Stars and planets grow rapidly together

View larger. | Artist’s concept of a comet-like object falling towards a white dwarf star. Astronomers recently analyzed the atmospheres of 237 white dwarfs. Material from comets and asteroids had collided with stars and “polluted” their atmospheres. From this study, the researchers learned that stars and planets form simultaneously and rapidly. Image via ESA/Hubble/Wikipedia (CC BY 4.0).

Planetary systems, like ours, are born in clouds of dust and gas that surround their nascent 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 Nov. 14, 2022 that, indeed, stars and planets 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 Nature astronomy on November 14th. You can also download a free PDF copy of the document.

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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 newborn stars. These discs are composed of hydrogen, helium, and ice and dust particles. Dust particles, along with other material, gradually aggregate over millions of years. These lumps eventually become planetesimals, or tiny planetlets, if you will. Any leftover material becomes asteroids and comets.

However, scientists still argue about the timing. Do stars and planets form at the same time or do planets start taking shape millions of years after the star? The new study shows it’s the former: Stars and planets tend to form at virtually the same time. Amy Bonsor, lead author from the University of Cambridge, said:

We have a pretty good idea of ​​how planets form, but one lingering question we’ve had is when do planets 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 dwarfs, 237 to be exact. White dwarfs are basically dead stars. They still have atmospheres, however, and astronomers can scan those atmospheres to see what they contain. And often, those atmospheres are polluted with heavier elements such as silicon, magnesium, iron, oxygen, calcium, carbon, chromium and nickel. These elements are unrelated to the normal hydrogen and helium that white dwarfs are made of.

As Bonsor noted:

Some white dwarfs are amazing laboratories, because their thin atmospheres are almost like celestial graveyards.

Melting of asteroids

Where do those elements come from? Most likely, small bodies like asteroids. The asteroids collided with the white dwarfs and burned up in their atmospheres. As a result, the elements present in the asteroids polluted the atmospheres 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 in the core while the lighter elements floated to the surface. This process, known as differentiation, also occurred on Earth. In fact, it created the iron-rich core of the Earth. bonsore said:

The cause of the melting can only be attributed to very short-lived radioactive elements, which existed in the early stages of the planetary system but decompose in just a million years. In other words, if these asteroids were merged from something that existed only for a very short time at the dawn of the planetary system, then the planetary process must begin very quickly.

Glowing rings of material around a bright white star.
Planets are born in clouds of dust and gas around newly forming stars called protoplanetary disks or circumstellar disks. The new study shows that stars and planets form at the same time and grow together at the same time. Image via ESO/L. Calçada.

Iron-core planet simals

Co-author Tim Lichtenberg, previously at Oxford University when the research began and now at the University of Groningen, added:

Enrichment with heavy elements indicates that iron-core planetesimals have fallen on the star. And such an iron core can probably only form if the fragment was previously strongly heated. This is because that is where iron, rock and more volatile elements are separated.

The heat released during the decay of short-lived radioactive elements likely created the iron cores, as with asteroids in our solar system. Suppose the element in question is aluminum-26. That element has also driven the formation of planetary nuclei in our solar system.

Learn how stars and planets form

The findings shed new light on how planetary systems form and evolve. Future observations can now further build on these findings. bonsore said:

Our study complements a growing consensus in the field that planet formation began early, with the first bodies forming concurrently with the star. Analyzes of polluted white dwarfs tell us that this radioactive fusion 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 gather more evidence and learn more about how planets form. We can track 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.

Bottom line: An international team of researchers has published a new study of “polluted” white dwarf stars that shows that stars and planets likely form simultaneously.

Source: Rapid formation of exoplanetesimals revealed by white dwarfs

Source (PDF): Rapid formation of exoplanetesimals revealed by white dwarfs

Through the University of Cambridge



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