Dying stars may still give birth to planets

If confirmed, it will change the prevailing theories about planet formation forever.

Planets are usually not much older than the stars they orbit. Take the sun. Our parent star was born 4.6 billion years ago, and not long after, Earth was born. However, astronomers make a surprising discovery in a new study. Because possibly even dying stars can still give birth to planets.

planet formation

After our sun started to ‘burn’, a flattened disk formed around the young protostar; the protoplanetary disk. Over the millions of years that follow, particles in the disk collide, causing these particles to clump together into increasingly larger objects. And so the planets in our solar system were born. The birth of planets in that protoplanetary disk—a giant pancake of dust and gas with the sun at its center—explains why they all revolve in the same plane.

double stars

But such disks of dust and gas don’t necessarily surround just newborn stars. They can also develop independently of star formation, for example around binary stars, one of which is dying. How does that work? As it nears its end for a medium-sized star (like our sun), it catapults the outermost part of its atmosphere into space, where it slowly dies. In the case of binary stars, however, the gravitational pull of the second star causes the matter ejected from the dying star to form a flat, rotating disk. And this disk is very similar to the protoplanetary disks that astronomers observe around young stars.

Birth around death

So far nothing new. But what researchers have now discovered is that the disks around so-called “evolved binary stars” often show signs of planet formation. This even turns out to be the case for one in ten of these double stars. “Ten percent of the evolved binary stars we’ve studied have a large cavity in the disk,” said researcher Jacques Kluska. “This betrays that something is floating around there that has scraped together all the matter in the region of the cavity.”

Discs surrounding so-called evolved binary stars often show signs of planet formation. Image: N. Stecki

The researchers hypothesize that this could well be the work of a protoplanet. So that planet may not have formed at the very beginning of the life of the double stars, but precisely at the end.

Iron

The astronomers also found strong indications for the presence of such planets. “We saw that heavy elements, such as iron, were very scarce on the surface of a dying star,” said Kluska. “This suggests that dust particles rich in those elements have been trapped by a planet.” The Leuven astronomer does not rule out the possibility that several planets could form around these double stars in this way.

To confirm
The results from the study are very interesting. For if new observations confirm the existence of planets around evolved binary stars – and so it turns out that the observed planets did not form until after one of the stars had reached the end of its life – the prevailing theories about planet formation will 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 researcher Hans van Winckel.

The astronomers will soon want to verify their hypothesis themselves. To do this, they will take a closer look at the ten pairs of binary stars whose disks have a large cavity with large telescopes from the European Southern Observatory in Chile.