WD 1856 b is an extraordinary planet that has left astronomers scratching their heads. This Jupiter-sized world orbits a white dwarf, the burned-out remnant of a Sun-like star.
The discovery was made by chance when a team of researchers pointed the TESS observatory at a sample of roughly 2,000 white dwarfs in 2020. They were searching for small objects like comets or asteroids that might transit across the face of these dead stars.
The white dwarf is about seven times smaller than the gas giant circling around it. Its brightness should be dropping to nearly nothing each time the planet crosses in front of it, but instead, it’s dipping by about half. The team thinks the reason for this unusual behavior is a grazing transit, where only the edge of the planetary disk clips the face of the star.
The planet orbits at about 0.02 AU from the white dwarf, which goes against our ideas of how the death of a star should reshape its system. When a star expands to become a red giant, it consumes the inner planets. Then, in the process of shrinking down to a white dwarf, it loses about half of its original mass, making its gravitational pull weaker. The outer planets, like gas giants, should migrate outward by about a factor of two.
However, WD 1856 b did not migrate outward; instead, it got closer to its star. This has the science community buzzing with excitement and curiosity. ‘It sent theoretical astrophysicists into a feeding frenzy,’ said Christopher O’Connor, a theoretical astrophysicist at Cornell University and co-author of the recent Nature study on WD 1856 b.
To get more data to work with, O’Connor’s team booked time on the James Webb Space Telescope to take a closer look at what was going on in the WD 1856 system. The JWST observations were done on April 27, 2023, and captured a single transit that lasted just eight minutes.
The viewing angle and the unusual size mismatch between the star and its planet posed an immediate technical problem. Standard exoplanet transmission spectroscopy assumes a smaller planet is entirely silhouetted against the face of a much larger star, which was not the case here. To get around it, the team developed new equations to express the transmission spectrum as the time-varying area of the planet overlapping the star’s disk.
When the scientists were done crunching numbers, WD 1856 b’s atmosphere proved somewhat surprising. It turned out the planet is shrouded in aerosol hazes, and its atmosphere contains methane. It is also far hotter than the team expected. WD 1856 b apparently emits roughly 25 times more energy into space than it receives from its cooling host star.
The extraordinary temperature of WD 1856 b tells us a lot about its history. ‘We expected this planet to be roughly as hot as Jupiter, but it wasn’t,’ O’Connor said. At about 0.02 AU from a white dwarf that has been cooling for 6 billion years, WD 1856 b should be somewhere between 150 and 200 Kelvin, close to the temperature of Jupiter’s cloud tops.
Instead, it is around 400 Kelvin. ‘Whatever is causing this planet to glow, it must be an internally derived heat rather than just re-radiating energy from the star,’ O’Connor said. The team managed to estimate when it happened by working backward through planetary cooling models.
The scientists figured out the most probable reason why WD 1856 b got so close to its star. They initially came up with two competing scenarios to explain how WD 1856 b ended up in its current orbit. The first is a common-envelope model, where the planet was originally in a close orbit and survived being engulfed when its star expanded into a red giant.
In the second, a high-eccentricity migration model, the planet started farther out, had its orbit destabilized by gravitational interactions with companion objects (WD 1856 has two distant stellar companions) and then spiraled inward over billions of years through gravitational interactions.
The discovery of WD 1856 b challenges our current understanding of planetary formation and evolution. It also highlights the importance of continued research into these complex systems, which can provide valuable insights into the history of our own solar system.
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