The mechanism of planetary accretion and the evolution of the planets’ geophysics and composition are all influenced by the timing of the first planetesimals’ formation. Planetesimal formation during molecular cloud collapse is demonstrated by astronomical studies of circumstellar discs and Solar System geochronology far earlier than previously thought.

A new study by the University of Cambridge has offered distinct observational evidence from white dwarf planetary systems for planetesimal formation. A team of astronomers has found that planet formation in our young Solar System started much earlier than previously thought.

The findings change our understanding of how planetary systems, including our Solar System, formed, potentially solving a significant puzzle in astronomy.

Dr. Amy Bonsor from Cambridge’s Institute of Astronomy, the study’s first author, said, “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?”

To find the answer to this question, astronomers studied the atmospheres of white dwarf stars to investigate the building blocks of planet formation. These white dwarfs are amazing laboratories because their thin atmospheres are almost like celestial graveyards.

Normally, 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.

These elements must have originated from tiny objects like asteroids left over during planet formation and collided with the white dwarfs before igniting in their atmospheres. Therefore, the interiors of those fragmented asteroids can be explored through spectroscopic investigations of contaminated white dwarfs, providing astronomers with a clear understanding of the conditions under which they evolved.

The scientists examined spectroscopic data from 200 contaminated white dwarfs in surrounding galaxies. They found that the mixing of elements in the atmospheres of these white dwarfs could only be explained if many of the original asteroids had once melted, causing heavy iron to sink to the core and lighter metals to float on the surface. The Earth’s iron-rich core resulted from a process called differentiation.

Dr. Bonsor said, “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. In other words, if these asteroids were melted by something which only existed for a very brief time at the dawn of the planetary system, then the process of planet formation must kick off very quickly.”

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

“Our study complements a growing consensus that planet formation got going early, with the first bodies forming concurrently with the star. 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 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 trace elements like nickel and chromium and say how big an asteroid must have been when it formed its iron core. Amazingly, we can probe processes like this in exoplanetary systems.”

Journal Reference:

  1. Bonsor, A., Lichtenberg, T., Dra̧żkowska, J. et al. Rapid formation of exoplanetesimals revealed by white dwarfs. Nat Astron (2022). DOI: 10.1038/s41550-022-01815-8