Study says stars can form planets even when they are dying
Research by astronomers at the Catholic University of Leuven in Belgium suggests that even close to death, some types of stars can still form planets. If this is confirmed, theories about the formation of planets will probably have to be adjusted.
Worlds like the Earth, and all the others that are part of our solar system, were formed shortly after the Sun. Our "star-king" began to burn 4.6 billion years ago, and in the following millions of years, the matter around it turned into protoplanets.
The birth of the planets in that protoplanetary disk, a gigantic 'pancake' made of dust and gas with the Sun in the middle, explains why they all orbit in the same plane.
However, such disks of dust and gas need not necessarily surround only newborn stars. They can also develop independently from star formation, for example around binary stars of which one is dying – binary stars are two stars orbiting each other, also called a binary system.
Binary stars form disks similar to protoplanetaries
As the end approaches for a medium-sized star (such as the Sun), it catapults the outer part of its atmosphere into space, after which it slowly dies, like white dwarfs.
However, in the case of binary stars, the gravitational pull of the second star causes the matter ejected by the dying star to form a flat, rotating disk. Furthermore, this disk strongly resembles the protoplanetary disks that astronomers observe around young stars elsewhere in the Milky Way.
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According to the researchers, the disks around the so-called evolved binary stars do not show signs that could point to the formation of planets. Furthermore, their observations show that this is the case for one in ten of these binary stars.
“In 10% of the disk-evolved binary stars that we study, we see a large cavity in the disk,” says astronomer Jacques Kluska, first author of the paper describing the research and published in the journal Astronomy & Astrophysics. "This is an indication that something floating around there has collected all the matter in the cavity area."
And this collection could be the work of a planet that may not have formed early in the life of one of the binary stars, but at the end. In addition, astronomers have found stronger indications of the presence of such planets. “In the evolved binary stars with a large cavity in the disk, we saw that heavy elements like iron were very scarce on the surface of the dying star,” says Kluska. “This observation leads to the suspicion that dust particles rich in these elements were fixed by a planet”, added Kluska, who does not rule out the possibility that, in this way, several planets could be formed around these binary stars.
Milky Way Binary Star Inventory
The discovery was made when astronomers were drawing up an inventory of evolved binary stars in the Milky Way. They did this based on existing and publicly available observations. Kluska and his colleagues counted 85 of these binary star pairs. In ten pairs, they identified a disk with a large cavity in the infrared images.
The astronomers involved in the discovery say it will be useful for further investigations, but say they want to verify their hypotheses themselves. To do this, they will use the large telescopes at the European Southern Observatory in Chile to take a closer look at the ten pairs of binary stars whose disks show a large cavity.
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