as an accretion rate,” Dr Cunnigham continued. “You can then work backwards and work out how much of an element was in the parent body, whether a planet, moon or asteroid.”
A white dwarf is a star like our Sun at the end of its life. It is not massive enough to end up in a supernova, so its core collapses into itself once it can no longer fuse. Its outer shells had previously expanded during the red giant phase and they are blown away and nearby planets reduced to chunks can spiral in and be cannibalized.
As the debris gets close enough it is turned into plasma, with temperatures between 100,000 and 1,000,000 °C (180,000 and 1,800,000 °F). Once it reaches the dead star surface, it begins to cool, with the emission of x-rays that scientists detected in this case.
“This detection provides the first direct evidence that white dwarfs are currently accreting the remnants of old planetary systems. Probing accretion in this way provides a new technique by which we can study these systems, offering a glimpse into the likely fate of the thousands of known exoplanetary systems, including our own Solar system.”
But worry not, the Sun is not expected to become a white dwarf for many billions of years.