Dying stars can still give birth to planets

Until now, astronomers thought that in most planetary systems, planets were born shortly after their host stars. This was certainly the case for the planets and moons of our Solar System, which formed around 4.6 billion years ago out of the disc of gas and dust spinning around our newborn Sun.

But now, a new study published in Astronomy & Astrophysics has found that dying stars can give birth to planets, too.

An international team of astronomers led by KU Leuven in Belgium made this discovery while looking at evolved binary stars in the Milky Way. Binary systems contain two stars orbiting around each other; it’s estimated that at least three-quarters of all stars are found in such pairs.

Previous studies found that protoplanetary discs – the debris out of which planets form – can be found in binary systems, around not just newborn stars but older stars too. For example, one of the stars could shed its outer layers as it reaches the end of its life, in the process of becoming a cold, dim stellar corpse called a white dwarf. The second star could then gravitationally suck in this ejected material to form a disc of matter around itself, resembling a protoplanetary disc.

Artist's illustration with labelled parts, showing a binary star, a gap, and then a disc of gas and dust
Credit: N. Stecki

This new study looked at 85 binary star systems with these discs. In ten of these pairs, they found signs of planetary formation.

“In 10% of the evolved binary stars with discs we studied, we see a large cavity (a void/opening) in the disc,” explains KU Leuven astronomer Jacques Kluska, first author of the paper. “This is an indication that something is floating around there that has collected all matter in the area of the cavity.”

Perhaps something like a planet.

This is how planets form around newborn stars: essentially, gas and dust collide to form small pebbles of material, that then stick together into rocks, then boulders, then eventually 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,” adds Kluska. “This observation leads one to suspect that dust particles rich in these elements were trapped by a planet.”

These observations will pave the way for new studies into potential planetary formation in binary systems – and if confirmed, we may need to tweak our theories of how solar systems are born.

On the left, a complete disc around a star. On the right, a gap between the star and the disc
A full disc (left) compared to a disc with a large cavity possibly made by planets (right). Credit: N. Stecki

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

So what next?

Observe the 10 binary pairs again – but with a bigger telescope. The team aim to get time on the telescopes of the European Southern Observatory, in Chile, to take a close look at these cavities.

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