Mystery of a Warm Planet Orbiting a Dead Star Deepens

Eighty light-years away, a Jupiter-sized world called WD 1856 b is baking at 260 degrees Fahrenheit—hot enough to boil water—while orbiting a dead star so faint it could vanish in a blink. The white dwarf it circles, barely larger than Earth, is far too dim to account for that heat. The star died 10 billion years ago. The planet should be frozen solid. Instead, it glows in infrared light like a living world orbiting a living sun. Something is keeping it warm, and astronomers still don't know what.

Ryan MacDonald, an astronomer at the University of St. Andrews, led observations of the system using the James Webb Space Telescope in April 2023. His team watched for just under two hours as the planet crossed in front of its star—a transit that took eight minutes. The planet is so enormous relative to the white dwarf, seven times larger, that it blocked more than half the starlight as it passed. That size difference allowed Webb to detect the planet's own thermal glow, something no one had managed before on a world orbiting a dead star. "Seeing thermal emission from the planet was very surprising," MacDonald told Earth.com. The crossing was so brief that Victoria Boehm of Cornell University, who measured the light at each wavelength, described it as something you could miss if you blinked.

The spectrum revealed an atmosphere rich with surprises. Methane appeared at three distinct wavelengths, making up roughly 7 percent of the air—comparable to the 4 percent found deep inside Neptune. The planet's carbon content is about 100 times what the Sun contains. Above a thick cloud layer that begins near 100 millibars of pressure, fine particles scatter light and create a haze. The team found hints of ethane and phosphine but no ammonia or water anywhere in the spectrum. No one had ever measured an atmosphere on a planet transiting a white dwarf before.

The mystery deepens when you do the math. The planet's mass, measured for the first time from the spectrum, falls between 4.3 and 10.9 times Jupiter's mass. Nothing inside the planet generates that heat today. To stay this warm on its own for 10 billion years, it would need to weigh about 24 Jupiters—far heavier than it actually is. Tidal heating from orbital friction could work, but only if the orbit were nearly circular, which would have smoothed out any elliptical path within 75 million years. The planet is too warm now for that timeline to fit.

Christopher O'Connor of Northwestern University used cooling models to work backward from the planet's current temperature. He found that the planet was last heated between 3.0 and 5.5 billion years after its star died. That timing points to a dramatic history. When the white dwarf was still alive, it swelled into a red giant that could have engulfed the planet entirely. But that bloated phase lasted less than 2 million years—far too brief. The planet must have arrived late, on a long, stretched orbit that brought it close only after the star had already cooled to a white dwarf. Tidal friction then pulled it into the tight circle it follows today, orbiting about eight times farther from its star than the Moon is from Earth. Two red dwarf stars circling the white dwarf at vast distances likely provided the gravitational nudge that started the inward migration.

The haze composition remains unsolved. Three candidates fit the data—potassium chloride, ammonia ice, or a water-rich organic haze created in laboratory conditions—but none stands out. A plain grey cloud with haze above it still fits better than any single explanation. Complicating matters further, when astronomers measured the system again in 2025 at longer wavelengths, the planet appeared much cooler, near minus 118 degrees Fahrenheit. The cloud tops may shift between the planet's day and night sides, or one of the instruments may need recalibration. Some simulations predict the planet should be wider than the 0.91 Jupiter widths measured today, and no published models yet exist for an object this massive and this rich in heavy elements.

MacDonald's team has already recorded four additional transits with Webb and has two more scheduled at wavelengths where most of the planet's remaining heat escapes. They hope to pin down the haze composition and search for ammonia and phosphine, clues that might reveal how the planet formed 10 billion years ago. The work carries implications beyond this single system. White dwarfs are small enough that an Earth-sized planet orbiting one would make signs of life relatively easy to detect—a fact MacDonald's team demonstrated in 2020. No such planet has been found yet. "We just need to find those planets first," he said. The research also offers a glimpse of Jupiter's possible future: when our Sun becomes a white dwarf, it will lose mass, and Jupiter will first drift outward, then potentially spiral inward on a stretched orbit, following the same path that brought WD 1856 b to its mysterious fate.