A study of white dwarfs suggests that planets are as old as their stars

A study of white dwarfs suggests that planets are as old as their stars

Astronomers have known for a long time how form planets, but the if it was always unclear. If a team at Cambridge University’s conclusion from a study of white dwarf stars proves correct, that question will be answered.

The debate essentially revolves around whether stars formed first and planets followed millions of years later, or whether stars and planets grew up together. According to Amy Bonsor of the Cambridge Institute of Astronomy, the question can be answered by looking at the atmospheres surrounding “polluted” white dwarfs. dr Bonsor is the lead author of a study just published in Nature.

“Some white dwarfs are amazing laboratories because their thin atmospheres are almost like celestial cemeteries,” Bonsor said.

These cemeteries are haunted by the remains of material that once surrounded the stars, Bonsor said. These stars are similar to our Sun, which is too small to produce a supernova. It will likely shed its outer layers as it ages, shrinking to a fraction of its size while continuing to emit faint light before eventually (theoretically) fading into a dead, cold black dwarf.

So-called polluted white dwarfs contain many heavy elements – such as magnesium, iron and calcium – and come from asteroids left over from planet formation. Looking at these asteroid remnants, Bonsor said, gives astronomers a look inside asteroids, which helps to understand how they formed.

By examining the elements in these stellar cemeteries, Bonsor and her team came to a single conclusion: planets must have formed early, when young stars ejected short-lived radioactive isotopes that melted tiny but growing planetesimals like early Earth.

Such radioactive isotopes, Bonsor said, burn up in about a million years. “In other words, if these asteroids were melted by something that existed for a very short time at the beginning of the planetary system, then the process of planet formation must start very quickly,” Bonsor said.

Layers don’t happen by accident

Earth is known as a differentiated planet, meaning it has different compositional layers. To get these layers, planets have to accumulate a lot of matter from a growing star’s molecular cloud.

Gravity does some of the work to move heavier elements — like the iron in Earth’s core — toward the center of the planet while lighter elements move toward the surface. This differentiation in planetary layers is aided by gravity, but intense heat from short-lived radioactive isotopes plays a central role, the research says.

“Analysis of polluted white dwarfs tells us that this radioactive melting process is a potentially ubiquitous mechanism affecting the formation of all extrasolar planets,” Bonsor said.

Bonsor’s team highlights 26Al as the element that does the most melting work, at least in our solar system. The researchers believe the short-lived aluminum isotope may also have played a role in stimulating the environmental conditions on Earth that led to the emergence of life.

Researchers from the University of Oxford, the Ludwig-Maximilians-Universität Munich, the University of Groningen and the Max Planck Institute for Solar System Research were also involved in the project.

Bonsor tells The registry that the data they use was collected by ground-based spectrographs, implying that “much can be done with relatively modest telescopes”. Good news for future studies on the formation of white dwarfs and planets.

“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. It’s amazing that we can study such processes in exoplanetary systems,” Bonsor said. ®

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