The James Webb telescope has obtained the first spectrum of the atmosphere of a planet orbiting a white dwarf — the remnant of a Sun-like star that has already finished its main phase of life.
The planet is WD 1856 b. Astronomers used transmission spectroscopy: when a planet passes in front of its star, part of the light travels through its atmosphere, and the spectrum reveals the composition. The NIRSpec PRISM instrument worked across a range from 0.5 to 5 micrometres.
The spectrum shows signs of hydrocarbons, with methane (CH₄) the most likely candidate at an abundance of about 7%. Aerosols (particles suspended in the atmosphere that form a haze) and thermal emission from the planet's nightside were also detected. The mass of WD 1856 b is estimated between 4.3 and 10.9 Jupiter masses.
The most notable result is the temperature. The calculated equilibrium temperature should have been around 160 K, but observations point to 390–412 K. The authors suggest the planet underwent reheating during migration to its present orbit with a radius of 0.02 astronomical units. According to cooling models, this happened 3 to 5.5 billion years after the star became a white dwarf.
Such planets show that gas giants can survive the transformation of their star. It is a preliminary look at what awaits the planets around the Sun.