Planetary systems, like ours, are born in clouds of dust and gas around their young stars. But do stars and planets form together, at about the same rate? Or do they form at different rates? An international team of astronomers, led by the University of Cambridge in England, reported on November 14, 2022, that stars and planets are indeed forming together, and quite quickly. The researchers came to this conclusion after observations and simulations of 237 “polluted” white dwarf stars.
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. Those disks are composed of hydrogen, helium and ice and dust particles. The dust particles, along with the other material, gradually clump together over millions of years. These clumps eventually become planetesimals, or minor baby planets, if you will. Any leftover material becomes asteroids and comets.
However, scientists are still debating the timing. Do the stars and planets form simultaneously, or do planets begin to form millions of years after the star? The new study shows it’s the former: Stars and planets form at nearly the same time. Amy Bonsorlead author from the University of Cambridge, mention:
We have a pretty good idea of how planets form, but an open question we’ve had is when they form: Does planet formation begin early, when the parent star is still growing, or millions of years later?
‘Dirty’ white dwarf stars
So, how did the researchers determine that stars and planets grow together? They looked at white dwarf stars, 237 to be exact. White dwarfs are actually dead stars. However, they still have atmospheres, and astronomers can analyze those atmospheres to see what they contain. And often those spheres are contaminated with heavier elements such as silicon, magnesium, iron, oxygen, calcium, carbon, chromium and nickel. Those elements are foreign to the normal hydrogen and helium that make up the white dwarfs.
As a bonsor noted:
Some white dwarfs are amazing laboratories, as their thin atmospheres almost resemble celestial burial grounds.
Where did those elements come from? Most likely small bodies like asteroids. The asteroids collided with the white dwarfs and burned up in their atmospheres. The elements in the asteroids therefore polluted the atmosphere of the white dwarfs.
The analysis results are intriguing. They show that the asteroids had melted. This melting caused heavy iron to sink to the core while the lighter elements floated on the surface. This process, known as differentiation, also happened on Earth. In fact, it created the iron-rich core of the Earth. 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 decomposed in just a million years. In other words, if these asteroids have been melted by something that has only existed for a very short time at the beginning of the planetary system, then the process of planet formation must start very quickly.
Planetesimals with iron core
The heavy element enrichment indicates that planetesimals with an iron core have fallen on the star. And such an iron core can probably only form if the fragment has previously been strongly heated. This is because then iron, stone and more volatile elements are separated.
Heat released during the decay of short-lived radioactive elements likely created the iron cores, as in asteroids in our own solar system. We assume that the element in question is aluminium-26. That element also caused the formation of planetary nuclei in our own solar system.
Learn how stars and planets form
The findings shed new light on how planetary systems form and evolve. Future observations can now build on these results. Bonsor said:
Our study adds to a growing consensus in the field that planetary formation started early, with the first bodies forming simultaneously with the star. Analyzes of polluted white dwarfs tell us that this radioactive melting process is a potentially ubiquitous mechanism influencing the formation of all exoplanets.
This is just the beginning; every time we find a new white dwarf we can collect 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 is amazing that we can investigate such processes in exoplanetary systems.
Bottom line: An international team of researchers has published a new study of “polluted” white dwarf stars, showing that stars and planets likely form at the same time.