A dead star has been caught tearing its planetary system apart

What happens to its planets when a star dies? Well, if that star is a particular white dwarf 86 light-years away, those planets are currently being torn apart and eaten by the star, like a grotesque cosmic representation of Kronos devours his children

This is not entirely unusual for white dwarfs. But this particular star, called G238-44, is a glutton: For the first time, astronomers have seen one of these stars simultaneously gobble up material from both the inner and outer regions of its planetary system, in the most far-reaching view of stellar childlike cannibalism observed to date.

In the atmosphere of G238-44, astronomers have discovered traces of elements indicating that the dead star has recently accumulated material that is metallic and rocky, such as asteroids in the inner solar system, as well as material that is icy, such as the frozen bodies found. are in the Kuiper belt of the outer solar system.

“We’ve never seen both types of objects growing on a white dwarf at the same time,” said physicist and astronomer Ted Johnson from the University of California at Los Angeles. “By studying these white dwarfs, we hope to gain a better understanding of planetary systems that are still intact.”

White dwarfs are what happens when an ordinary star up to eight times the mass of the Sun reaches the end of its life. Once such a star runs out of material to fuse, it inflates to the size of a red giant before ejecting its outer material, and the stellar core collapses under gravity, forming a dense object that shines brightly with the light. of the residual heat. That’s the white dwarf.

While this process appears to be quite rough on the planets orbiting the star, the sun can inflate big enough to engulf Mars when it reaches the red giant hour in a few billion years — but lately, astronomers have found evidence suggesting that some parts of planetary systems may actually survive.

Exoplanets have been spotted orbiting white dwarfs† And then there is necroplanetology – the study of the remains of white dwarf exoplanets based on traces of the heavy elements they contain “polluting” white dwarf atmospheres.

Because white dwarfs are so close (think something like the mass of the sun packed into an Earth-sized sphere), heavy elements should sink out of sight pretty quickly, meaning any heavy element pollution in a white dwarf atmosphere must have been deposited recently.

This is exciting because it means we have an indirect probe into exoplanetary interiors. We know what the Earth is made of, and we’re pretty sure we understand the makeup of other planets in the solar system to some degree, but exoplanets orbiting distant stars are impossible to investigate as we can the Earth. , or even other planets in the solar system.

Because other planetary systems detected so far are very similar to the Solar System in many ways, examining the guts of exoplanets eaten by white dwarfs could help scientists determine whether exoplanetary interiors are also different. Which brings us back to G238-44.

g 238 44 diagramDiagram illustrating what scientists think is happening around G238-44. (NASA, ESA, Joseph Olmsted/STScI)

The pollution in this white dwarf’s atmosphere is unparalleled, Johnson and his colleagues found. Ten elements heavier than helium were detected: carbon, nitrogen, oxygen, magnesium, aluminum, silicon, phosphorus, sulfur, calcium and iron.

Iron and nitrogen concentrations were particularly high; the former, the team said, suggests a body with a differentiated iron core, while the latter suggests the presence of icy bodies.

“The best fit for our data was an almost two-to-one mix of Mercury-like material and comet-like material, which consists of ice and dust,” johnson said:† “Iron metal and nitrogen ice each suggest completely different conditions of planetary formation. No object in the solar system is known to have so much of both.”

The results also suggest that the ingredients for making a habitable world may not be so rare in the Milky Way galaxy. The Earth is a rocky world and is thought to be dotted with the elements essential to life, such as water, by asteroid bombardment. The detection of nitrogen-rich material could mean that frozen reservoirs of these elements are common.

“Life as we know it requires a rocky planet covered with a variety of volatile elements such as carbon, nitrogen and oxygen,” said physicist and astronomer Benjamin Zuckerman from UCLA.

“The abundance of elements we see on this white dwarf appears to come from both a rocky parent body and a volatile-rich parent body — the first example we’ve found in studies of hundreds of white dwarfs.”

In fact, aliens peering at the sun from a distance, once it evolved into a white dwarf in about 5 billion years, could expect something similar. Although the objects in the inner solar system are being vaporized by the expanding white dwarfs, the asteroid belt between Mars and Jupiter could survive, be disrupted by a destabilized Jupiter and rain on the dead star.

The team’s research was presented at the 240th Meeting of the American Astronomical Society

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