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Planets may still be born from fading stars


Planets are often younger than the stars in which they orbit. Take the Sun, for example: it was created 4.6 billion years ago, and Earth was formed not long after. However, astronomers at KU Leuven have determined that another possibility is equally plausible. Some kinds of stars may still generate planets even if they are approaching death. If this is confirmed, theories about the formation of planets will have to be revised.

Planets like Earth, as well as the rest of our solar system’s planets, formed not long after the Sun. Our Sun began to burn 4.6 billion years ago, and the stuff surrounding it clumped into protoplanets during the following million years. The formation of the planets in that protoplanetary disc, a massive pancake of dust and gas with the Sun in the center, explains why they all circle in the same plane.

However, such dust and gas discs may not have to be found solely around nascent stars. They may also originate independently of star formation, such as in the vicinity of double stars, one of which is dying (binary stars are two stars that orbit each other, also called a binary system). When a medium-sized star (like the Sun) reaches the end of its life, it catapults the outer portion of its atmosphere into space, following which it slowly dies as a so-called white dwarf. The gravitational attraction of the second star, on the other hand, causes the matter expelled by the dying star to create a flat, revolving disc in binary stars. Furthermore, this disc closely mimics the protoplanetary discs seen surrounding newborn stars across the Milky Way.

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This was something we already knew. However, as observed by an international team of astronomers headed by KU Leuven researchers, the discs around so-called evolved binary stars often display indications that might hint to planet formation. Furthermore, their findings show that one out of every ten binary stars is like this. “We observe a significant cavity (a void/opening, ed.) in the disc in 10% of the evolved binary stars with discs we analyzed,” explains KU Leuven astronomer Jacques Kluska, first author of the paper in the journal Astronomy & Astrophysics in which the discovery is presented. “This is a sign that something is floating about there collecting all of the materials in the cavity region.”

Planets of the second generation

The matter cleanup might be the responsibility of a planet. That planet might have formed near the very end of one of the twin stars’ lives, rather than at the very beginning. Furthermore, the scientists discovered further compelling evidence for the existence of such worlds. “We discovered that heavy elements like iron were relatively sparse on the surface of the dying star in evolved binary stars with a wide hole in the disc,” Kluska explains. “This finding leads one to believe that a planet captured dust particles rich in these components.” By the way, the Leuven scientist does not rule out the potential of several planets forming around these binary stars in this manner.

Astronomers found the finding while compiling a list of evolved binary stars in our Milky Way galaxy. They did so based on data that was already publicly accessible. A total of 85 binary star pairings were discovered by Kluska and his colleagues. On 10 sets of infrared photos, the researchers discovered a disc with a big hollow.

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Current hypotheses are put to the test.

If fresh studies confirm the presence of planets orbiting evolved binary stars, and it turns out that the planets formed only after one of the stars had reached the end of its life, planet formation theories will need to be revised. According to Professor Hans Van Winckel, director of the KU Leuven Institute of Astronomy, “the confirmation or denial of this unusual process of planet creation will be an unparalleled challenge for the present ideas.”

The astronomers at KU Leuven are eager to test their idea. To this goal, they will examine the 10 pairs of binary stars whose discs exhibit a huge cavity using the European Southern Observatory’s massive telescopes in Chile.

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