Even dying stars can still give birth to planets

Date:
February 1, 2022
Source:
KU Leuven
Summary:
Planets are usually not much older than the stars around which
they revolve. Take the Sun: it was born 4.6 billion years ago, and
not long after that, Earth came into the world. But astronomers
have discovered that a completely different scenario is also
possible. Even if they are near death, some types of stars can
possibly still form planets. If this is confirmed, theories on
planet formation will need to be adjusted.



FULL STORY ========================================================================== Planets are usually not much older than the stars around which they
revolve.

Take the Sun: it was born 4.6 billion years ago, and not long after that,
Earth came into the world. But KU Leuven astronomers have discovered that
a completely different scenario is also possible. Even if they are near
death, some types of stars can possibly still form planets. If this is confirmed, theories on planet formation will need to be adjusted.


========================================================================== Planets such as Earth, and all other planets in our solar system, were
formed not long after the Sun. Our Sun started to burn 4.6 billion years
ago, and in the next million years, the matter around it clumped into protoplanets. The birth of the planets in that protoplanetary disc,
a gigantic pancake made of dust and gas, so to speak, with the Sun in
the middle, explains why they all orbit in the same plane.

But such discs of dust and gas needn't necessarily only surround 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 that orbit each other, also called a binary system). When the end approaches
for a medium-sized star (like the Sun), it catapults the outer part of
its atmosphere into space, after which it slowly dies out as a so-called
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.

This we already knew. However, what is new is that the discs surrounding
so- called evolved binary stars not uncommonly show signs that could
point to planet formation, as discovered by an international team of astronomers led by KU Leuven researchers. What's more, their observations
show that this is the case for one in ten of these binary stars. "In ten
per cent of the evolved binary stars with discs we studied, we see a large cavity (a void/opening, ed.) in the disc," says KU Leuven astronomer
Jacques Kluska, 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 planets The clean-up of the
matter could be the work of a planet. That planet might not have formed
at the very beginning of one of the binary stars' life, but at the very
end. The astronomers moreover found further strong 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," says Kluska. "This observation
leads one to suspect that dust particles rich in these elements were
trapped by a planet." By the way, the Leuven astronomer doesn't rule out
the possibility that in this way, several planets can be formed around
these binary stars.

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

Current theories put to the test If new observations confirm the existence
of planets around evolved binary stars, and if it turns out the planets
were only formed after one of the stars had reached the end of its
life, the theories on planet formation will need to be adjusted. "The confirmation or refutation of this extraordinary way of planet formation
will be an unprecedented test for the current theories," according to
Professor Hans Van Winckel, head of the KU Leuven Institute of Astronomy.

The KU Leuven astronomers soon want to verify their hypothesis
themselves. To this end, they will use the big telescopes of the European Southern Observatory in Chile to take a closer look at the ten pairs of
binary stars whose discs show a large cavity.

========================================================================== Story Source: Materials provided by KU_Leuven. Note: Content may be
edited for style and length.


========================================================================== Journal Reference:
1. J. Kluska, H. Van Winckel, Q. Coppe'e, G.-M. Oomen, K. Dsilva,
D. Kamath,
V. Bujarrabal, M. Min. A population of transition disks around
evolved stars: Fingerprints of planets. Astronomy & Astrophysics,
2022; 658: A36 DOI: 10.1051/0004-6361/202141690 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220201074533.htm

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