Planetary systems, like ours, are born in clouds of dust and gas surrounding their fledgling stars. But do stars and planets form together, at roughly the same rate? Or do they form at different speeds? An international team of astronomers, led by the University of Cambridge in England, reported on November 14, 2022 that stars and planets do indeed form together, and quite quickly. The researchers came to this conclusion after conducting observations and simulations of 237 “polluted” white dwarf stars.
The researchers published their peer-reviewed findings in the journal natural astronomy November 14, 2022. You can also download a free PDF copy of the article.
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Stars and planets grow together
We know that planets form in clouds of dust and gas — called protoplanetary disks or circumstellar disks — around newly born stars. These disks are made up of hydrogen, helium, and particles of ice and dust. Dust particles, along with other materials, gradually clump together over millions of years. These clusters eventually become planetesimals, or little baby planets, if you will. Any remaining material becomes asteroids and comets.
However, scientists are still debating the timing. Do stars and planets form simultaneously, or do planets begin to form millions of years after the star? The new study shows it’s the first: Stars and planets tend to form around the same time. Amy Bonsor, lead author from the University of Cambridge, said:
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?
“Polluted” White Dwarf Stars
So how did the researchers determine that stars and planets grow together? They observed white dwarf stars, 237 of them to be exact. White dwarfs are essentially dead stars. However, they still have atmospheres, and astronomers can analyze those atmospheres to see what’s in them. And often these atmospheres are polluted with heavier elements such as silicon, magnesium, iron, oxygen, calcium, carbon, chromium and nickel. These elements are foreign to the normal hydrogen and helium of which white dwarfs are composed.
As Bonsor noted:
Some white dwarfs are amazing laboratories, as their thin atmospheres almost look like celestial graveyards.
Molten asteroids
Where do these elements come from? Most likely, small bodies such as asteroids. Asteroids collided with white dwarfs and burned up in their atmospheres. The elements of the asteroids have thus polluted the atmosphere of the white dwarfs.
The results of the analysis are intriguing. They show that the asteroids had melted. This fusion caused the heavy iron to sink to the core while the lighter elements floated to the surface. This process, known as differentiation, also happened on Earth. In fact, he created the iron-rich core of the Earth. Bonsor said:
The cause of the meltdown can only be attributed 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 existed for a very short time at the dawn of the planetary system, then the process of planet formation must start very quickly.
Iron-core planetesimals
Co-author Tim Lichtenberg, previously at Oxford University when the research began and now at the University of Groningen, added:
The heavy element enrichment indicates that iron-core planetesimals fell on the star. And such an iron core can probably form only if the fragment has previously been strongly heated. This is because this is when the iron, rock and more volatile elements are separated.
The heat released during the decay of short-lived radioactive elements likely created the iron cores, similar to asteroids in our own solar system. We assume that the element in question is aluminum-26. This element has also led to the formation of planetary cores in our own solar system.
Learn how stars and planets are formed
The findings shed new light on how planetary systems form and evolve. Future observations can now build on these results. Bonsor said:
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. 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 gather more evidence and learn more about planet formation. We can trace elements like nickel and chromium and tell how big an asteroid must have been when it formed its iron core. It’s amazing that we are able to probe processes like this in exoplanetary systems.
Conclusion: An international team of researchers has published a new study on “polluted” white dwarf stars showing that stars and planets probably form simultaneously.
Source: Rapid formation of exoplanetsimals revealed by white dwarfs
Source (PDF): Rapid formation of exoplanetsimals revealed by white dwarfs
Via the University of Cambridge
Via Astronomy.nl
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