The mechanism of planetary accretion and the evolution of planetary geophysics and composition are all influenced by the timing of the formation of the first planetesimals. The formation of planetesimals during molecular cloud collapse is evidenced by astronomical studies of circumstellar disks and solar system geochronology much earlier than previously thought.
A new study from the University of Cambridge has provided unequivocal observational evidence from white dwarf planetary systems for the formation of planetesimals. A team of astronomers has found that planet formation began much earlier in our young solar system than previously thought.
The findings are changing our understanding of how planetary systems, including our solar system, formed and potentially solving an important mystery in astronomy.
dr Amy Bonsor of the Cambridge Institute of Astronomy, the study’s first author, said: “We have a pretty good idea of how planets form, but one open question we had was when they form: does planet formation start early, when the parent star is still growing, or millions of years later?”
To find the answer to this question, astronomers studied the atmospheres of white dwarf stars to study the building blocks of planet formation. These white dwarfs are amazing laboratories because their thin atmospheres are almost like celestial cemeteries.
Normally, telescopes cannot observe the interiors of planets. However, a certain group of white dwarfs, dubbed “polluted” systems, have heavy elements like calcium, magnesium, and iron in their normally pure atmosphere.
These elements must have come from tiny objects like asteroids left over from planet formation that collided with the white dwarfs before igniting in their atmosphere. Therefore, the interior of these fragmented asteroids can be probed through spectroscopic studies of contaminated white dwarfs, giving astronomers a clear understanding of the conditions under which they formed.
The scientists examined spectroscopic data from 200 contaminated white dwarfs in surrounding galaxies. They found that the mixing of elements in these white dwarfs’ atmospheres can only be explained if many of the original asteroids had once melted, causing heavy iron to sink into the core and lighter metals to float to the surface. Earth’s iron-rich core is the result of a process called differentiation.
dr Bonsor said: “The cause of the melting can only be traced back to very short-lived radioactive elements that existed in the earliest stages of the planetary system but decay in just a million years. In other words, if these asteroids were melted by something that only existed for a very short time at the beginning of the planetary system, then the process of planet formation must start very quickly.”
The study suggests that the early formation picture is probably correct, meaning that Jupiter and Saturn have had plenty of time to reach their current magnitudes.
“Our study adds to a growing consensus that planet formation began early, with the first bodies forming concurrently with the star. Analysis of polluted white dwarfs tells us that this radioactive melting process is a potentially ubiquitous mechanism affecting the formation of all extrasolar planets.”
“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. We can detect elements like nickel and chromium and tell how big an asteroid must have been when it formed its iron core. Amazingly, we can study such processes in exoplanetary systems.”
- Bonsor, A., Lichtenberg, T., Dra̧żkowska, J. et al. Rapid Formation of Exoplanet Ensimals Revealed by White Dwarfs. Nat Astron (2022). DOI: 10.1038/s41550-022-01815-8
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