JWST Reveals Aerosols and Hydrocarbons in the Atmosphere of a Planet Orbiting a Dead Star
Featured image: Artist’s impression of a Jupiter-sized planet transiting a white dwarf; credit: NASA/ESA/Joseph Olmsted (STScI)
For the first time, astronomers have detected an atmosphere on a planet transiting a dead star. Using the James Webb Space Telescope, an international team led by the University of St Andrews has identified small cloud particles (aerosols) and hydrocarbons, most likely methane, in the atmosphere of WD 1856 b — a Jupiter-sized world orbiting a white dwarf 80 light-years from Earth. The findings were published July 1 in Nature.
The discovery is a milestone for exoplanet science. White dwarfs are the embers of Sun-like stars that have exhausted their nuclear fuel and shed their outer layers, collapsing into Earth-sized stellar remnants. Until now, no one had successfully characterized the atmosphere of a planet caught in such a system.
WD 1856 b completes an orbit every 34 hours at a distance of just 0.02 astronomical units, more than 50 times closer than Earth is to the Sun. Despite the tight orbit, the planet is not heated by its host star alone: JWST measured a dayside temperature of roughly 400 Kelvin (127 degrees Celsius), significantly hotter than passive stellar heating would predict. The excess warmth is a relic of tidal reheating that occurred when gravitational perturbations from companion stars in this triple-star system kicked the planet into its current orbit billions of years after the white dwarf formed.
“The big question is how WD 1856 b ended up where it is today,” said Christopher O’Connor of Northwestern University, a coauthor on the study.
The answer appears to be high-eccentricity migration. The planet originally orbited far from its parent star and survived the red giant phase that would have engulfed any worlds in the inner system. Some 3 to 5 billion years after the star became a white dwarf, gravitational nudges from stellar companions sent the planet plunging toward its current close-in orbit, where tidal forces circularized the path and reheated the atmosphere. The timing of this reheating rules out both engulfment during the red giant phase and passive cooling over 10 billion years.
The transmission spectrum, obtained during brief 8-minute transits using JWST’s NIRSpec PRISM instrument, reveals a carbon-enriched hydrogen/helium envelope with roughly 7 percent methane by volume and a carbon-to-hydrogen ratio 100 times that of the Sun. An optically thick cloud deck of potassium chloride particles sits at an altitude of roughly 100 millibars. The detection of hydrocarbons at a combined 3.6 to 4.5 sigma confidence marks the first time any such compounds have been identified in a white dwarf planet atmosphere.
“It’s like using a time machine to peer into the distant future of our Solar System,” said lead author Ryan MacDonald of the University of St Andrews.
In roughly 5 billion years, the Sun will become a red giant, engulfing Mercury, Venus, and possibly Earth. Jupiter and Saturn may survive, migrating inward through a similar mechanism and ending up as objects much like WD 1856 b — hot Jupiters orbiting a white dwarf ember. The carbon enrichment of this planet’s atmosphere also suggests it accreted volatile-rich material, including ice and organics, during or after its migration.
The research was conducted under JWST program GO-2358 (PI: MacDonald) using data from April 2023. Two independent reduction pipelines, FIREFLy and Juniper, produced consistent results. Four additional transits have already been observed with JWST for deeper characterization of the planet’s atmospheric chemistry.
Draft for 1ban.news – Space Desk