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 burned-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 directed the TESS observatory to a sample of about 2,000 white dwarfs in 2020. These stars are the remains of a Sun-like star that has already gone through a red giant phase, leaving behind an Earth-sized body that is primarily composed of elements such as carbon and oxygen. The TESS team was looking for small objects, such as comets or asteroids, that could transit the surface 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, okay, that’s weird,” said Christopher O’Connor, a theoretical astrophysicist at Cornell University and co-author of the recent Nature study on WD 1856 b.
The white dwarf is about seven times smaller than the gas giant that surrounds it. Its brightness should drop to almost nothing every time the planet crosses in front of it, but instead it is decreasing by about half. O’Connor believes the reason is a grazing transit, where only the edge of the planetary disk cuts the face of the star. “That’s a very unlikely angle of view,” he said, “but it’s the only way to explain what we actually see.”
What’s more, the planet orbits about 0.02 AU from the white dwarf, which runs counter to our ideas about how the death of a star should reshape its system. “When the 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 its gravitational pull becomes weaker. “Outer planets, like gas giants, should migrate outward by about a factor of two,” O’Connor said.
