Astronomers question whether dying stars can still cause planet formation

Astronomers have discovered that stars, even when near death, can potentially still cause planet formation.

Planets are typically not a lot older than the stars in which they revolve around. For example, the Sun was born 4.6 bn years ago, and not long after that the Earth was formed.

However, KU Leuven astronomers have discovered that a completely different scenario could also be possible. Even if a star is near to death, some types of stars can still possibly form planets, suggesting that theories on planet formation will need to be adjusted.

Planets, such as Earth, were formed not long after the Sun. Our Sun started to burn 4.6 bn years ago, and in the next million years, the matter around it was clumped into protoplanets. The birth of the planets in that protoplanetary disc, with the Sun in the middle, explains why they all orbit the same plane.

Binary stars

However, such discs of dust and gas need not necessarily surround only newborn stars. They can also develop independently from star formation, for instance around binary stars of which one is dying. When 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 develops into a white dwarf.

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 disc. Moreover, this disc strongly resembles the protoplanetary discs that astronomers observe around young stars elsewhere in the Milky Way.

Although, an international team of astronomers led by KU Leuven researchers discovered that the discs surrounding evolved binary stars display signs that could point to planet formation. What is more, their observations highlights that this is the case for one in 10 of these binary stars.

“In 10% of the evolved binary stars with discs that we studied, we can see a large cavity in the disc,” explained Jacques Kluska, KU Leuven astronomer and first author of the article in the journal Astronomy & Astrophysics in which the discovery is described. “This is an indication that something is floating around there that has collected all matter in the area of the cavity.”

Second-generation planet

The clean-up of the matter could be the work of a planet, indicating that the planet might not have formed at the very beginning of one of the binary stars’ life, but instead at the end. The astronomers moreover observed further indications for the presence of such planets.

“In the evolved binary stars with a large cavity in the disc, we saw that heavy elements such as iron were very scarce on the surface of the dying star,” commented Kluska. “This observation leads one to suspect that dust particles rich in these elements were trapped by a planet.”

The discovery was made when the astronomers were drawing up an inventory of evolved binary stars in our Milky Way. They carried out this discovery and formed observations based on existing observations that were publicly available. Kluska and his colleagues counted 85 of such binary star pairs. In 10 pairs, the researchers came across a disc with a large cavity on the infrared images.

Current theories of planet formation

If these new observations confirm the existence of planets around evolved binary stars, and if the planets were only formed after one of the stars had reached the end of its life, then the astronomers suggest that theories on planet formation should be adjusted.

“The confirmation or refutation of this extraordinary way of planet formation will be an unprecedented test for the current theories,” concluded Professor Hans Van Winckel, head of the KU Leuven Institute of Astronomy.

The KU Leuven astronomers intend to verify their hypothesis themselves; they will utilise the telescopes of the European Southern Observatory in Chile to take a closer look at the ten pairs of binary stars whose discs exhibit a large cavity.

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