Astronomers using the European Southern Observatory's Very Large Telescope have discovered a rare debris disk around a dead star. It shows what could happen to our Solar System. Unlike similar systems found earlier, the debris - the remains of planets that once orbited the star - is organized into a double disk, two parts of which are separated by a clear gap.
"We have known for some time that planets can survive the transformation of their central star, but this is the first example of a complex system with a double debris disk and possibly two surviving planetesimals," says Nicola Gentile Fusillo, an astronomer at the University of Trieste (Italy) and lead author of the study published in the journal Nature Astronomy. Planetesimals are small bodies from which planets form. The system's central star is the white dwarf J1328-1450: a dense, burnt-out core left after a Sun-like star exhausted its fuel.
When stars like the Sun enter the final stage of their lives, they expand and shed their outer layers, engulfing and destroying nearby planets. Surviving rocky bodies can break apart, forming a disk around the dead star. However, the disk around J1328-1450 differs from those observed around other dead stars: instead of a single ring of vaporized planetary debris, two concentric rings surround the star.
"The structure we see in this system is particularly intriguing: the two gas rings are surprisingly stable and remain separated by a gap," says co-author Felipe Lagos-Vilches, an astronomer at the University of Warwick (UK) and the Federico Santa Maria Technical University in Chile.
According to the team, the complex double structure is probably maintained by two surviving planetary bodies in different orbits. The researchers estimate that the body in the inner ring is the size of a typical asteroid - several kilometers in diameter - while the outer one may be the size of Pluto or even our Moon.
All known debris disks around white dwarfs have been identified by the same observational signature, indicating the presence of a single flat rotating disk. But new observations made with the X-Shooter instrument on the telescope in Chile's Atacama Desert show that the J1328-1450 system completely breaks this pattern, displaying a unique double-disk signature.
The observations also revealed elements such as calcium, oxygen, magnesium and iron in the disk, indicating a composition similar to rocky planetary material in our Solar System. Moreover, the data suggest that the two rings of the disk have fairly different compositions: the inner one is rich in gaseous magnesium, while the outer one largely lacks it. This suggests that the planetesimals feeding the rings originated from two planets with different compositions. An alternative explanation is that they are the result of different parts of a single celestial body, such as a planet's crust and core.
Our Solar System may meet a similar end: in the distant future the Sun will become a white dwarf in a process that could destroy Earth. "Thus, the system offers a remarkable glimpse of what the future of planetary systems, including systems like ours, may look like," Lagos-Vilches concludes.