A planet orbiting extremely close to a white dwarf may have formed inside its star – this could be the origin of some of the most promising worlds beyond our solar system to search for life.
A distant world may have formed inside a doomed star. This star was ripped apart by its smaller, denser companion – a dense stellar corpse called a white dwarf – and the planet could have formed from the star’s remains. These planets or their moons could be some of the most promising places beyond our solar system to search for life.
 |
| A Jupiter-sized planet orbits very close to a white dwarf Goddard Space Flight Center/NASA |
“I never expected that it would be possible to form a planet inside a star,” says Jason Nordhaus at the Rochester Institute of Technology in New York. He and his colleagues tested this unlikely idea using models of a planet called WD 1856+534 b, which orbits a white dwarf about 80 light years from Earth. The planet is about the size of Jupiter, but resides extraordinarily close to its star – just 2 per cent Earth’s distance from the sun.
Usually, planets form in orbit around their star from the same disc of dust that creates the star itself, as they did in our own solar system. But this process cannot create a world so close to a white dwarf, because its gravitational pull is so intense that it would rip a young planet apart.
However, many white dwarfs form in binary systems where they and another larger star orbit one another, so the researchers posited that the death of this secondary star may have led to the birth of the Jupiter-sized planet. Their modelling showed that if the star was just the right size, a bit smaller than the sun, the white dwarf and the star would come close enough together that the white dwarf’s orbit would end up entirely inside the star. As the dense white dwarf made its way around the diffuse outer edges of its companion star, it would slowly suck up the star’s plasma.
The white dwarf’s spin would also generate a disc of material around it called an accretion disc. This process would begin to blow away the wispy outer layers of gas surrounding the pair of stars. The planet could then form from the accretion disc – not dissimilar to how regular planets form – like a phoenix rising from the ashes of a devoured star. The left-over gas would be swept away by the energy from the motion of the white dwarf and the planet.
The ideas behind this mechanism aren’t entirely new, says Philipp Podsiadlowski at the University of Oxford, but in the past they have been mostly applied to neutron stars rather than white dwarfs because the latter do not tend to have nearby planets. “There have been various claims [of exoplanets] around white dwarfs, and it has mostly turned out that these were not planets, they were just oscillations of the star,” says Podsiadlowski. “This is a much stronger case.”
Unfortunately, while the evidence for the planet itself is strong, proof that it formed from its dying star, which would make it a second-generation planet, is nearly impossible to find. There could be slight hints in the amounts of various elements inside the planet, but we have no instrument precise enough to measure them.
Planets near white dwarfs are particularly interesting because these stellar corpses are relatively cool, making their habitable zones – the “Goldilocks region” around a star where the temperature is just right for liquid water on an orbiting planet – very close in. “In principle, this planet is sitting in the habitable zone, although it is a gas giant – but it could have potentially habitable moons,” says Nordhaus. White dwarfs tend not to be very active, so WD 1856+534 b could remain in a stable orbit for many billions of years, making worlds like this even more enticing for their potential to host life.
Reference: