It turns out that we can actually measure the amount of planetary debris in the outer layers of white dwarf stars.

In a new paper published in Nature Astronomy, astronomers from the University of Warwick report on findings from observations of white dwarfs using data from the Gaia telescope. First, they found one unusual signal in the spectroscopic data, which they were using to analyze the composition of the outer layers. The signal turned out to be lithium, and as we have repeatedly mentioned, when you find one example, there are likely more out there. So they dug through the data and found three more white dwarfs with a lithium signal. And found one of those white dwarfs had potassium as well.

The team then compared the amounts of lithium and potassium they found with the amounts of sodium and calcium observed. All of these comparisons led to the conclusion that the chemical composition matched the crusts of rocky planets like Earth and Mars — planets that had been vaporized by the stars’ novae.

Lead author Dr. Mark Hollands said: 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. Lithium and potassium are good indicators of crust material, they are not present in high concentrations in the mantle or core. Now we know what chemical signature to look for to detect these elements, we have the opportunity to look at a huge number of white dwarfs and find more of these. Then we can look at the distribution of that signature and see how often we detect these planetary crusts and how that compares to our predictions.

These results are pretty amazing on a couple of levels. One, we’d not yet seen evidence of crustal material in these stars, although we had seen evidence of mantle and core material. And two, the four stars sampled burned out their fuel up to 10 billion years ago, making them some of the oldest in the Milky Way. Co-author Dr. Pier-Emmanuel Tremblay said: 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. 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.

These new findings once again change ideas we’ve harbored about planetary formation and system evolution. But you knew that was coming, didn’t you?

More Information

University of Warwick press release

“Alkali metals in white dwarf atmospheres as tracers of ancient planetary crusts,” Mark A. Hollands et al., 2021 February 11, Nature Astronomy