A dead star was caught tearing apart its planetary system

When a star dies, what happens to its planets? Well, if that star is some white dwarf 86 light years away, those planets are currently being torn apart and devoured by the star, like some grotesque cosmic performance of Kronos devouring his children.

This is not entirely unusual for white dwarfs. But this particular star, named G238-44, is a glutton: for the first time, astronomers have seen one of these stars swallow material both inside and outside its planetary system at the same time. time, in the most far-reaching manifestation of stellar filial cannibalism observed to date.

In the atmosphere of G238-44, astronomers have detected traces of elements suggesting that the dead star has recently accumulated metallic and rocky materials, such as the solar system’s inner asteroids, as well as icy materials, such as bodies. frosts that can be found in the Kuiper belt of the outer solar system.

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

White dwarfs are what happens when a regular 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 swells to the size of a red giant before ejecting its outer material, and the stellar core collapses under gravity to form a dense, glowing object. with residual heat light. It’s the white dwarf.

Although this process seems to be quite difficult for the planets orbiting the star – the Sun could swell big enough to gobble up Mars when it reaches the red giant hour in a few billion years – but recently the Astronomers have found evidence to suggest that parts of planetary systems may in fact survive there.

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

Because white dwarfs are so dense (think something the mass of the Sun, packed into an Earth-sized sphere), the heavy elements should disappear fairly quickly, which means that any heavy element pollution in a white dwarf atmosphere must have deposited recently.

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

Since the other planetary systems detected so far look very different from the solar system in many ways, probing the innards of exoplanets nibbled away by white dwarfs can help scientists determine whether exoplanet interiors are different as well. Which brings us back to G238-44.

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

The pollution in the atmosphere of this white dwarf is unlike any other observed to date, Johnson and his colleagues found. Ten elements heavier than helium were detected: carbon, nitrogen, oxygen, magnesium, aluminum, silicon, phosphorus, sulphur, calcium and iron.

Iron and nitrogen abundances were particularly high; the first, according to the team, suggests a body with a differentiated iron core, while the second suggests the presence of icy bodies.

“The best fit for our data was a nearly two-to-one mixture of mercury-like materials and comet-like materials, which are made up of ice and dust,” Johnson said. “Metallic iron and nitrogen ice each suggest wildly different conditions for planetary formation. There is no known solar system object with so many of the two.”

The results also suggest that the ingredients needed to create a habitable world might not be so scarce in the Milky Way galaxy. Earth is a rocky world and is believed to have been seeded with elements vital to life, such as water, by asteroid bombardment. Detection of nitrogen-rich materials could mean that frozen reservoirs of these elements could be common.

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

“The abundance of elements we see on this white dwarf appear to come from both a rocky parent body and a volatile-rich parent body – the first example we have found among studies of hundreds of white dwarfs. “

In fact, aliens peering at the Sun from afar, once it has transformed into a white dwarf in about 5 billion years, might expect to see something similar. Although inner solar system objects may be vaporized by expanding white dwarfs, the asteroid belt between Mars and Jupiter could survive being disrupted by a destabilized Jupiter and raining down on the dead star.

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

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