These exo-rocks are stranger than fiction

2 November 2021

We know there are thousands of planets orbiting distant stars in our galaxy: the famous exoplanets. However, knowing what these exoplanets are made of has always been tricky — until now. By studying ‘polluted’ white dwarfs, astronomers have found that most rocky exoplanets are made of rocks we can’t find anywhere in our Solar System.

White dwarfs are what stars like our Sun become at the end of their lives. As they die, they expel the gas around them until all that remains is the dense, collapsed hot core that used to be the star’s heart — a white dwarf. The ‘polluted’ types contain material from planets or asteroids that used to orbit the former star and eventually fell into the white dwarf, ‘contaminating’ its atmosphere mostly made of hydrogen and helium.

Astronomer Siyi Xu (NOIRLab) and geologist Keith Putirka (California State University, Fresno) studied 23 polluted white dwarfs around 650 light-years from our Sun. They looked for elements that were not hydrogen or helium to see what they would find. By measuring how abundant these other elements are, it is possible to ‘reconstruct’ the minerals and rocks that formed the rocky planets around these distant, dead stars. 

And guess what? The researchers found much more than our everyday calcium, magnesium or iron. Some combinations of minerals do not even exist in our Solar System and the team had to come up with (quite interesting) names to classify these new rock types.

For example: on our Earth we can find periclase (a magnesium mineral from rocks that have changed along the ages with high heat and pressure), pyroxene (a silicate mineral found in volcanic rocks) and quartz (a crystalline mineral commonly used in the making of jewellery). But you probably won’t hear about researchers finding “quartz pyroxenites” or “periclase dunites” anywhere on our planet. 

These rocky exoplanets, however, are filled with them. Some of these rocks could even dissolve more water and some might melt at much lower temperatures than the rocks we have on Earth. How could that affect the formation of oceans and plate tectonics in those worlds? This can be a very interesting next step for scientists to figure out. 

So if you think all rocky planets look like our Earth, think again!

Image: in this illustration, rocky debris surround a white dwarf. 
Image credit: NOIRLab/NSF/AURA/J. da Silva 
Image processing: M. Zamani and M. Kosari (NSF's NOIRLab)

Cool Fact

The atmospheres of these polluted white dwarfs were rich in magnesium and poor in silicon, suggesting the debris the team was looking at came from the interior of rocky planets, not their surface.