The Heat Memory of a Planet Orbiting a Dead Star ✦
Spectroscopy with JWST has allowed the first detailed study of a planet's atmosphere around a white dwarf (the remnant of a Sun-like star). Methane (about 7%), aerosols, and thermal radiation from the nightside were detected. The planet's mass is between 4 and 11 Jupiter masses, and its temperature (390–412 K) is almost three times higher than predicted, indicating a "heat-up" due to migration after the star became a white dwarf. This event occurred billions of years after the star's death, revealing an unexpected scenario of planetary evolution.
When a Sun-like star sheds its shells and contracts into a white dwarf, its system sinks into a prolonged twilight. Outer worlds drift away, inner ones evaporate — giant planets, it seemed, were doomed to eternal cold in interstellar emptiness. But the discovery of exoplanet WD 1856 b, orbiting just 0.02 AU from its dead star, showed that stellar evolution writes far more bizarre scripts. Now, thanks to prism spectroscopy with NIRSpec on JWST, we have, for the first time, peered into its atmosphere — and found something astonishing.
The transmission spectrum, obtained via the transit method (to which David Charbonneau made enormous contributions), brought several surprises. First, the geometry: the planet is seven times larger than the star, so its passage across the disk is just a grazing touch.
Second, the chemical composition revealed a strong enrichment in carbon: methane CH₄ fills about 7% of the gas envelope — a hundred times more than the solar value. And most importantly: the planet's night side glows in the infrared like a smoldering ember, far brighter than expected from the feeble irradiation of a white dwarf. The effective temperature via the Stefan–Boltzmann law is 390–412 K, whereas the equilibrium temperature doesn't exceed 160 K. The planet blazes more than twice as hot as it should.
This heat excess is not a glitch, but an archive of the past. Numerical simulations of giant planet evolution and white dwarf cooling (the physical foundations of which were laid by Subrahmanyan Chandrasekhar) showed: the planet didn't heat up during the recent red giant phase — it was roasted by tidal forces during a high-eccentricity migration that took place 3–5.5 billion years after the star became a white dwarf. In other words, long after the system's "death", some dynamical kick flung the giant onto an elongated orbit, and then tidal interaction squeezed and heated it, turning it into something like a blazing flywheel, whose heat we are now detecting.
This is thermal memory that has lasted billions of years — as if the planet still hasn't cooled down from an ancient catastrophe.
This discovery ushers the study of exoplanet atmospheres into a new era. White dwarf systems are no longer just strange curiosities, but full-fledged laboratories of late stellar evolution. Upcoming observations with MIRI on JWST, and in the longer term with the Ariel mission and ground-based ELTs, will allow us to determine the aerosol composition, catch signatures of ethane and phosphine, and build thermal maps of such worlds. This is a step toward understanding how giant planets survive even the fiery finale of their star, and perhaps a hint at what awaits our own Jupiter billions of years from now, when the Sun becomes a white dwarf. The thermal memory of a distant world becomes a mirror of our own cosmic fate — an archive we are just beginning to read.
🎯 WD 1856 b is seven times larger than its white dwarf — if it took Earth's place, it would cover nearly the entire sky. Yet during transit we see only a fleeting touch due to the orbit's inclination.