‘Polluted’ white dwarfs could unveil the secrets of how planetary systems are formed

The idea that planets only develop when a star reaches its maximum size has been challenged by the observation that stars and planets "grow up" together.
Sade Agard
Study suggests that planets grow at the same time as their parent star
Study suggests that planets grow at the same time as their parent star

Amanda Smith 

Some of the oldest stars in the universe, the building blocks for planets like Jupiter and Saturn start to form when a young star is growing, suggests a new study published on Nov. 14 in the journal Nature Astronomy.

The recent findings indicate that stars and planets ‘grow up' together, challenging a leading belief that planets only form once a star has reached its final size.

Significantly, it alters our understanding of how planetary systems, including our own Solar System, came to be and may help to resolve a significant astronomical enigma.

"Some white dwarfs are amazing laboratories, because their thin atmospheres are almost like celestial graveyards"

"We have a pretty good idea of how planets form, but one outstanding question we've had is when they form: does planet formation start early, when the parent star is still growing, or millions of years later?" said the study's lead author Dr. Amy Bonsor from Cambridge's Institute of Astronomy in a press release.

To understand the fundamental elements of planet formation, Bonsor and her coworkers examined the atmospheres of white dwarf stars, the ancient, dim remains of stars like our Sun.

"Some white dwarfs are amazing laboratories, because their thin atmospheres are almost like celestial graveyards," said Bonsor.

Usually, telescopes are unable to observe the interiors of planets. However, a particular group of white dwarfs, referred to as "polluted" systems, have heavy elements like calcium, magnesium, and iron in their ordinarily pure atmospheres.

Two hundred polluted white dwarfs from nearby galaxies had their atmospheres' spectroscopic measurements examined

Apparently these substances must have originated from tiny objects like asteroids left over during planet formation. They collided with the white dwarfs before igniting in their atmospheres.

Therefore, the interiors of those fragmented asteroids can be explored through spectroscopic investigations of polluted white dwarfs, providing astronomers with a clear understanding of the conditions under which they evolved.

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In the new study, two hundred polluted white dwarfs from nearby galaxies had their atmospheres' spectroscopic measurements examined. According to their analysis, the composition white dwarf atmospheres can only be explained if many of the original asteroids had once melted.

This would have caused heavy iron to sink to the core and lighter components to float on the surface. Known as differentiation, this is the same process that gave rise to the Earth's iron-rich core.

The study demonstrated that "the process of planet formation must kick off very quickly" supporting a growing theory in the field

"The cause of the melting can only be attributed to very short-lived radioactive elements, which existed in the earliest stages of the planetary system but decay away in just a million years," said Bonsor.

"In other words, if these asteroids were melted by something which only exists for a very brief time at the dawn of the planetary system, then the process of planet formation must kick off very quickly," she explained.

According to the study, the early-formation model is likely accurate, which means Jupiter and Saturn had plenty of time to expand to their present sizes.

"Our study complements a growing consensus in the field that planet formation got going early, with the first bodies forming concurrently with the star," said Bonsor.

Future white dwarf discoveries will aid more understanding of how planets are formed

"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," stated Bonsor.

"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."

Researchers from the Max Planck Institute for Solar System Research in Gottingen, the University of Oxford, Ludwig-Maximilians-Universität in Munich, the University of Groningen, and other institutions participated in the study.

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