Remains of a Planet Found Orbiting Dead Star

Solar System
This planetary fragment is orbiting around the white dwarf among the debris from the earlier solar system. The image is an artistic impression. Illustration: University of Warwick/Mark Garlick

Astronomers have discovered what appears to be the remnants of a planet orbiting a dead star in a disc of debris formed from destroyed planets. The planetary fragment could offer clues into the fate of our own Solar System in the far-off future.

According to a new study published in Science, researchers have found a small celestial body, a so-called planetesimal, orbiting a white dwarf approximately 400 light years from Earth. Like asteroids or comets, planetesimals are composed of stone or ice and are anything from a few kilometres to a few hundred kilometres in size. They can be either building blocks for future planet formation, or remnants of a previous planet that has broken apart or been in a collision.

“This is only the second planetesimal we know of that orbits around a white dwarf”, says Alexander Mustill, astronomy researcher at Lund University and one of the authors of the study. 

The fact that the newly discovered planetary fragment orbits around a white dwarf is particularly interesting, as it can increase our knowledge of older solar systems. When a star runs out of fuel it transforms into a white dwarf; but what happens to any planets around this sun? According to previous theoretical calculations by astronomers, planets in such a solar system could very well continue to exist and orbit around the white dwarf, depending on the conditions. 

In the research community, it is therefore worth focussing on the universe’s white dwarves in the search for concrete evidence of these kinds of events, that is, to find out what happens to the orbiting planets when their star dies.  Our own star, the sun, will suffer the same fate in the future and become a white dwarf – luckily, in a relatively distant future, or approximately six billion years. 

According to observations, the newly discovered planetesimal has an orbital period of just two hours, which means it is orbiting very close to its white dwarf. This indicates that the small planetary fragment must have a very high density, that is, it is composed of very compact material. Otherwise, it would not be able to circulate so close to the white dwarf and would instead be torn apart by the dying star’s enormous gravity.  

“It is likely this planetesimal is composed mainly of iron”, says Melvyn Davies, professor of astronomy at Lund University and co-author of the study.

The results from the current study surprise the researchers. The extremely short orbital period, the high density and the supposed high-iron content raise many questions. Why does the observed planetesimal not contain other materials like many of the planets and asteroids in our own solar system? Why is it similar in composition to the earth’s iron core? 

“In the same way archaeologists can learn about history by digging in the ground, astronomers can learn about planets around dead stars by studying planetesimals that orbit white dwarves”, concludes Alexander Mustill.

The current study was carried out in collaboration with a large number of researchers in several countries. The observation work was conducted at Gran Telescopio Canarias on the island of La Palma.