Why is niobium so revealing?

"Niobium and other elements heavier than iron are astronomically special because, unlike many common elements, they are not formed in the cores of stars by thermonuclear fusion," said Nicholas Stone, a theoretical astrophysicist at the University of Wisconsin–Madison and a co-author of the study.

"Instead, these heavy elements can only be synthesized in the exotic conditions that briefly emerge inside dying stars. The presence of niobium is a signpost of these 'death' throes, and the expulsion of the dying star's innards into space," Stone said.

The team's scenario runs like this. After the original star puffed off its outer layers, some of that chemically enriched debris coalesced into a gas giant planet. The rest of the disk has long since dispersed, but the planet remains.

What does the candidate planet look like?

NASA's Transiting Exoplanet Survey Satellite (TESS) monitored the white dwarf for four months. The spacecraft detected periodic brightness variations that suggest a gas giant roughly the size of Jupiter orbits the stellar remnant at about 3.7 million miles (6 million kilometers) — much closer than Mercury orbits the Sun.

The white dwarf is still relatively young and very hot, so it likely blasts the close-in planet and strips gas from its outer layers. The result could be a comet-like tail of material trailing the planet.

Lost gas would form a disk around the white dwarf and slowly rain down onto its surface, dumping the chemical signatures Hubble picked up. Researchers confirmed the Hubble observations with archival data from NASA's retired Far Ultraviolet Spectroscopic Explorer (FUSE) mission, which also showed strong niobium lines.

Is the planet likely to survive?

Williams thinks so. Eventually, the white dwarf will cool to a steady temperature. The planet would then sit in a stable habitable region for millions of years.

Still, the work is preliminary. Scientists have never directly imaged a second-generation world around a white dwarf, and the team has yet to rule out alternative explanations for the unusual chemistry.

Williams plans to keep using Hubble for several years to search for more such systems. The goal: build up statistics on how common — or rare — these unusual worlds might be.

Co-author Boris Gaensicke, also at the University of Warwick, captured the spirit of the work: "When Jamie asked me about niobium in relation to this study I was truly gobsmacked, as that element had not been reported in any other white dwarf analyzed to date. Once we realized it was there, everything fell into place."