The interiors of some exoplanets have Earth-like geochemistry, according to a new study, which finds that extrasolar rocks share similar levels of oxidation, or oxygen fugacity, to those in the Solar System. The results suggest that some of the rocky exoplanets orbiting distant stars have similar internal properties to Earth and Mars. Alexandra Doyle and colleagues were able to probe the geochemistry of distant rocks by evaluating their elemental remains, imprinted in the atmospheres of white dwarfs after rocky bodies crashed into them. 

Most rocky materials in the Solar System have a high degree of oxidation, known as oxygen fugacity (fO2), which reflects conditions during the earliest stages of protoplanetary rock formation around the Sun. fO2 influences a planet’s overall geochemical and geophysical properties – including the composition of any atmosphere it produces. However, determining these properties in exoplanets is both difficult and often unreliable, so it has been uncertain whether the same processes which oxidized planet-forming material in our Solar System occurred elsewhere, in solar systems around other stars. A white dwarf (WD) is the dense, exhausted core of a dead star; due to its strong gravitational pull, WD surface atmospheres should be composed almost entirely of the lightest elements, hydrogen and helium. 

However, some WDs have been identified with atmospheres “polluted” by heavier elements, including magnesium, iron and oxygen, introduced by rocky exoplanets and asteroids that have crashed into the WD. Spectroscopic observations of six polluted WDs allowed Doyle et al. to measure the constituent elements of these extrasolar rocks and to determine the abundance of oxidized iron within the rocks. The results probe the bulk composition of the former exoplanets, including their interiors. They find that rocky bodies that once orbited these long-dead stars had high fO2, similar to Earth, Mars and the asteroids in our Solar System, but unlike Mercury.