Study finds that stars and planets grow together

Astronomers have found that planet formation in the young Solar System started much earlier than previously thought.

According to the research, the building blocks of planets started growing at the same time as their parent star.

A study of some of the oldest stars in the Universe suggests planets like Jupiter and Saturn begin to form while a young star is growing.

It had been previously thought that planets only form once a star has reached its final size, but the new research suggests stars and planets grow up together.

Researchers say the study, 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.

The study’s first author, Dr Amy Bonsor from the University of Cambridge’s Institute of Astronomy, 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 answer the question, researchers studied the atmospheres of white dwarf stars – the ancient, faint remnants of stars like our Sun – to investigate the building blocks of planet formation.

Dr Bonsor said: “Some white dwarfs are amazing laboratories because their thin atmospheres are almost like celestial graveyard.”

While the interiors of planets are out of reach of telescopes, a special class of white dwarfs – known as polluted systems – have heavy elements such as magnesium, iron, and calcium in their normally clean atmospheres.

These elements must have come from small bodies like asteroids left over from planet formation, which crashed into the white dwarfs and burned up in their atmospheres.

Therefore, observations of polluted white dwarfs can probe the interiors of those torn-apart asteroids, giving astronomers direct insight into the conditions in which they formed.

Scientists analysed observations from the atmospheres of 200 polluted white dwarfs from nearby galaxies.

According to their findings, the mixture of elements seen in the atmospheres of these white dwarfs can only be explained if many of the original asteroids had once melted.

This would have caused heavy iron to sink to the core while the lighter elements floated on the surface.

This process, known as differentiation, is what caused the Earth to have an iron-rich core.

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

Dr Bonsor added: “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.”

The study, which 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, is published in the journal Nature Astronomy.

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