White dwarf study suggests planets are as old as their stars

Scientists have long been unsure of the 'when' of planetary formation – but that question may have been answered

Astronomers have long known how planets form, but the when of it has always been unclear. If a Cambridge University team's conclusion from a study of white dwarf stars proves correct, that question has been answered.

The debate is essentially over whether stars formed first, with planets following millions of years later, or whether stars and planets grow up together. According to Cambridge Institute of Astronomy's Amy Bonsor, the question can be settled by looking at the atmospheres surrounding "polluted" white dwarfs. Dr Bonsor is the first author on a study just published in Nature.

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

Haunting those graveyards are the remnants of material that once surrounded the stars, Bonsor said. Those stars are much like our Sun, which is too small to create a supernova. It will likely shed its outer layers as it ages, shrinking to a fraction of its size while it continues to radiate faint light before eventually (theoretically) fading into a dead, cold black dwarf.

So-called polluted white dwarfs contain lots of heavy elements – like magnesium, iron and calcium – and come from asteroids left over from planetary formation. Looking at those asteroid remnants, Bonsor said, gives astronomers a glimpse into the interior of asteroids, which helps understand how they were formed. 

By examining the elements in those stellar graveyards, Bonsor and her team reached a single conclusion: Planets have to have formed early, when young stars were ejecting short-lived radioactive isotopes that caused tiny-but-growing planetesimals, like early Earth, to melt. 

Such radioactive isotopes, Bonsor said, burn off in around 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," Bonsor said.

Layers don't form by accident

Earth is known as a differentiated planet, meaning it has distinct compositional layers. In order to gain those layers, planets have to accrete 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 – towards the middle of the planet, while lighter elements move toward the surface. That differentiation in planetary layers is aided by gravity, but intense heat from short-lived radioactive isotopes plays a central role, the research suggests.

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

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

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

Bonsor told The Register that the data they used was collected from ground-based spectrographs, meaning "a lot can be done with relatively modest telescopes." Good news for future white dwarf and planetary formation studies. 

"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. It's amazing that we're able to probe processes like this in exoplanetary systems," Bonsor said. ®