A planet seven times larger than its star: the anomaly discovered by the James Webb Space Telescope
The James Webb Space Telescope has observed the transit of a giant planet around the remnants of its star, solving a mystery that astronomers have grappled with since 2020: how could a Jupiter-sized planet survive the death of its sun? The study, led by Ryan MacDonald from the University of St Andrews, was published on July 1 in the journal Nature.
The main subject is WD 1856 b, an exoplanet discovered in 2020 thanks to data from the TESS satellite and NASA's Spitzer telescope. It orbits the white dwarf WD 1856+534, located about 80 light-years from Earth, completing a full orbit every 34 hours at a distance of less than 3 million kilometers. A detail that illustrates the proportions at play: the white dwarf is the size of Earth, while the planet orbiting it is seven times larger than its own star.
The point is that a white dwarf does not start out that way. It is what remains of a Sun-like star after it has reached the end of its life, swollen into a red giant, and then shed its outer layers. During that phase, any planet orbiting at the current distance of WD 1856 b would have been swallowed and destroyed. In fact, the planet orbits 50 times closer to its star than Earth does to the Sun, a value incompatible with its survival if that had always been its position.
The temperature that unveiled the mystery of WD 1856 b
By observing the transit with Webb's infrared instruments, the team measured the planet's mass, ranging from four to eleven times that of Jupiter, and, most importantly, its temperature: about 126°C. This value is too high to be explained solely by the residual radiation of the white dwarf, and this anomaly proved to be the key to reconstructing the planet's orbital history.
Christopher O'Connor from Northwestern University, a co-author of the study, reconstructed the temperature trend over time. The hypotheses on the table were two: that the planet had been engulfed by the dying star and somehow survived inside it, or that it had moved inward due to the gravitational effects of other bodies in the system. It should be noted that WD 1856+534 is part of a triple star system, and the companion stars may have influenced the planet's orbit over time.
By applying cooling models to sub-stellar bodies like WD 1856 b, the team calculated that the observed heating dates back to a period between 3 and 5.5 billion years after the formation of the white dwarf. This data shifts the balance in favor of the second hypothesis: the planet orbited far away, sheltered from the wrath of the red giant, and only later moved closer, warming up due to the gravitational interaction with the white dwarf. Since then, it has been slowly cooling down.
The same observations allowed for the detection, for the first time on a planet transiting around a dead star, traces of its atmosphere. Victoria Boehm from Cornell University, another co-author of the research, spoke of signals attributed to small cloud particles and hydrocarbons, likely methane.
The team has already collected four more transits of the planet with Webb to further investigate the chemical composition of its atmosphere.
The research has implications closely related to the future of our solar system. In about five billion years, the Sun will exhaust the hydrogen in its core, swell to over a hundred times its current size becoming a red giant, and then shed its outer layers to become a white dwarf. Mercury, Venus, and possibly Earth are destined to be destroyed in that phase. The fate of the outer planets, led by Jupiter and Saturn, remains open: WD 1856 b provides the first concrete clue about how their orbits might evolve when the Sun is no longer there.