An international team of astronomers using the NASA/ESA/CSA James Webb Space Telescope has captured unprecedented details of a Jupiter-sized exoplanet orbiting a dead star, shedding light on how such a world could survive the violent death of its sun. The findings offer a rare window into what may happen to the outer planets of our own solar system billions of years from now, when the Sun exhausts its nuclear fuel and swells into a red giant.
The exoplanet, designated WD 1856 b, orbits the white dwarf WD 1856+534, a stellar remnant located about 80 light-years from Earth. White dwarfs are the dense, cooling cores left behind after stars like the Sun shed their outer layers at the end of their lives. The planet is roughly the size of Jupiter, while its host star is only about the size of Earth — making the planet seven times larger than its star, an extreme contrast that has puzzled astronomers since the planet's discovery in 2020 by NASA's Transiting Exoplanet Survey Satellite (TESS) and the Spitzer Space Telescope.
What makes WD 1856 b particularly unusual is its extremely tight orbit. The planet circles its white dwarf at a distance about 50 times smaller than the distance between Earth and the Sun. If it had been that close before the star died, it would have been engulfed and destroyed during the red giant phase. The key question has been: how did it survive and end up in such a close orbit?
Using Webb's infrared capabilities, the team observed the planet during a grazing transit — a configuration in which only the upper edge of the planet partially crossed the star's disk. This allowed them to measure the planet's mass, estimated between four and eleven times that of Jupiter, and its temperature. The data revealed that WD 1856 b has a temperature of about 126 degrees Celsius, roughly 240 degrees warmer than expected if it were only heated by the faint light of the white dwarf.
This excess heat is a critical clue. The researchers concluded that the warmth is residual, left over from a past episode of intense heating. By combining cooling models for substellar objects like WD 1856 b with the new Webb measurements of mass and temperature, they determined that the heating event most likely occurred between 3 and 5.5 billion years after the star evolved into a white dwarf. This suggests that the planet originally orbited at a safe distance, survived the red giant phase, and only later migrated inward to its current tight orbit.
Two main scenarios could explain the migration. One possibility is that the planet was engulfed by the star during its red giant phase and somehow survived. The other, favored by the new data, is that gravitational interactions with other objects in the system — the white dwarf belongs to a triple-star system, and the outer companion stars may have perturbed the planet's orbit — gradually pulled it inward. The residual heat would then be a consequence of tidal heating or frictional forces during that migration.
The study, published in the journal Nature, was led by Ryan MacDonald of the University of St Andrews in Scotland. MacDonald described the planet as «really out of the ordinary» and noted that the Webb observations represent the first time a planet has been seen in such a close orbit around a white dwarf. The transmission spectrum obtained during the transit also revealed chemical signatures in the planet's atmosphere, offering further clues about its composition and history.
These results have profound implications for understanding the long-term fate of planetary systems. In about five billion years, the Sun will exhaust its hydrogen fuel and expand into a red giant, likely engulfing Mercury, Venus, and possibly Earth. The fate of the outer gas giants — Jupiter, Saturn, and Neptune — has remained uncertain. WD 1856 b demonstrates that a Jupiter-sized planet can survive such a cataclysm and even end up in a close orbit around the stellar remnant, providing a possible analog for what may happen to Jupiter and its siblings.
«This is our first window into the future of planets like Jupiter after the death of the Sun, billions of years into the future,» the team stated. The Webb telescope's ability to probe the atmosphere and thermal properties of such an exotic world marks a significant step forward in exoplanet science, and the findings will help refine models of planetary system evolution across cosmic timescales.