Hubble data hint a dead star may be hosting a second-generation planet

Niobium, a heavy metal never before found in a white dwarf, shows up in the atmosphere of HS 0209+0832 at more than 1,000 times the Sun’s level. That oddity, found in an old Hubble Space Telescope spectrum, is the main evidence that the star may host a planet born after the star itself died.

The planet is a candidate. It has not been imaged directly; it is inferred from the chemistry and from a brightness signal, and the team that proposed it calls the explanation the most likely one, not a settled fact. The paper itself carries the word candidate in its title.

Hubble observed the star in 1999, and NASA’s account of the work says the spectrum contained roughly 100 features no one could identify. Jamie Williams, a doctoral candidate at the University of Warwick in England, returned to the archived data with an updated chemical database and found that niobium matched many of them. NASA’s shorter piece, published October 6, frames it as a cold case solved, and says the sleuthing uncovered a chemical clue indicating the white dwarf may host a second-generation planet.

Niobium as the fingerprint

A white dwarf is the dead core of a star that has burned through its nuclear fuel and shed its outer envelope of gas and dust. NASA’s artist’s concept, credited to NASA, ESA and Leah Hustak of the Space Telescope Science Institute, shows a gaseous planet in silhouette against a small white star with a thin, bright disk. A planet formed from that cast-off material would carry different chemistry from one that grew up beside a living star, and niobium is the telltale. NASA describes it as a heavy element that forms in dying stars, not through fusion in stellar cores. The University of Warwick, which led the team, reports that the white dwarf’s atmosphere also holds zinc and copper, that niobium has never before been found in a white dwarf, and that the pattern matches the s-process, a nuclear reaction that builds heavy elements in dying stars.

The spectrum shows other marks too. NASA says it has dips for nickel, which is much dimmer than the model predicts, and for calcium, which is slightly dimmer. Williams says it is a bit like finding a planet that has risen from the ashes of the very star it once orbited, as quoted in the University of Warwick’s press release. Dr Nicholas Stone of the University of Wisconsin-Madison put the chemistry plainly: it is a signature no ordinary, first-generation planet should carry.

TESS and the 4.4-day signal

Chemistry alone could not pin down a planet, so the team went looking for a brightness change in the star’s light. NASA’s TESS satellite watched the star for four months and detected a faint, regular signal repeating every 4.4 days, which Warwick says is consistent with a tidally locked planet in a tight orbit. NASA’s retired FUSE mission, an ultraviolet observatory, confirmed strong niobium signatures independently of the Hubble data.

The candidate is a gas giant about the size of Jupiter, circling about 3.7 million miles (6 million kilometers) from the white dwarf, closer than Mercury orbits the Sun. NASA says it is rapidly losing its atmosphere and probably trails a comet-like tail of material that falls back onto the star. Science News reports the orbit as about 4 percent of the Earth-Sun distance and says the 4.4-day period is too slow to be the white dwarf’s own rotation.

Candidate status and the rarity argument

The result appears in Nature Astronomy as “Discovery of a second-generation planet candidate accreting onto a white dwarf”, and the word candidate is doing real work. Williams told Science News the team is not really sure what it would be if it is not a planet, which concedes that other explanations are not ruled out. Zifan Lin, an independent scientist, called it the first evidence for this kind of planet formation, which had been mostly theoretical.

NASA adds that the team thinks the planet is likely to survive, and that Williams suggests it could eventually sit in a stable habitable zone once the white dwarf cools. That is speculation about the far future, flagged as such, and it rests on the planet being real. The team also offers an account of why such planets should be rare. The disc of ejected material probably needed a companion star to pull it back into orbit, Warwick says. Boris Gänsicke, a Warwick physics professor and European Research Council grantee, said the system has given birth to a new world using the foundations of the old one.

Everything now rests on whether the 4.4-day brightness signal holds up, since that is the one piece of evidence that does not come from the star’s chemistry.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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