Remains of asteroids in a white dwarf star could help astronomers find ‘missing’ lithium 

Crushed up asteroids found in the atmosphere of a long-dead white dwarf star could help astronomers find and measure the universe’s missing lithium.

Lithium measurements in stars like our own Sun have never added up to the amount scientists predict should exist – suggesting there is much more than we can find. 

The Big Bang, the leading explanation for how the universe began 13.8 billion years ago, produced three elements: hydrogen, helium and lithium. 

Of the three elements, lithium presents the biggest mystery. But the new study by University of North Carolina astronomers provides clues for tracking its evolution. 

Finding traces of the element in the rocky remains of an asteroid in the atmosphere of a nine billion year old white dwarf could help scientists estimate the total amount of lithium in the universe as it suggests it may be dispersed to rocky bodies. 

This is the first time the hard-to-find element has been identified in the burned out remains of a dead star, according to University of North Carolina researchers. 

Despite its many uses on Earth to power electronics and stabilise moods, scientists have been stumped by what’s become of the lithium expected from the Big Bang, a discrepancy known as the ‘cosmological lithium problem.’

Nobody knows exactly how much lithium there is in the universe, but these new findings mean white dwarf stars could be used to estimate the total amount. 

The discovery was made possible by using a unique spectrograph mounted on the Southern Astrophysical Research telescope. 

Study author, astrophysicist J. Christopher Clemens, led the design of the Goldman Spectrograph which measures how much light is emitted by a white dwarf.

White dwarfs are the leftover cores that remain when stars die, and they can be surrounded by rocky worlds. Our Sun will become a white dwarf when it dies.

The high surface gravities of these stars should cause elements heavier than hydrogen and helium to rapidly sink below the surface. 

Nevertheless, some ‘polluted’ white dwarf stars show evidence for heavier elements on their surfaces, thought to be due to recent accretion of rocky bodies.

In the study, researchers describe detecting the crushed-up remains of large asteroid-like objects in the atmospheres of two very old white dwarfs.

The planets of these dead stars first formed nine billion years ago – our Sun and the planets formed just 4.6 billion years ago.

The team measured the chemical make-up of the asteroids, and for the first time identified and measured both lithium and potassium from an extrasolar rocky body. 

Theory predicts that lithium was mostly formed in the first five minutes after the Big Bang. Its subsequent history is different from other elements and is more uncertain because lithium is consumed by nuclear reactions in stars.

Finding it in the white dwarf stars provides a record of the original rocky bodies that formed nine billion years ago – and therefore the galactic lithium abundance at the time they formed – within the first few billion years of the universe. 

The authors note that accreted bodies such as those that polluted this star ‘represent an alternative to old stars for gaining insight into the primordial [lithium] abundance, the earliest epochs of chemical enrichment in our Galaxy, and the properties of ancient exoplanets.’

‘Our measurement of lithium from a rocky body in another solar system lays the foundation for a more reliable method of tracking the amount of lithium in our galaxy over time,’ Clemens said. 

‘Eventually with enough of these white dwarfs that had asteroids fall on them, we will be able to test the prediction of the amount of lithium formed in the Big Bang. 

The findings have been published in the journal Science.