Dying Stars Caught Snacking On The Vaporized Crusts Of Earth-Like Planets They Destroyed

Artist impression of debris from a planetary crust disintegrating under the tidal forces around a cool white dwarf.  Image Credit: University of Warwick/Mark Garlick

Astronomers have detected the vaporized remnants of Earth-like planetary crusts in the atmosphere of four nearby white dwarfs, the final stage of evolution for stars up to 10 times the mass of our Sun. These incredible observations suggest that rocky planets like Earth and Mars form roughly in the same way across the galaxy and also rather quickly.

Reporting in the journal Nature Astronomy, the team found the outer layers of the white dwarfs to contain an estimated 300,000 gigatonnes of rocky debris, which included up to 60 gigatonnes of lithium and around 3,000 gigatonnes of potassium. The comparison with other elements detected, such as sodium and calcium, showed that this rocky debris had a similar composition to the crusts of Earth and Mars, if theirs had been vaporized and mixed with the gas outer layers of a star for millions of years. This the first time material from a planetary crust has been discovered in the atmosphere of white dwarfs.  

“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,” lead author Dr Mark Hollands from the University of Warwick, the UK explained in a statement.

“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.”

The four white dwarfs could be some of the oldest to form in our galaxy, burning up their fuel 10 billion years ago. Among them is one that is 70 percent more massive than average. Usually, "extra-large" white dwarfs are pretty quick at making rocky material disappear. The fact that the team observed the material suggests that the white dwarf is still snacking on what remains of the planets that once surrounded it.

“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,” said co-author Dr Pier-Emmanuel Tremblay also from the University of Warwick.

These findings are painting a clearer picture of how rocky planets form. The composition of this rocky debris is similar to what we see in the Solar System, meaning rocky planets can form very quickly, and they were the same from the very beginning of our galaxy, meaning their formation is a very standard process.

“As we understand it, rocky planet formation happens in a similar way in different planetary systems. Initially, they are formed from similar material composition to the star, but over time those materials separate and you end up with different chemical compositions in different parts of the planets. We can see that at some point that these objects have undergone differentiation, where the composition is different to the starting composition of the star,” Dr Hollands continued.

“It is now well understood that most normal stars like the Sun harbour planets, but now there’s the opportunity to look at the frequency of different types of material as well.”

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