Scientists from the Rochester Institute of Technology in New York have discovered evidence of a distant planet forming within a dying star. According to the researchers, the star was torn apart by its smaller, denser companion—a white dwarf, which is a dense stellar remnant. The planet is thought to have formed from the remnants of the disintegrated star. New Scientist reported that these newly-formed planets or their moons could be among the most promising places beyond our solar system to search for life.
“I never expected that it would be possible to form a planet… inside a star,” said Jason Nordhaus of the Rochester Institute of Technology in New York. He and his team explored this unexpected possibility using models of a planet named WD 1856+534 b, which orbits a white dwarf about 80 light-years from Earth. The planet is roughly the size of Jupiter but orbits extremely close to its star—only 2% of the distance between Earth and the Sun.
Typically, planets form from a disc of dust surrounding a star, the same disc that creates the star itself, as seen in our solar system. However, this process cannot produce a planet so close to a white dwarf, as the star's intense gravitational pull would destroy a young planet.
Many white dwarfs are part of binary systems, where they orbit another larger star. The researchers theorized that the death of the secondary star in such a system could lead to the birth of a Jupiter-sized planet. Their modelling indicated that if the secondary star was just the right size, slightly smaller than the Sun, the white dwarf's orbit would eventually move entirely inside the larger star. As the dense white dwarf orbited through the outer layers of the larger star, it would gradually consume the star's plasma.
This process would also cause the white dwarf to generate a disc of material around it, known as an accretion disc. As the white dwarf continued its orbit, it would begin to blow away the star's outer layers of gas. The planet could then form from this accretion disc, much like planets form from a disc of dust around young stars, rising from the remains of the consumed star. The residual gas would be dispersed by the white dwarf's energy and the forming planet.
While the concept isn't entirely new, as similar ideas have been applied to neutron stars, it is more convincing in the case of white dwarfs, which typically do not have nearby planets. Philipp Podsiadlowski of the University of Oxford noted, “There have been various claims [of exoplanets] around white dwarfs, but they mostly turned out to be just oscillations of the star, not planets. This is a much stronger case.”
Unfortunately, while there is strong evidence for the planet's existence, proving that it formed from its dying star—and thus would be a second-generation planet—remains nearly impossible. Detecting slight variations in the elements within the planet could provide clues, but current instruments lack the precision to make such measurements.
Planets orbiting near white dwarfs are of particular interest because these stellar remnants are relatively cool, placing their habitable zones—the "Goldilocks region" where conditions might allow liquid water—very close to the star. “In principle, this planet is sitting in the habitable zone, although it is a gas giant—but it could have potentially habitable moons,” said Nordhaus. Given that white dwarfs tend to be stable and inactive, WD 1856+534 b could maintain its orbit for billions of years, making such worlds particularly intriguing in the search for life.
