Jupiter-Size Candidate Planet Found Orbiting Dead Star WD 1856, Defying Expectations

Astronomers reported on September 16, 2020, that they had found signs of a giant planet circling a white dwarf, the superdense remnant of a dead star. The star, called WD 1856, is part of a three-star system about 80 light-years from Earth. The planet candidate, WD 1856 b, is roughly Jupiter-size, about seven times larger than the star it orbits, and it completes a lap around it every 34 hours.

The finding, published in the journal Nature, matters because of how white dwarfs are made. When a sunlike star runs out of hydrogen fuel, it swells into a red giant that engulfs and incinerates anything orbiting nearby. Our own sun is expected to destroy Mercury, Venus and perhaps Earth when it does this, about 5 billion years from now. The red giant then collapses into a white dwarf, which typically packs the mass of the sun into a sphere only slightly bigger than Earth.

That history means WD 1856 b cannot have formed where it is today. It would not have survived the red-giant phase. The study team's calculations suggest it was born about 50 times farther from the star than its present orbit, then migrated inward. Lead author Andrew Vanderburg, an assistant professor of astronomy at the University of Wisconsin-Madison, said the planet somehow got very close to its white dwarf and stayed in one piece. The creation of a white dwarf destroys nearby planets, he noted, and anything that later strays too close is usually torn apart by the star's gravity. Many questions remain about how it arrived without meeting either fate.

The team found the object with NASA's Transiting Exoplanet Survey Satellite, TESS, which looks for the small dips in brightness that occur when a planet crosses its star's face. They then observed the system in infrared light with NASA's Spitzer Space Telescope, shortly before Spitzer was decommissioned in January 2020. Spitzer saw no infrared glow from the object itself. That suggests it is a planet rather than a low-mass star or a brown dwarf, a body that straddles the line between the two. Even so, WD 1856 b remains a candidate, awaiting confirmation from further analysis or observation.

Coauthor Siyi Xu, an assistant astronomer at the Gemini Observatory in Hawaii, explained why the result excited her. Astronomers have long known that asteroids and comets can scatter inward toward white dwarfs after the stars form, where they are usually pulled apart into debris disks. There have been hints that planets might scatter inward too, she said, but this appears to be the first time a planet has been seen that made the whole journey intact.

What pushed the planet inward is not known. The authors list possibilities including nudges from the system's other two stars and a brief encounter with an intruding rogue star. Coauthor Juliette Becker, a planetary scientist at Caltech, said the most likely case involves several other Jupiter-size bodies near the planet's original orbit, whose gravity could easily have produced the instability needed to knock it inward. She added that at this point there are more theories than data points. No other planets have been spotted in the system, though the team says that does not mean none exist.

A companion paper, published the same day in The Astrophysical Journal Letters, looked at a further possibility: if a gas giant can survive and sit close enough to a dead star to absorb meaningful warmth, perhaps a rocky world could too. Led by Cornell University researchers Lisa Kaltenegger and Ryan MacDonald, with Vanderburg among the contributors, the team modeled what NASA's James Webb Space Telescope could see from a hypothetical rocky planet in WD 1856's habitable zone, the range of distances where liquid water could be stable on a surface.

Their simulations indicated that Webb, a $9.8 billion observatory scheduled to launch in October 2021, could detect oxygen and carbon dioxide in such a planet's atmosphere after observing just five transits. Gas combinations that might indicate biological activity could be found in as few as 25 transits, said Kaltenegger, director of Cornell's Carl Sagan Institute. For now, the next step is confirming that WD 1856 b is a planet, with Webb's potential look at rocky worlds around white dwarfs coming afterward.