Hubble Finds Clues That a White Dwarf May Host a Second-Generation Planet

Astronomers reviewing decades-old observations from NASA’s Hubble Space Telescope have uncovered new clues in a long-running cosmic cold case. Unusual chemical signatures suggest that the white dwarf star HS 0209+0832 may host a second-generation planet, according to a study published Oct. 5 in Nature Astronomy.

A white dwarf is the remaining core of a low-mass star that has exhausted its nuclear fuel and expelled its outer layer of gas and dust into space. Unlike ordinary planets, which form around young stars, second-generation planets may form later from material ejected by a dying star.

“Rather than the white dwarf phase being a kind of epilogue to the story of stars and their planets, this study shows that the star systems we know so well are just the first chapter of a potentially much longer story, with some new characters introduced. That’s a very exciting prospect to pursue,” said astronomer and first author Jamie Williams, a PhD candidate at the University of Warwick in the UK.

Earth and the other worlds in our solar system are first-generation planets. They formed from material left behind when the Sun was born.

Hubble Detects an Unusual Niobium Signature

Hubble observed HS 0209+0832 in 1999, but scientists were unable to identify about 100 chemical signatures in the data at the time. Williams revisited the archival observations using modern chemical databases and found that niobium explained many of the previously unidentified features.

“What Hubble is showing us in this white dwarf system is something we’ve never seen before: the presence of the element niobium in large quantities, a feature I wasn’t familiar with when I first found it in archival data,” Williams said.

Niobium exists in the solar system and is used on Earth in applications including jewelry and medical imaging equipment. However, the unusually large number of niobium detections in the HS 0209+0832 system may point to a very different origin. Instead of coming from material associated with a star’s birth, the element may trace material expelled during a star’s death.

“Niobium and other elements heavier than iron are astronomically special because, unlike many common elements, they do not form in the centers of stars through thermonuclear fusion,” said Nicholas Stone, a theoretical astrophysicist at the University of Wisconsin-Madison and a member of the research team. “Instead, these heavy elements can only be synthesized under unusual conditions, where they appear temporarily inside a dying star. The presence of niobium is a signpost to the throes of these ‘deaths’ and the ejection of the dying star’s interior into space.”

Researchers propose that after a star ejected this chemically concentrated material, some of it collected and eventually formed a gas giant planet. Most of the remaining material would have dispersed into space, while the planet remained in orbit around the white dwarf.

“When Jamie asked me about niobium in relation to this study, I was really taken aback, because that element had not been reported in any other white dwarf stars analyzed so far. Once I realized it was there, everything fell into place,” said Boris Gencik, an astronomer and study co-author from the University of Warwick.

NASA Missions Provide Additional Evidence

The research team compared Hubble’s findings with observations from NASA’s retired FUSE, or Far Ultraviolet Spectroscopy Explorer, mission. The FUSE data also showed a strong niobium signature in the HS 0209+0832 system.

NASA’s Transiting Exoplanet Survey Satellite, or TESS, provided another important clue. TESS monitored the white dwarf for four months and detected repeated changes in its brightness. These fluctuations indicate that the candidate planet may orbit about 3.7 million miles (6 million kilometers) from its star—much closer than Mercury orbits the Sun.

Researchers estimate that the candidate world is a gas giant approximately the size of Jupiter. Its atmosphere also appears to be rapidly disappearing.

A Gas Giant Losing Its Atmosphere

Because the white dwarf formed relatively recently, it remains extremely hot. Intense energy from stellar debris may be stripping material from the planet’s outer atmosphere. The escaping gas could form a comet-like tail and create a disk around the white dwarf before some of the material falls onto the star’s surface. This process could explain why Hubble detected niobium while observing the system.

Despite the continuing loss of atmosphere, Williams does not expect the potential planet to disappear soon.

“If there’s a second-generation planet out there, I think it’s likely to survive. Eventually the white dwarf will cool down and maintain a constant temperature, and the planet will remain in a stable habitable zone for millions of years,” Williams said.

Searching for Planets Born After a Star Dies

Many questions remain about second-generation planetary systems. Scientists do not yet know how these planets form, whether they are common or exceptionally rare, or how they change while orbiting a white dwarf.

Williams plans to continue using Hubble over the next few years to investigate these questions and collect enough observations to begin building meaningful statistics about this unusual class of objects.

“I think this study is an important example of the fact that scientific discovery is not a straight path,” Gaensicke said. “It often takes a magical moment when people talk about the big questions in their hearts and realize that together they can find unexpected answers.”