In our early Solar System, planet formation began far earlier than previously believed, with the planets’ building components developing at the same rate as their parent star. This discovery was made by a team of astronomers.
The building blocks of planets like Jupiter and Saturn are thought to start forming when a young star is growing, according to research on some of the earliest stars in the universe. This research, published in the journal Nature Astronomy, challenges the conventional wisdom that planets only originate when a star reaches its full size and implies that stars and planets ‘grow up’ concurrently.
The research, led by the University of Cambridge, changes our understanding of how planetary systems, including our own Solar System, formed, potentially solving a major puzzle in astronomy.
“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 Dr. Amy Bonsor from Cambridge’s Institute of Astronomy, the study’s first author.
In order to examine the fundamental elements of planet formation, Bonsor, and her coworkers looked at the atmospheres of white dwarf stars, the ancient, dim remains of stars like our Sun. 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, Gottingen.
“Some white dwarfs are amazing laboratories because their thin atmospheres are almost like celestial graveyards,” said Bonsor.
Typically, telescopes are unable to observe the interiors of planets. However, a distinct group of white dwarfs known as “polluted” systems contains heavy metals like calcium, magnesium, and iron in their generally pure atmospheres.
Our study complements a growing consensus in the field 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.
Dr. Amy Bonsor
These substances must have originated from tiny objects like asteroids that were left over during planet formation and collided with the white dwarfs before igniting in their atmospheres.
So, 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.
It is thought that planet formation starts in a protoplanetary disc around a young star that is mostly composed of hydrogen, helium, and minute particles of ice and dust. According to the most widely accepted hypothesis now in use, dust particles attach to one another and eventually grow larger and larger solid bodies.
Some of these larger bodies will continue to accrete and form planets, while others, like the asteroids that collided with the white dwarfs in the current study, will remain as asteroids. 200 contaminated white dwarfs from adjacent galaxies had their atmospheres’ spectroscopic measurements examined by the researchers.
According to their analysis, the composition of these white dwarf atmospheres can only be explained if many of the original asteroids had once melted, causing heavy iron to sink to the core and lighter components to float on the surface. This process, known as differentiation, is what caused the Earth to have an iron-rich core.
“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.”
According to the findings, the early-formation picture is probably 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. “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. It’s amazing that we’re able to probe processes like this in exoplanetary systems.”
Amy Bonsor is a Royal Society University Research Fellow at the University of Cambridge. The research was supported in part by the Royal Society, the Simons Foundation, and the European Research Council.
