Stars and planets grow at the same time, new study finds

A new study has found that the building blocks of planets grow at the same time as their parent star. Previously, it had been thought that planets only form once a star has reached its final size, but new results suggest that stars and planets grow up together.

The research, led by the University of Cambridge, is published in the journal Nature Astronomy.

Dr Amy Bonsor from Cambridge's Institute of Astronomy, the study's first author, and her colleagues studied the atmospheres of white dwarf stars - the ancient, faint remnants of Sun-like stars - to investigate the building blocks of planet formation. 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, were also involved in this study.

"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?" Dr Amy Bonsor said in a statement.

The team analyzed spectroscopic observations from the atmospheres of 200 polluted white dwarfs from nearby galaxies. Polluted white dwarfs are a special class of white dwarfs that have heavy elements such as magnesium, iron, and calcium in their normally clean atmospheres.

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, which caused heavy iron to sink to the core while the lighter elements floated on the surface, the researchers explained. This process, known as differentiation, is what caused the Earth to have an iron-rich core.

The research suggests that the early-formation picture is likely to be correct, which means that planets like Jupiter and Saturn had plenty of time to grow to their current sizes.

"Our study complements a growing consensus in the field that planet formation got going early, with the first bodies forming concurrently with the star. Analyses of polluted white dwarfs tell us that this radioactive melting process is a potentially ubiquitous mechanism affecting the formation of all extrasolar planets," said Bonsor.