Study of 'polluted' white dwarfs reveals interesting facts about planet formation

Study of ‘polluted’ white dwarfs reveals interesting facts about planet formation

A team of astronomers from the University of Cambridge have found that the formation of planets in our young solar system began much earlier than previously thought, with the building blocks of planets developing alongside their parent star.

A study of some of the oldest stars in the Universe suggests that the formation of early planets, such as Jupiter and Saturn, is linked to the birth of young stars. Previously, astronomers believed that planets only formed once a new star had reached its final size; however, these new results suggest that planet formation is occurring simultaneously.

The research, published in natural astronomy, is changing our understanding of how planetary systems form, potentially solving a major puzzle in astronomy.

What can white dwarfs tell us about the formation of the first planets?

“We have a pretty good idea of ​​how planets form, but a lingering question is when do they form: does planet formation start early, when the parent star is still growing, or millions of years later?” said Dr Amy Bonsor of the Cambridge Institute of Astronomy, first author of the study.

To answer that question, Bonsor and his colleagues studied the atmospheres of white dwarf stars — the faint old remnants of stars like our Sun — to investigate the building blocks of planet formation. The study also involved researchers from the University of Oxford, the Ludwig-Maximilians-Universität in Munich, the University of Groningen and the Max Planck Institute for Solar System Research in Gottingen.

“Some white dwarfs are amazing laboratories, because their thin atmospheres almost look like celestial graveyards,” Bonsor explained.

Normally, the interiors of planets are beyond the reach of telescopes. But a special class of white dwarfs – called “polluted” systems – contain heavy elements such as magnesium, iron and calcium in their normally clean atmospheres.

These elements must come from small bodies like asteroids left over from the formation of the planets, which crashed into the white dwarfs and burned up in their atmospheres. As a result, spectroscopic observations of polluted white dwarfs can probe the interiors of these ripped asteroids, giving astronomers direct insight into the conditions under which they formed.

Planet formation is thought to begin in a protoplanetary disk — made up mostly of hydrogen, helium, and tiny particles of ice and dust — orbiting a young star. According to the current mainstream theory of planet formation, dust particles stick together, eventually forming larger and larger solid bodies. Some of these larger bodies will continue to accrete, becoming planets, and some will remain as asteroids, like those that crashed into the white dwarfs in the current study.

The process of differentiation

The researchers analyzed spectroscopic observations of the atmospheres of 200 polluted white dwarfs from nearby galaxies. According to their analysis, the mixing of elements observed in the atmospheres of these white dwarfs can only be explained if many of the original asteroids had once melted, causing heavy iron to flow into the core while the elements lighter ones floated to the surface.

This process, known as differentiation, is what caused the Earth to have an iron-rich core.

Bonsor said: “The cause of the melting can only be traced to very short-lived radioactive elements, which existed in the early stages of the planetary system but decay in just a million years.

“In other words, if these asteroids were melted by something that only exists for a very short time at the dawn of the planetary system, then the process of planet formation must start very quickly.”

The study suggests that the picture of early formation is probably correct, meaning Jupiter and Saturn have had plenty of time to grow to their current size.

“Our study complements a growing consensus in the field that the formation of planets began early, with the first bodies forming at the same time as the star,” Bonsor concluded.

“Analyzes of polluted white dwarfs tell us that this process of radioactive fusion is a potentially ubiquitous mechanism affecting the formation of all extrasolar planets.

“This is just the beginning – each time we find a new white dwarf, we can collect more evidence and learn more about planet formation. We can trace elements like nickel and chromium, and tell what must have been the size of an asteroid when it formed its iron core.It’s amazing that we are able to probe processes like this in exoplanetary systems.

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