A Dead Star May Be Eating A Planet Born From Its Own Remains

Strange Metals Around a White Dwarf Point to a Planet Born After Its Star Died

A dead star 270 light-years from Earth appears to be feeding on a planet that did not exist until the star itself died, swallowing the gas boiling off it. If the idea holds up, it would be the first strong evidence of a planet forming around a white dwarf after the star’s death rather than before it, built from wreckage the star blew into space during its final years.

Called HS 0209+0832, the star is a white dwarf, the collapsed core left behind when a sun-like star runs out of fuel. The sun itself will meet the same fate in billions of years. Many white dwarfs are polluted by debris from broken-up planets, and that debris generally resembles solar-system material. This one looks nothing like that.

Its atmosphere is loaded with unusual metals, including zinc, copper and niobium. Niobium is more than a thousand times more plentiful, compared with calcium, than in the sun. Silicon shows up only as a trace, and no iron was detected, the opposite of what rocky debris should look like. That chemical fingerprint, researchers report in the journal Nature Astronomy, fits a second-generation planet candidate, one built from the star’s own cast-off material rather than the stuff that formed its first planets.

One White Dwarf Shows Niobium That 33 Others Lack

HS 0209+0832 has puzzled astronomers for 25 years. It is young for a white dwarf, dead for only about 5 million years, and hot, near 64,000 degrees Fahrenheit. Its atmosphere holds a little helium, which should sink out of sight within months at that temperature, so something must keep topping it off. A 1999 reading of its light from the Hubble Space Telescope found several metals. Each element absorbs light at telltale colors, leaving dark lines like fingerprints, and about 100 of those lines could not be identified.

For the new study, researchers revisited old data from Hubble, an ultraviolet telescope called FUSE, and the Very Large Telescope, using updated computer models. Most of the mystery lines turned out to belong to copper and niobium, a heavy metal the team found in none of the 33 other metal-polluted white dwarfs it checked.

Chemistry Points to a Second-Generation Planet, Not Solar System Debris

Next, the team compared the incoming material with meteorites and with debris around other white dwarfs. Nothing matched. Instead, nickel, a metal chemically similar to iron, was abundant, with a nickel-to-iron ratio more than 40 times higher than in Earth or primitive meteorites.

That pattern fits material forged inside aging stars, which build elements like nickel, copper, zinc and niobium by slowly adding neutrons to atoms. A planet assembled from a dying star’s leftovers should carry that signature, and the paper names a pileup of those elements as the key sign separating a second-generation planet from a first-generation one.

Material Shed by a Dying Star May Have Formed the Planet

Stars like this one swell into giants near the end of their lives and shed their outer layers. The researchers propose that a close companion could have triggered a violent shedding event, leaving a disk of material rich in heavy elements near the white dwarf. A giant planet could then have formed inside that disk.

Because helium is also falling onto the star, the source is more likely a gas giant’s atmosphere than a rocky body. Infrared data from the Spitzer Space Telescope make a brown dwarf, a failed star, look unlikely as the source, and suggest no large dusty disk. Planets around pulsars, another kind of dead star, are generally thought to be second-generation, but none had been identified around a white dwarf.

A 4.4-Day Flicker Hints at a Planet Near the White Dwarf

Data from NASA’s planet-hunting TESS satellite revealed a second clue, a faint rise and fall in the star’s brightness that repeats every 4.399 days and shifts the light by only about 0.12%. Typical white dwarfs spin about every 1.25 days, and among 20 watched by the Kepler telescope, only one rotated slower than once every 4 days, so a spin that slow would be rare. The pattern can instead be explained by a giant planet orbiting at roughly 0.04 times the Earth-sun distance, close enough to be scorched by the star.

Two explanations are on the table. The planet could be tidally locked, always showing one face to the star the way the moon does to Earth, creating a hot day and cool night. Or a comet-like tail of gas boiling off the planet could be passing in front of the star, an idea backed by helium levels that have shifted by a factor of two to three over time.

Planet formation may not end when a star does, and HS 0209+0832 may now be consuming the escaping atmosphere of a world born from its remains. Until the candidate is seen directly, the evidence stays mostly chemical and circumstantial, but no solar-system material the team tested comes close to matching it.

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