Hubble cold case: Second-generation planet found around white dwarf star HS 0209+0832

Astronomers have cracked a 25-year-old cold case in the Hubble Space Telescope archive, identifying a chemical clue that points to a second-generation planet orbiting the white dwarf star HS 0209+0832. The findings, published Monday in Nature Astronomy, show that the system may host a gas giant about the size of Jupiter—a world formed not from a star’s birth but from its cast-off material as it died.

A white dwarf is the dense core left behind when a low-mass star burns through its fuel and sheds its outer layers. A second-generation planet is one that assembles from that ejected debris, rather than from the leftover disk of a star’s formation. Earth and the other planets in our solar system are first-generation worlds.

Lead author Jamie Williams, an astronomer and doctoral candidate at the University of Warwick in the United Kingdom, was combing through old Hubble data when something caught his eye. The 1999 observations of HS 0209+0832 contained about 100 chemical features that could not be identified at the time. Armed with an updated database, Williams matched many of those mystery features to niobium—a heavy element that, unlike common elements forged in stellar cores, is only produced in the exotic conditions inside dying stars.

“Rather than the white dwarf stage being a kind of epilogue to the story of a star and its planets, this research points to the systems we are familiar with only being the first chapter of a potentially much longer tale, with some new characters showing up,” Williams said in a statement. “That’s a really exciting prospect to pursue.” The research team, which includes co-author Boris Gaensicke also at the University of Warwick, confirmed the Hubble findings with data from NASA’s retired FUSE (Far Ultraviolet Spectroscopic Explorer) mission, which also showed strong signatures of niobium in the system. NASA’s TESS (Transiting Exoplanet Survey Satellite) observed the white dwarf for four months and detected periodic brightness variations suggesting a planet orbits at about 3.7 million miles (6 million kilometers)—much closer than Mercury is to the Sun.

Because the white dwarf is still young and incredibly hot, it is likely blasting the planet with energy that strips its outer atmosphere, possibly creating a comet-like tail of material that falls back onto the star’s surface. That would explain how Hubble detected niobium when studying the star. Despite the mass loss, Williams said the planet is probably not a temporary feature.

“If the second-generation planet is there, I think it is likely to survive. Eventually the white dwarf will cool and then maintain a consistent temperature, with the planet in its stable habitable zone for millions of years,” Williams said.

The team plans to use Hubble in the coming years to study how second-generation planets form, how common they are, and how they evolve around “dead” stars. Co-author Nicholas Stone, a theoretical astrophysicist at the University of Wisconsin–Madison, noted that the presence of niobium is a signpost of a dying star’s death throes and the expulsion of its innards into space, adding that the discovery was a moment of “truly gobsmacked” realization when the pieces fell into place.

• HS 0209+0832
• Hubble Space Telescope
• NASA FUSE
• niobium
• second-generation planet
• TESS
• University of Warwick
• white dwarf