Some rocky exoplanets bear a striking resemblance to Earth, according to Alexandra Doyle, Edward Young and colleagues at the University of California at Los Angeles. The team used the properties of light coming from six white-dwarf stars to calculate how much oxygen, iron and other elements were present in planets that once orbited the stars. Their observations suggest that these planets – which were consumed by their stars long ago – have the same geophysical and geochemical properties as Earth. While astronomers are able to observe rocky exoplanets, working out what they are made of is difficult and this research provides important clues regarding the composition of these Earth-like objects.
White dwarfs are the ancient remnants of stars that had masses less than about 10 Suns. This means that most stars in the Milky Way will eventually become white dwarfs – including the Sun. Many white dwarfs would have had planets, which would have been consumed by the stars at some point in their stellar evolution. The atmosphere of a white dwarf is expected to comprise only the lightest elements – hydrogen and helium – so the presence of heavier substances in the stellar atmosphere such as magnesium, iron and oxygen means that the star has probably ingested rocky planets or asteroids.
Minerals and metallic iron
Doyle, Young and colleagues did spectroscopic studies of light from six white dwarfs to determine the amount of oxygen, iron, magnesium, silicon, calcium and aluminium present in their atmospheres. In particular, they were interested in the abundance of oxygen relative to the other elements. This allowed them to work-out how much oxygen in a planet would be bound-up in minerals such as magnesium and silicon oxides – and also how much unreacted metallic iron would have been present in a planet. Earth, for example, has a huge amount of metallic iron in its core.
The study revealed that the white-dwarfs’ planets had elemental compositions similar to Earth, Mars and the asteroids. Describing their observations in Science, the researchers conclude that “at least some rocky exoplanets are geophysically and geophysically similar to Earth”. The abundance of oxygen determines the relative size of the metallic core of a planet; the geochemistry of its mantle and crust; the composition of its atmosphere; and the processes involved in mountain building. As a result, these observations suggest that some of the many rocky exoplanets expected to populate the Milky Way should be very similar to Earth.
