Composition of exoplanets: autopsy of the white dwarfs

Researchers have investigated the materials from which exoplanets are composed. The team headed by Alexandra Doyle from the University of California at Los Angeles has analyzed debris from destroyed planetary systems. Thus, among the rocky planets, stars could be many with earthlike properties.

In the investigation, they focused on six so-called white dwarfs. These are remnants of burned out suns, in which no more atomic nuclei merge. They collapse under their own gravity into compact balls and become so hot that they glow white and then slowly anneal.

The heavy chemical elements sink when they collapse by gravity into the interior of the white dwarf star. In the spectrum of the star-coral almost only hydrogen and helium from the former sun can be seen. But only almost: The strong gravity of a white dwarf ruptures even asteroids and planets that revolve around him. Its atmosphere is so contaminated by debris from its former planetary system.

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To find out how a stellar atmosphere is chemically composed, astronomers split the starlight into the rainbow colors. Concise lines reveal the presence of individual chemical elements in this spectrum. It creates a kind of chemical fingerprint.

In the spectrum of investigated corals, the researchers searched for the six most common elements in rocks: iron, oxygen, silicon, magnesium, calcium and aluminum. It showed that the planetary debris of the white dwarfs are very similar to terrestrial and Mars rocks, the researchers report in the journal "Science".

Meaning for the search for a second earth

"We've found that rock everywhere has a very similar geophysics and geochemistry," said researcher Doyle. That means there could actually be life on the distant planet. In particular, according to the researchers, the degree of oxidation of the rock has an important influence on the atmosphere, the core and the surface of a rocky planet.

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"All of the chemistry that takes place at the surface of the earth can be traced back to the degree of oxidation of a planet," says Edward Young, who also participated in the study. "The fact that there are oceans on Earth and all the necessary ingredients for life can be traced back to the fact that the Earth is as oxidized as it is."

The autopsy of the planetary debris therefore also has significance for the search for a second earth in the cosmos, the scientists report. If extraterrestrial rocks have a degree of oxidation similar to that of the earth, one could assume that the planet has similar plate tectonics and magnetic field potentials as Earth. These properties are considered key ingredients in the development of life.