KU Leuven: Dying stars can still produce planets

Planets like Earth, as well as all the others in our solar system, formed not long after the sun. It started burning 4.6 billion years ago, and over the millions of years that followed, the matter around it clumped together into protoplanets. The birth of the planets in that protoplanetary disk, a gigantic pancake of dust and gas with the sun in the middle, explains why they all revolve in the same plane.

But such disks of dust and gas don’t necessarily have to hang around newborn stars. They can also form completely separate from star formation, for example around binary stars of which one is dying (binary stars are two stars that revolve around their common center of gravity, also called a binary system). As a medium-sized star (such as the Sun) nears its end, it catapults the outermost part of its atmosphere into space, after which it slowly fades out as a so-called white dwarf. In binary stars, however, the gravitational gravitational pull of the living star forces the matter ejected from the dying star into a flat rotating disk. This disk of matter also closely resembles the protoplanetary disks observed by astronomers around young stars elsewhere in the Milky Way.

That was all already known. What is new, however, is that the discs of matter around such so-called evolved binary stars often show fingerprints of, yes, planet formation. This has been discovered by an international team of astronomers led by researchers from KU Leuven. In fact, their observations indicate that this is the case with one in ten of these binary stars. “In ten percent of evolved binary stars we see a large cavity (a gap/opening, ed.) in the disc of matter”, says KU Leuven astronomer Jacques Kluska, first author of the article in the journal Astronomy & Astrophysics, in which the discovery is described. “That indicates that something is floating around there that has gathered all the matter at the height of the cavity.”

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Second Generation Planets
The clearing of matter may be the work of a planet. It might not have formed at the very beginning of the life of one of the double stars, but at the very end of it. The astronomers also found strong evidence for the formation of such second-generation planets. “In the evolved binary stars with a large disc of matter cavity, we saw that heavy elements such as iron are very scarce in the outer regions of the binary stars,” says Kluska. “That strongly suggests that matter particles rich in these elements were scavenged by a planet.” The Leuven astronomer does not rule out the possibility that several planets could form in this way in this type of binary star.

In the evolved binary stars with a large cavity in the matter disk, we saw that heavy elements such as iron are very scarce in their outer regions. This strongly suggests that these matter particles were captured by a planet.

– astronomer Jacques Kluska

The discovery came during an inventory by astronomers of evolved binary stars in our Milky Way. They did so on the basis of existing, publicly available observations. Kluska and his colleagues turfed 85 such binary star pairs. In ten pairs, the researchers found a disk of matter with a large cavity on infrared images.

Existing theories to the test
If the existence of second-generation planets around evolved binary stars is soon confirmed by new observations, and if it turns out that they only formed after one of the stars had already reached the end of its life, then the theories about planet formation have to be adjusted. “The confirmation or refutation of this very special way of planet formation will be an unprecedented test for current theories,” said Professor Hans Van Winckel, head of the Institute of Astronomy at KU Leuven.

The Leuven astronomers want to verify their hypothesis themselves. To do this, they will take a closer look at the ten binary star pairs whose disc of matter shows a large cavity with the large telescopes of the European Southern Observatory in Chile.

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