Astronomers have detected lithium in the atmospheres of four cool and old white dwarfs, of which one also displayed atmospheric potassium. The relative abundances of these two alkali elements with respect to sodium and calcium strongly suggest that all four stars have accreted fragments of rocky planets similar to Earth and Mars.
“In the past, we’ve seen all sorts of things like mantle and core material, but we’ve not had a definitive detection of planetary crust,” said Dr. Mark Hollands, an astronomer in the Department of Physics at the University of Warwick.
“Lithium and potassium are good indicators of crust material, they are not present in high concentrations in the mantle or core.”
Dr. Hollands and his colleagues analyzed data on over 1,000 nearby white dwarfs from ESA’s Gaia telescope.
They also inspected 30,000 white dwarf spectra from the Sloan Digital Sky Survey (SDSS) published over the last 20 years.
They detected lithium in the atmospheres of four white dwarfs, aged between 5-10 billion years: LHS 2534, WD J231726.74+183052.75, WD J182458.45+121316.82, and SDSS J133001.17+643523.69.
They also detected potassium in one of these stars, LHS 2534.
By comparing the amount of lithium and potassium with the other elements they detected, the astronomers found that the ratio of elements matched the chemical composition of the crust of rocky planets, if those crusts and been vaporized and mixed within the gaseous outer layers of the star for two million years.
The outer layers of the white dwarfs contain up to 300,000 gigatons of rocky debris, which includes up to 60 gigatons of lithium and 3,000 gigatons of potassium, equivalent to a 60 km (37 miles) sphere of similar density to Earth’s crust.
The amount of crust material detected is similar in mass to that of the asteroids we see in our own Solar System, leading the researchers to believe that what they are seeing around all four white dwarfs is material broken off from a planet, rather than an entire planet itself.
“In one case, we are looking at planet formation around a star that was formed in the Galactic halo, 11-12.5 billion years ago, hence it must be one of the oldest planetary systems known so far,” said Dr. Pier-Emmanuel Tremblay, also from the Department of Physics at the University of Warwick.
“Another of these systems formed around a short-lived star that was initially more than four times the mass of the Sun, a record-breaking discovery delivering important constraints on how fast planets can form around their host stars.”
The results were published online in the journal Nature Astronomy.
M.A. Hollands et al. Alkali metals in white dwarf atmospheres as tracers of ancient planetary crusts. Nat Astron, published online February 11, 2021; 10.1038/s41550-020-01296-7