Evaporated crusts of Earth-like planets found in dying stars

Remains of Earth-crusted planets have been discovered in the atmospheres of four nearby white dwarf stars by astronomers at the University of Warwick, who have glimpsed the planets that orbited them billions of years ago.

These crusts come from the outer layers of rocky planets, similar to Earth and Mars, and could give astronomers more insight into the chemistry of the planets that once harbored these dying stars.

The discovery is reported today in the journal Nature Astronomy and includes one of the oldest planetary systems astronomers have seen to date.

The University of Warwick-led team analyzed data from the European Space Agency’s Gaia telescope of more than 1,000 nearby white dwarf stars when they encountered an unusual signal from a particular white dwarf. The University of Warwick researchers have received funding from the European Research Council and the Science and Technology Facilities Council (STFC).

They used spectroscopy to analyze the star’s light at different wavelengths, which allowed them to detect when elements in the star’s atmosphere absorb light of different colors and which elements they are and how much is present. They also inspected the Sloan Digital Sky Survey’s 30,000 white dwarf spectra published over the past 20 years.

The signal matched the wavelength of lithium, and the astronomers soon discovered three more white dwarfs with the same signal, one of which was also observed with potassium in its atmosphere. By comparing the amount of lithium and potassium with the other elements they discovered – sodium and calcium – they found that the ratio of the elements matched the chemical makeup of the crusts of rocky planets like Earth and Mars, when those crusts had evaporated and mixed in. the gaseous outer layers of the star for 2 million years.

Lead author Dr. Mark Hollands of the University of Warwick’s Department of Physics said: “In the past we’ve seen all kinds of things like mantle and nuclear material, but we haven’t 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 shell or core.

“Now that we know what chemical signature to look for to detect these elements, we have the ability to look at a large number of white dwarfs and find more of them. Then we can look at the spread of that signature and see how. often we detect these planetary crusts and how that relates to our predictions. “

The outer layers of the white dwarfs contain up to 300,000 gigatons of rocky debris, including up to 60 gigatons of lithium and 3,000 gigatons of potassium, which corresponds to a 60 km sphere of similar density to the Earth’s crust. The amount of crustal material detected is similar in mass to that of the asteroids we see in our own solar system, leading astronomers to believe that what they see around all four stars is material that has broken off a planet rather than an entire planet. self.

Previous observations of white dwarfs have found evidence of material from the inner core and mantle of planets, but no definitive evidence of crustal material. Crust is a small fraction of a planet’s mass, and the elements detected in this study are only detectable when the star is very cool. White dwarfs are in the final stages of their life cycle as they have burned out their fuel and cooled for billions of years. These four white dwarfs are said to have burned out their fuel up to 10 billion years ago and could be among the oldest white dwarfs to form in our galaxy.

Co-author Dr. Pier-Emmanuel Tremblay of the University of Warwick said, “In one case we are looking at planet formation around a star that formed in the galactic halo 11-12.5 billion years ago, so it must be one of the oldest planetary stars. systems known so far.Another of these systems formed around a short-lived star that initially had more than four times the mass of the Sun, a record-breaking discovery that provided important limitations on how quickly planets can form around their host stars . “

Of the oldest of these white dwarfs, one is 70% more massive than average and so its sheer mass would normally cause any material in its atmosphere to disappear relatively quickly, leading the astronomers to conclude that the crustal material must be replenished from a surrounding debris disk. In addition, the astronomers detected more infrared light than expected for the white dwarf alone, indicating that a disk is heated by its star and then re-radiated at longer wavelengths.

Dr. Hollands adds, “As we understand it, the formation of rocky planets takes place in a similar way in different planetary systems. Initially, they are formed from a similar material composition to the star, but over time those materials separate and you get finally different chemical compositions in different parts of the planets. We can see that these objects have undergone differentiation at some point, the composition being different from the starting composition of the star.

“It is now clear that most normal stars, such as the Sun, harbor planets, but now there is the opportunity to also look at the frequency of different types of material.”