Planetary science: Exoplanet composition more varied than previously thought

Exoplanets in our solar neighbourhood may have more varied geological and mineral compositions than previously thought, suggests a paper in Nature Communications. The findings indicate that some exoplanets may have unique planetary accretion and differentiation paths, which are not similar in composition to Earth and lack direct counterparts in our own solar system.

Once a main sequence star, such as the Sun, has used up its energy, it will first swell to a red giant, then shrink and transform into a white dwarf, capturing nearby orbiting exoplanets in the process. These white dwarfs can be polluted with planetary, rocky material. Previous research has indicated that these polluted white dwarfs may contain traces of continent-like granitic crust of former exoplanets, which is also abundant on Earth, but these earlier findings do not use analyses of silicon, an essential element for evaluating rock type.

Keith Putirka and Siyi Xu analysed the atmospheres of 23 nearby white dwarfs, and determine that some contain high amounts of calcium (Ca), but all have very low silicon (Si) and high magnesium (Mg) and iron (Fe) amounts, which, the authors suggest, reflects the mantle compositions of the exoplanets that once orbited these stars. The authors also compared the polluted white dwarfs to the bulk compositions of the terrestrial planets in our Solar System, including planet Earth, and found no evidence of granitic crust, or crust-like rocks of any kind. The authors suggest that other stellar systems may contain a much wider variety of planetary compositions and rock types than previously thought.

The findings raise questions about why the composition of Earth is different compared to its planetary neighbours and whether such contrasts are typical or inevitable, the authors conclude.