James Webb Telescope Detects Atmosphere on a Planet Orbiting a Dead Star

A Planet That Survived Its Star's Death

About 80 light-years from Earth, a Jupiter-sized planet orbits a dead star — and it still has an atmosphere. That remarkable discovery, published July 1 in the journal Nature, marks the first time scientists have detected an atmosphere on a planet transiting a white dwarf, offering a stunning preview of what could eventually happen in our own solar system.

An international team of astronomers used the James Webb Space Telescope to observe the exoplanet WD 1856 b pass in front of its host star — a white dwarf — offering the first window into the future of planets like Jupiter after the death of the Sun. Lead author Dr. Ryan MacDonald from the University of St Andrews noted that "the planet is quite the oddball," explaining that it's about the size of Jupiter, while the white dwarf it orbits is the size of Earth — meaning the planet is seven times larger than its star.

What Webb Actually Found

The study used Webb to watch WD 1856 b pass in front of its star in a so-called grazing transit, where the very top of the planet partly overlapped the star. Light from the star passing through the planet's atmosphere picked up information about its chemical composition. Scientists saw "the telltale signatures of small cloud particles and hydrocarbons, most likely methane" — the first time an atmosphere has been detected on a planet transiting a dead star.

The 0.5–5.0-μm spectrum revealed the presence of hydrocarbons, aerosols, and thermal emission from the planetary nightside. The transit also yielded unique information about the planet's mass and temperature, enabling scientists to estimate the planet at between four and eleven times Jupiter's mass. But perhaps the most puzzling finding was the planet's temperature. While the expected planetary equilibrium temperature is about 160 Kelvin, the measured temperature came in between 390 and 412 Kelvin — far hotter than it should be if it were heated only by starlight.

How Did It Get So Close — and So Hot?

WD 1856 b orbits extremely close to its host star, at a distance 50 times closer than Earth orbits the Sun. If it had originally been orbiting at that distance, it would have been obliterated while the star was a red giant. So how did it end up there? As the planet moved inward, its interactions with the strong gravity of the white dwarf caused it to warm up considerably, and it has been cooling ever since.

The team's data suggests that heating of the planet occurred about 1 billion years ago, which may rule out the chance that it was engulfed during the red giant phase. The abundance of methane adds another line of evidence that the planet didn't go through engulfment during the red giant phase, since that would have diluted the gas's abundance as the planet accreted hydrogen from the star. Taken together, these observations show that the planet survived the red giant phase, migrated inward, and experienced heating — which also helps explain its tight orbit.

A Preview of Our Own Solar System's Fate

In approximately five billion years, the Sun will run out of hydrogen fuel and swell up more than 100 times larger than it is now into a red giant star, then shed its outer layers and end its life as a white dwarf. Mercury, Venus, and possibly Earth will be destroyed in the process. WD 1856 b shows that larger, outer planets — like Jupiter — may have a fighting chance of survival, and could even retain their atmospheres long after their star has died.

Co-author Victoria Boehm of Cornell University noted that the team "recently observed four more transits of WD 1856 b with Webb to take a deeper look into its atmospheric chemistry" and expressed excitement about seeing those results. Each new observation promises to sharpen our understanding of planetary survival — and what the distant future of our own cosmic neighborhood might actually look like.