Free Astronomy Magazine September-October 2026

49 ASTRO PUBLISHING nificantly warmer than expected and determined how it most likely reached its very tight orbit around the white dwarf star. The results, published in the journal Nature , are a window into the future of planets like Jupiter after the death of the Sun, billions of years into the future. WD 1856 b was discovered in 2020 by scientists using NASA's TESS (Transiting Exoplanet Survey Satel- lite) and Spitzer space telescopes. It orbits the white dwarf WD 1856 +534, which is located about 80 light-years from Earth. “The planet is about the size of Jupiter, but the white dwarf it orbits is the size of Earth, so the planet is seven times larger than its star,” said lead author Ryan MacDonald of the University of St. Andrews in the United Kingdom. WD 1856 b orbits extremely close to its host star, a distance 50 times closer than Earth orbits the Sun. If WD 1856 b had originally been or- biting at that distance, it would have been obliterated while the star was a red giant. How did it survive the death of its host star and end up in its current position? The new study used Webb to watch the planet passing in front of its star. This transit yielded unique informa- tion about the planet’s mass, which is between four and eleven times the mass of Jupiter. The team also was able to determine the planet’s temperature. During the transit, light from the star was partly blocked, but infrared light was reduced less than other wave- lengths. The difference was infrared light emitted by the planet from its own heat. The data indicated that the planet has a temperature of about 260 degrees Fahrenheit (126 degrees Celsius) — significantly hot- ter than it would be if its only source of heat was the light from the white dwarf. This puzzling discovery turned out to be the key fact that proved how the planet must have reached its current orbit. Christopher O’Connor of North- western University in Illinois, a co- author on the paper, was responsi- ble for tracing the temperature of the planet back in time. O’Connor said, “The big question is how WD 1856 b ended up where it is today, and there are two theories. One is that the planet was swallowed by the host star as it was dying, and managed to survive on the inside. The other is that migration took place due to the gravitational ef- fect of other objects in the system. The white dwarf is part of a triple star system, and the companion stars could have influenced WD 1856 b’s orbit.” The researchers realized that there was no source of energy present to generate that heat today, so it must be residual energy from an earlier time when the planet was heated. Using models of how sub-stellar ob- jects like WD 1856 b cool down over time, coupled with the new data from Webb, the team was able to project its temperature back in time and deduce how long ago the heat- ing must have happened. The timing SEPTEMBER-OCTOBER 2026 E xoplanet WD 1856 b, shown in this artist’s concept, is a gas giant that sur- vived the death of its star. It now orbits a white dwarf at a distance 50 times closer than Earth orbits the Sun. Observations by NASA’s James Webb Space Tel- escope not only determined the planet’s temperature but also detected mole- cules in its atmosphere. The former measurement provides evidence that WD 1856 b migrated to its current location billions of years after its star became a white dwarf. [Artwork: NASA, ESA, CSA, Ralf Crawford (STScI)]