A Jupiter-size planet orbiting the corpse of a Sun-like star is hotter than it ought to be, and that heat may be the forensic trace of how it got there.
In a study published in Nature, astronomers used the James Webb Space Telescope to study WD 1856 b, the only confirmed planet known to have survived the death of a Sun-like star. The planet circles a white dwarf, the compact remnant left after such a star swells into a red giant and sheds much of its mass.
That survival is already awkward for standard planetary bookkeeping. Christopher O’Connor, a theoretical astrophysicist at Cornell University and a co-author of the study, said planets close to a Sun-like star should be swallowed during the red-giant phase. More distant gas giants should drift outward as the dying star loses mass and its gravity weakens. WD 1856 b is instead parked very close to the remnant, at about 0.02 astronomical units.
A strange transit gets stranger
WD 1856 b was found in 2020 after astronomers used NASA’s TESS observatory to examine about 2,000 white dwarfs. The survey was looking for smaller debris, such as comets or asteroids, crossing in front of dead stars. It found a gas giant.
The geometry is perverse. The white dwarf is about seven times smaller than the planet. If the planet crossed the star head-on, the star’s light should almost vanish during transit. Instead, the observed brightness drop is about half. O’Connor attributes that to a grazing transit, where only part of the planet’s disk passes over the star from Earth’s line of sight.
Webb observed one such crossing on April 27, 2023. The transit lasted about eight minutes. Because usual exoplanet transmission spectroscopy assumes a small planet passing across a much larger star, the team had to rework the math. According to the study, they modeled the spectrum as the changing overlap between the planet and the star, then adapted the POSEIDON atmospheric retrieval software developed by lead author Ryan MacDonald.
The planet is too warm
The Webb data indicate that WD 1856 b has aerosol hazes and methane in its atmosphere. The bigger surprise is its temperature. The team reports that the planet gives off about 25 times more energy than it receives from the white dwarf.
O’Connor said a planet at that distance from a white dwarf that has been cooling for roughly 6 billion years should be around 150 to 200 kelvin, similar to Jupiter’s cloud tops. The team instead found a temperature near 400 kelvin. O’Connor argues that the excess energy must come from internal heat rather than starlight being absorbed and reradiated.
That heat is the clue. The researchers considered two routes into the current orbit. In one, the planet began close in and survived inside the star’s outer layers during the red-giant phase, a common-envelope scenario. In the other, the planet began farther away, was disturbed by gravitational interactions with companion objects, and later migrated inward on highly stretched orbits. WD 1856 has two distant stellar companions.
The timing favors the second explanation, according to the team. Common-envelope evolution would have ended when the red-giant phase ended, about 5.4 billion years ago. By running cooling models backward from the planet’s measured temperature, the researchers estimate the reheating event happened 3 billion to 5.5 billion years after that phase, too late for the common-envelope explanation.
One model caveat
The conclusion depends on cooling models built for Jupiter-like atmospheres. Those models assume methane makes up about 0.3 percent of the atmosphere, while the team estimates roughly 7 percent for WD 1856 b. Methane is a strong greenhouse gas, so O’Connor said models tailored to the planet’s composition may be needed.
WD 1856 is about 75 light-years from Earth. O’Connor said finding one survivor that close suggests more such planets may be awaiting detection. The team has already taken additional Webb observations of the system, according to O’Connor, but those data were collected after the Nature paper was submitted.
This story draws on original reporting from Ars Technica.
