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White Dwarfs With “Polluted” Atmospheres Reveal Stars And Planets Grow Up Together

Whether planets form when a star is still growing or has reached adulthood may now have been answered.


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockNov 14 2022, 17:23 UTC
artist impression showing a white dwarf and a molten planetesimal losing bits
Rocky fragments seen in the atmosphere of white dwarf tell a story of quick formation of planets. Image Credit:Amanda Smith

Astronomers have found evidence that planetary formation could begin extremely quickly after the formation of a star. This incredible insight into the histories of planetary systems doesn’t come from newborn stars but actually from the opposite end of the spectrum: white dwarfs, the dying remnants of the cores of stars like our Sun.

After Sun-like stars have turned into red giants, they are not massive enough to explode as a supernova. Their fate is to lose their outer layer and their core will eventually collapse into a white dwarf. These objects usually have pristine atmospheres of helium and hydrogen but some of them get polluted as bits of planets that used to orbit the stars in question fall onto them.


By studying these polluted white dwarfs it is possible to learn a lot: Last week researchers announced the discovery of the remains of the oldest known planet in the Milky Way. This new research has found evidence suggesting that planets and stars form pretty much at the same time.

Nebulae, the birthplace of stars, are mostly made of hydrogen and peppered with ice and dust grains. Gravitational instabilities lead to the eventual collapse of chunks of these nebulae into many stars. The grains eventually evolve into pebbles, then into planetesimals which eventually collide and merge into planets.

The team looked at 202 cool white dwarfs and discovered evidence consistent with the process of differentiation. This happens in melted objects in space and explains why there’s so much iron at the core of the Earth: it simply sinks there. For these asteroid-like chunks of future planets, the melting was probably caused by some short-lived radioactive elements, and they only exist for a few million years, an indication that this process must have happened very quickly.

“Our study complements a growing consensus in the field that planet formation got going early, with the first bodies forming concurrently with the star,” first author Dr Amy Bonsor from Cambridge’s Institute of Astronomy, said in a statement. “Analyses of polluted white dwarfs tell us that this radioactive melting process is a potentially ubiquitous mechanism affecting the formation of all extrasolar planets.”


This suggests that planets such as Jupiter and Saturn had plenty of time to grow big. And while there are still uncertainties on how planets have formed, research at both ends of stellar systems' timelines is providing new insights.

“This is just the beginning – every time we find a new white dwarf, we can gather more evidence and learn more about how planets form," Dr Bonsor explained. "We can trace elements like nickel and chromium and say how big an asteroid must have been when it formed its iron core. It’s amazing that we’re able to probe processes like this in exoplanetary systems."

The study is published in Nature Astronomy.

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