When the Sun dies, it will become what we call a white dwarf. White dwarfs are small and incredibly dense – a teaspoon of white dwarf matter would weigh the equivalent of an elephant on Earth! Studying white dwarfs helps us understand what will happen when the Sun dies. We can observe patterns amongst similar stars to the Sun throughout the universe, which helps us to make predictions about our Solar System’s fate. We spoke to Dr. Christopher Manser, a postdoc at the University of Warwick working on astronomy and astrophysics. His research studies remnant planetary systems around white dwarfs. We asked him what his studies tell us about exoplanets and the future of our own Sun.
What will happen to our Solar System after the Sun dies?
The Sun will run out of hydrogen in its core and start to expand into a red giant. During this phase, the Sun’s radius will increase, but it will also lose mass by blowing off its outer layers. The increase in radius will cause Mercury, Venus, and most likely Earth to be engulfed as the Sun becomes enormous. However, the surviving bodies – Mars, the asteroid belt, Jupiter, and beyond – will expand on their orbits as the Sun loses mass and therefore loses some of its gravitational attraction.
When the Sun finishes its destructive end and becomes a white dwarf, the bodies of the Solar System will be on more chaotic orbits. Some of these bodies may end up passing by the Sun and be ripped apart by its immense gravity, forming a disc and slowly consuming the body. Any other astronomers in our Solar neighborhood might gaze at the remnant of the Sun and learn about the composition of bodies in our Solar System!
What has studying remnant planetary systems surrounding white dwarfs taught us about the composition of exoplanets?
We can study the composition of planetary systems that have survived the death of their host star by looking at the dead remnant – the white dwarf. A white dwarf is so heavy and dense that the atmosphere is extremely stratified, with lighter materials rising to the surface – kind of like how oil floats on water. When a white dwarf consumes a planetary body (a process that can take almost one million years), the material takes some time (anywhere from a few days to more than 10,000 years) to sink out of the atmosphere. During this time, we can identify elements in the atmosphere that come from the planet and learn what it was made of.
We have seen that exoplanetary bodies have a wide range of varying compositions, similar to the variations we see in the different families of meteorites in the Solar System. We have also found that a lot of white dwarfs appear to host planetary systems with large amounts of water – enough to alter the atomic structure of the white dwarf atmosphere!
We have yet to find anything that is extremely different to the types of bodies we find in the Solar System; a recent search for carbon-rich planetary material which would be the building blocks for “carbon planets” turned up nothing. However, this field of research is relatively young, and I expect that we should have many more exciting discoveries to come!
Throughout your career, which research findings or discoveries are you most proud of?
I think the discovery I am proudest of is finding a small planetary body orbiting within a planetary debris disc around a white dwarf. We think that it might be the core of a larger body that was unfortunate enough to be knocked on an orbit bringing it too close to the white dwarf. In this scenario, the outer layers of the planet get stripped off, but the dense, iron-dominated core survives and orbits in its own remains. The body orbits the white dwarf once every two hours, and it is so interesting to find something that can survive in such a harsh environment. We hope to find more of these bodies, and we have a few candidate systems that we want to observe next!