In a cosmic first, scientists have detected a planet that may have formed from the burned remains of its dead host star, reports BritPanorama.
The planet is likely a gas giant orbiting a white dwarf called HS 0209+0832 located about 270 light-years from Earth, according to the authors of a study published Monday in the journal Nature Astronomy. After analyzing observations from NASA’s Hubble Space Telescope and other instruments, the team found chemical evidence suggesting that the planet could have condensed from material expelled by the star as it fizzled out of energy and collapsed into a white dwarf.
When a sunlike star dies, it first rapidly expands into a red giant and then sheds its outer layers, leaving behind a smaller but dense, hot core—known as a white dwarf. Some of the first-generation planets, which originally formed alongside the star, can survive this cataclysmic event if their orbit is wide enough. Astronomers have observed a handful of such survivors revolving around white dwarfs, but this is the first recorded instance of a so-called second-generation planet—an entirely new world made from a dead star’s debris.
The telltale sign that HS 0209+0832 might have birthed a new planet is unusual traces of heavy elements on the white dwarf’s surface that the study authors believe is planetary material raining down onto the leftover core. “This planetary material is very rich in an element called niobium,” said lead study author Jamie Williams, a doctoral student in the department of physics at the University of Warwick in England. “It’s the first time that this element is found in a white dwarf, and this implies that the planetary material is made from the ashes of the star as it died.”
Further analysis from NASA’s Transiting Exoplanet Survey Satellite (TESS) revealed a faint brightness signal repeating every 4.4 days—evidence consistent with a giant planet orbiting the white dwarf. “What’s interesting about planets orbiting close to white dwarfs is that because white dwarfs cool over time, their habitable zone is very stable. A second-generation planet could form and then be in the habitable zone for tens of billions of years,” Williams noted.
However, validation of the second-generation planet discovery is still pending. “It’s not a confirmed planet,” Williams stated. “It’s only a candidate for now.”
A new class of planet
After the red giant phase, a dying star becomes a nuclear furnace that can produce various elements. However, once that fuel runs out and the star transforms into a white dwarf, the heavier elements—including niobium—sink rapidly, leaving only lighter elements like hydrogen and helium on the surface. The researchers determined that the niobium detected on HS 0209+0832 must therefore come from the star’s surroundings. “We think these elements fell onto the white dwarf’s surface because the white dwarf is very hot and emitting loads of extreme ultraviolet radiation, which is stripping the atmosphere of a nearby planet,” Williams explained.
The concept of second-generation planets has been around since astronomers first identified exoplanets, noted coauthor David J. Wilson, a research associate at the Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder. “Although white dwarfs don’t form via supernova, they are surrounded by debris,” he said. “You have the planets that were lucky enough to survive the star’s giant phases, the shattered remains of those that weren’t, and leftover gas and dust ejected by the star as it turned into a white dwarf. So it’s a compelling idea that all that stuff might coalesce into new planets.”
Astronomers are still piecing together how first-generation planets form, which means the mechanisms behind this newly reported second-generation planet remain uncertain. Williams speculates that the planet-forming process might have resulted from a collision between the dying star and a second celestial body. “There could have been another object close to the core of the star, maybe a star around 20% the mass of the sun, or a brown dwarf,” he suggested, referring to a class of cool objects that lie between a star and a planet.
If a second object fell into the star during its giant phase, it could have prevented all the dust and gas from dispersing into a massive cloud, causing some to spin in a disk. This disk could then have rotated around the white dwarf, facilitating planet formation. “In a regular white dwarf there’s no disk, because the star’s material will just be ejected outwards during the red giant phase,” Williams added.
The researchers plan further observations of the system over the next year using Hubble and NASA’s Chandra X-ray Observatory. They have also requested time with the more powerful James Webb Space Telescope, which could provide more clues about the existence or features of the planet.
If confirmed, this distant world would belong to an entirely new planetary class and suggest that more second-generation planets could exist around white dwarfs, according to Williams. The implications for our solar system are profound, as Wilson noted: “The sun will eventually become a white dwarf, so we’re also looking at the future of the solar system here—maybe the Sun will get a new planet someday!”
The afterlives of exoplanets
The new discovery of a potential second-generation planet is incredibly exciting, said Sarah Casewell, a lecturer at the School of Physics and Astronomy at the University of Leicester, England, who was not involved in the study. “We know of a large number of white dwarfs that are polluted by planetary material similar in composition to rocks within our solar system,” she wrote in an email. “However, in this case, the white dwarf is polluted by incredibly unusual material, and the composition of this material is similar to atoms we see being created as stars end their lives.”
With over 95% of all stars in the universe expected to eventually become white dwarfs, astronomers have cataloged hundreds of thousands of these stars. A finding of a planet formed after the white dwarf would imply that second-generation planets around other white dwarfs are possible—an exciting prospect considering the fate of our own solar system after the sun dies, Casewell remarked.
This new evidence will likely shift our understanding of how planetary systems can evolve, even after the death of their host stars, as outlined by Susan Mullally, a mission scientist at the Space Telescope Science Institute, the operations center for Hubble. “If second-generation planets are possible, then many more white dwarf stars may have planets than we may otherwise expect,” she observed.
