Remains of planetary crust crumbling under the tidal forces around a cool white dwarf. Material in the disk evaporates near the central star and flows into the white dwarf atmosphere. Credit: University of Warwick / Mark Garlick
Remains of planets with earthy crusts were discovered in the atmosphere of four nearby white dwarf stars by University of Warwick astronomers, offering a glimpse of the planets that may have been orbiting them until billions of years ago.
- Observations of lithium and potassium around white dwarf stars indicate remnants of rocky planetary crusts
- Analysis by astronomers led by the University of Warwick shows that the chemical composition of crusts is very similar to the continental crust of the Earth
- The outer layers of the white dwarfs contain up to 300,000 gigatons of rocky debris, containing up to 60 gigatons of lithium and 3,000 gigatons of potassium.
- These white dwarfs are some of the oldest stars in our galaxy and may be one of the oldest planetary systems ever discovered.
Remains of planets with terrestrial crusts have been discovered in the atmosphere of four nearby white dwarf stars by astronomers from the University of Warwick, giving a glimpse of the planets they may have had around billions of years ago.
These crusts come from the outer layers of rocky planets similar to Earth and March and could give astronomers greater insight into the chemistry of the planets that these dying stars presented earlier.
The discovery is reported in the journal on February 11, 2021 Natural Astronomy and contains one of the oldest planetary systems seen so far by astronomers.
The team, led by the University of Warwick, analyzed data from the Gaia Telescope of the European Space Agency of more than 1,000 nearby white dwarf stars when they came across an unusual signal from one specific white dwarf. The researchers at the University of Warwick have received funding from the European Research Council and the Science and Technology Facilities Council (STFC).
They used spectroscopy to analyze the light of the star at different wavelengths, which enables them to detect when elements in the star’s atmosphere absorb light in different colors and determine which elements it is and how many there are. They also inspected the 30,000 white dwarf spectra from the Sloan Digital Sky Survey published over the past 20 years.
The signal corresponds to the wavelength of lithium and astronomers soon discover 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 detected – sodium and calcium – they found that the ratio of elements corresponds to the chemical composition of the crust of rocky planets such as Earth and Mars, as the crusts evaporate and have been mixed within the gaseous outer layers of the star for 2 million years.
Chief author, dr. Mark Hollands, from the Department of Physics at the University of Warwick, said: “In the past we have seen all sorts of things like mantle and nuclear material, but we have not yet definitively detected the planetary crust. Lithium and potassium are good indicators of crustal material, and is not present in high concentrations in the mantle or nucleus.
‘Now we know after what chemical signature to detect these elements. We have the opportunity to look at a large number of white dwarfs and find more of these. Then we can look at the distribution of the signature and see how often we detect these planetary crusts and how they compare to our predictions. ”
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, which is equivalent to a 60 km sphere of similar density to the earth’s crust. The amount of crustal material detected is similar to the mass of asteroids we see in our own solar system, leading astronomers to believe that what they see around all four stars is material that has been broken down from 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 the mass of a planet and the elements detected in this study are only observable when the star is very cool. White dwarfs are in the dying phase of their life cycle because they have burned their fuel and cooled it for more than billions of years. These four white dwarfs are thought to have burned out their fuel up to ten billion years ago and may be one of the oldest white dwarfs in our galaxy.
Co-author Dr Pier-Emmanuel Tremblay of the University of Warwick said: ‘In one case we are looking at the formation of a planet around a star that was formed 11-12.5 billion years ago in the galactic ray form , and it must therefore be one of the oldest planetary systems known so far. Another of these systems formed around a transient star that was initially more than four times the mass of the Sun, a record discovery that placed important limits on how fast planets could form around their host stars. ”
One of the oldest of these white dwarfs is one 70% massier than average, and its enormous mass will therefore normally cause any material in its atmosphere to disappear relatively quickly, leading astronomers to conclude that it is the crustal material of a surrounding junk disk. Furthermore, the astronomers detected more infrared light than expected for the white dwarf alone, indicating that a disk is heated by its star and then radiates again at longer wavelengths.
Dr Hollands adds: ‘As we understand it, rocky planet formation occurs in a similar way in different planetary systems. Initially it is formed from similar material composition as the star, but over time the materials separate and you have different chemical compositions in different parts of the planets. We can see that these objects have undergone differentiation at some point, where the composition differs from the initial composition of the star.
“It is now well understood that most normal stars are like the Sun planets, but there is now an opportunity to also look at the frequency of different types of material.”
Reference: “Alkali metals in white dwarf atmosphere as tracers of ancient planetary crusts” by Mark A. Hollands, Pier-Emmanuel Tremblay, Boris T. Gänsicke, Detlev Koester and Nicola Pietro Gentile-Fusillo, 11 February 2021, Natural Astronomy.
DOI: 10.1038 / s41550-020-01296-7
This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Program (Award Agreement No. 677706 (WD3D))