Planets are usually not much younger than their cosmic mothers – only young stars are considered childbearing. However, astronomers are now presenting evidence that under certain circumstances planets can form even in dying stars: if they form a binary system with an active star, the decaying material from the aging partner forms a kind of protoplanetary disk in which planets can form. This is suggested by suspicious traces in the disks around some binary systems. If the previous suspicion is confirmed, the theories about the formation of planets would have to be adjusted, say the scientists.
Shortly after igniting its stellar fire about 4.6 billion years ago, our Sun became the cosmic “mother”: its planets, including Earth, formed early in the history of our home system. This is considered typical – planets are believed to form from the residual material that still surrounds newly formed stars. The matter of dust and gas forms so-called protoplanetary discs, in which clumping processes ultimately lead to the formation of planets. At some point, however, the building material will be used up. Older or even decaying stars therefore no longer produce new companions, so the assumption. But as the international team of astronomers led by Jacques Kluska from the Catholic University of Leuven in Belgium is now reporting, there could be interesting exceptions to this rule.
Special double star systems in sight
It was already known that discs of dust and gas are not necessarily unique to newborn stars. They can also form independently of star formation in special systems of two stars orbiting each other – when one partner is “dying”. The reason: When a medium-sized star nears the end of its existence, it ejects the outer part of its atmosphere into space, after which it slowly fades into a white dwarf. In a binary system, however, the gravitational pull of the second star causes matter ejected from the dying partner to form a flat, rotating disk around the duo.
Kluska and his colleagues found such so-called evolved double star pairs with disks of matter as part of their research. They then took a closer look at their structures. As a basis, they used existing, publicly available observation data from various previous studies. As the scientists report, they found a total of 85 specimens of these so-called evolved double star pairs with disks of matter. In some of them, they encountered a characteristic structural feature.
Suspicious clearances in the panes
“In ten percent of the systems we examined, we see clear spaces in the pane,” reports Kluska. “This is an indication that there is something floating around that has picked up all the matter in the area of free space.” So this could be the work of a planet. It is possible that it is an ancient celestial body that formed at the very beginning of the life of one of the double stars. But it also seems possible that it is a second-generation planet, formed from the decay material of the dying star.
According to the astronomers, this scenario is also indicated by another piece of evidence, which can be seen in the spectral characteristics of the light: “In the evolved binary star systems with a large free area in the disk, we found that heavy elements such as iron on the surface of the dying star were unusually rare,” says Kluska. “This observation suggests that dust particles rich in these elements have been captured by a planet,” explains the astronomer. However, as he and his colleagues emphasize, the indications and the assumptions derived from them must first be substantiated by further investigations.
If the existence of planets around evolved binary star systems is confirmed and it turns out that they only formed after the beginning of the dying process of one of the partners, the ideas about planet formation would have to be adjusted. “Confirming or refuting this extraordinary mode of planet formation will be a special test for current theories,” says co-author Hans Van Winckel from KU Leuven. The astronomers now want to test their hypothesis themselves. To do this, they will use the large telescopes at the European Southern Observatory in Chile to take an even closer look at the ten pairs of double stars, whose discs have large open spaces.
Source: KU Leuven, professional article: Astronomy and Astrophysics, doi: 10.1051/0004-6361/202141690