If it weren’t for the sun constantly showering us with energy, there would be no life on Earth. But eventually the sun will run out of fuel, expand into a red giant and finally collapse into faint, white dwarf. What will happen to us and the other planets in the solar system when the sun dies? It’s not entirely clear.
Scientists think that they have spotted the possible core remnant of a planet orbiting the white dwarf SDSSJ122859.93+104032.9, residing some 410 light years away. The results, published in Science, offer important clues about the fate of the planets in our solar system.
The planetary fragment produced a stream of gas that could be detected by spectrometers. Researchers spotted it orbiting the star by looking at how its spectrum shifted in color as the body moved towards and away from Earth. This change in color is called a doppler shift, which is essentially a stretching or squashing of waves because of motion. It is similar to the pitch of the sound of an ambulance being higher when it is heading towards you, and lower when it is moving away.
The object completed one passage around its host star in just over two hours, orbiting at a distance that is smaller than the radius of the sun in a disc of gas and dust.
The discovery is surprising, since scientists didn’t think anything could survive so close to a white dwarf. A white dwarf is only about the size of the Earth but it contains around 60-70% of the sun’s mass, making it extremely dense. If a body orbits too close to a white dwarf, its immense gravity will rip it apart. This was likely the fate of the material that formed the disc around it.
So how did this object survive without getting ripped apart? It would have to either be very dense itself or have some amount of internal strength holding it together. Scientists calculated that it has a maximum diameter of 720km, which is the size of a small minor planet. The dwarf planet Ceres in our solar system has a diameter of 946km by comparison.
The origin of this object remains a mystery. One possibility is that this is the core of a minor planet that was pushed close to the white dwarf by a larger planet further out in the remnant planetary system, like a Jupiter. As the minor planet passed close to the white dwarf, its crust and mantle layers would have been ripped away.
All that would be left of the planet would be its dense, iron-dominated core. This kind of object is quite common, with one famous resident in our own solar system: the asteroid 16-Psyche.
Systems such as the one just discovered can help us understand the future of our own planetary system. In about five billion years, the sun will start to expand into a red giant. At this point, it will engulf Mercury, Venus and most likely Earth, unless we manage to move our planet into a wider orbit, which should be possible in theory. However, Mars, the asteroid belt and the rest of the solar system will survive engulfment and continue orbiting the sun as it then collapses into a white dwarf.
During this process, planets like Jupiter could also scatter asteroids, comets or even minor planets towards the white dwarf. There they would undergo partial or complete disruption, forming a disc like the one just discovered. It is unlikely that any living organisms on planetary or moon fragments could survive this process. Even if they did, they would struggle to live on in the faint light of a white dwarf.
This is not only the solar system’s fate, but that of practically all known exoplanet systems. In the much much closer future, scientists hope to find more planetary bodies around other white dwarfs. There are six candidate white dwarfs that are orbited by discs made of dust and gas, and researches want to test whether these discs are the “smoking gun” for the presence of minor planets. The more such planets are found, the more that can be learned about what happens to a planetary system as its star dies.