Around a dead star, two rocky bodies may still be orbiting amid the debris of its former planetary system. They could even be fragments of larger planets.
In several billion years, the Sun will swell as it becomes a red giant. It will then shed its outer layers, leaving behind a very dense core called a white dwarf. The innermost planets may not survive this transformation. But what will happen to bodies located farther away?
Artistâs impression of the white dwarf J1328â1450 surrounded by debris and two separate gas rings. Two small rocky bodies could explain this structure.
Illustration: ESO/L. Calçada, CC BY 4.0.
A team of astronomers has discovered an intriguing clue around J1328â1450, a white dwarf formed from a star that was once similar to the Sun. Using the Very Large Telescope, located in Chile, they studied the light emitted by the gas orbiting this star.
The measurements revealed an unusual arrangement: the gas appears to form two concentric rings, separated by a region almost devoid of gas.
Why two rings rather than one? The astronomers favor the idea that two solid bodies are each located within one of the rings. By colliding with the dust surrounding them, they could produce gas and help maintain this structure. Their presence remains a hypothesis: neither body has been observed directly.
Another artistic representation of the debris disc surrounding the white dwarf J1328â1450. The two rings and the small bodies that could be orbiting within them are not directly visible through telescopes.
Illustration: Fortuna and Gentile Fusillo, via ESO, CC BY 4.0.
In reality, their size is unknown. The researchers calculated the maximum diameters these bodies could have without being torn apart by the white dwarfâs gravity. For the inner ring, this limit ranges from about 45 to 172 kilometers depending on its distance from the star, assuming a dense, solid body. It could be much smaller. For the outer body, the limit could reach about 3,500 kilometers under the most favorable conditions, roughly the size of our Moon.
This difference is due to the starâs proximity. Its gravity pulls more strongly on the side of an object facing it than on the opposite side. These forces can eventually tear it apart. Closer to the white dwarf, a large body therefore struggles to withstand the forces, whereas a small, sufficiently solid asteroid could remain intact.
The team also identified several elements in the gas, including iron, calcium, oxygen and magnesium. These are also found in the rocks of planets in our Solar System. The two rings do not have the same composition, particularly in terms of magnesium. Their materials could come from two different bodies, or from distinct parts of the same planet, such as its crust and interior.
These results show how turbulent the end of a planetary system can be. Planets can be destroyed and their remains can form rings. This may offer a very distant glimpse of what awaits our own Solar System.
The researchers now want to better understand what sustains these rings and determine whether rocky bodies are actually responsible for them.