WD 1856 b is the only confirmed case of a planet surviving the death of a Sun-like star. It’s a Jupiter-sized world orbiting a white dwarf, the burnt-out remnant of a Sun-like star. Now, a team of astronomers has used the James Webb Space Telescope to take a closer look at this planet for the first time, and what they found makes an already strange system even stranger.
A feeding frenzy
WD 1856 b was an accidental discovery. Astronomers focused the TESS observatory on a sample of about 2,000 white dwarfs in 2020. These stars are the remnants of a Sun-like star that has already passed through its red-giant phase, leaving behind an Earth-sized body composed mostly of elements such as carbon and oxygen. The TESS team was looking for small objects, such as comets or asteroids, that might pass through the faces of these dead stars.
What they found in the WD 1856 system was a gas giant. “As soon as they looked at it, they said, OK, this is weird,” said Christopher O’Connor, a theoretical astrophysicist at Cornell University and co-author of the recent Nature study on WD 1856 b.
A white dwarf is about seven times smaller than the gas giant it orbits. Every time a planet passes in front of it, its brightness should drop to almost nothing, but instead it drops by about half. O’Connor thinks that this is only because of a grazing transit, where the edge of the planetary disk intersects the face of the star. “It’s a very difficult point of view,” he said, “but it’s the only way to explain what we’re actually seeing.”
Moreover, the planet orbits about 0.02 AU from the white dwarf, which contradicts our understanding of how the death of a star changes its system. “When a star expands to become a red giant, it consumes the inner planets,” explains O’Connor. Then, in the process of shrinking to a white dwarf, it loses about half of its original mass, meaning that its gravitational pull weakens. “The outer planets, like the gas giants, should migrate outward about twice as much,” O’Connor said.
