Among the most peculiar worlds in the Milky Way galaxy, some of the most enigmatic are those orbiting white dwarfs. These are not ordinary stars, busy fusing atoms in their cores, but rather ultra-dense remnants of sun-like stars that have undergone their demise—swelling into colossal red giants before shedding their outer layers and collapsing into compact stellar cores.
This is precisely what will happen to our Sun in roughly 5 billion years, so any exoplanets circling white dwarfs are naturally of immense interest to scientists seeking clues about the Solar System’s ultimate fate.
Now, using the JWST telescope, astronomers have, for the first time, peered into the atmosphere of the giant planet WD 1856b, which orbits a white dwarf, and discovered it is far hotter than anyone anticipated. Their findings are published in the journal Nature.
“The moment we saw the spectrum of WD 1856b, showing a huge drop in the planet’s effective size at longer infrared wavelengths, we exclaimed, ‘Wow! What is happening here?'” shared astronomer Ryan McDonald from the University of St Andrews in the UK. “The transit spectrum of WD 1856b captured by JWST is unlike any other planet we have ever observed.”
White dwarfs rank among the most extreme objects in the universe. They are what remains after a star up to eight times the mass of the Sun leaves the main sequence—a dead core that glows solely from residual heat for trillions of years.
We have a rough understanding of how this transformation unfolds. As a star nears the complete exhaustion of its nuclear fuel, it inflates to hundreds of times its original size.
Eventually, this bloated envelope expands and dissipates into nothing, while the core, no longer supported by the outward pressure of fusion, collapses under gravity.
The resulting white dwarf is incredibly dense, packing up to 1.4 solar masses into a body roughly the size of Earth.
Astronomers believe this process will be highly destructive for the Solar System, with the red giant phase potentially causing the Sun to swell as far as Mars’s orbit.
“When the Sun becomes a red giant, Mercury and Venus will be destroyed. Earth’s fate hangs in the balance—its survival depends on the tiniest details of stellar models. But the outer planets, like Jupiter and Saturn, will almost certainly survive the Sun’s demise,” McDonald explained. “When the Sun dies and its core remains as a white dwarf, the lost mass will alter the orbits of the remaining planets, pushing them outward. Since the white dwarf’s gravitational pull will be much weaker than the Sun’s, the Solar System will become less tightly bound, making it easier for dynamic interactions to shift planets.”
Numerous exoplanets have been found orbiting white dwarfs, naturally raising the question of how they survived the star’s death throes.
However, studying the atmosphere of an exoplanet around a white dwarf is far more challenging than examining one around a living star.
Astronomers analyze exoplanet atmospheres when planets pass between us and their stars. As starlight filters through the atmosphere, it picks up signatures of the gases present, allowing scientists to determine its composition.
Most stars are much larger than their planets. WD 1856b, located 82 light-years away, is seven times larger than the white dwarf it orbits.
“Normally during a transit, the transiting planet’s atmosphere sits entirely on its star’s disk, but with WD 1856b, only part of the atmosphere covers the white dwarf,” said McDonald. “This system has the largest known transit depth of any exoplanet—56 percent—and the transit lasts only 8 minutes. The white dwarf is also much dimmer than the main-sequence stars we usually observe. So this is a truly different system compared to those we typically study with JWST. We had to develop new models and completely rethink our approach to analyzing JWST spectra for this unusual planetary system around a dead star.”
What surprised the researchers was not the exoplanet’s atmospheric composition, but its temperature.
They expected temperatures around -113 degrees Celsius, similar to Jupiter, which is comparable in size and orbit to WD 1856b. Instead, the air temperature hovered around 126 degrees Celsius.
“We couldn’t understand how a planet orbiting an old, dim white dwarf—one that has been cooling since the star it formed from died 5.4 billion years ago—could be so warm?” McDonald said.
Giant planets cool at a predictable rate over billions of years. This means their current temperatures preserve traces of past events that heated them.
The researchers were also astonished to find that, although WD 1856b is only slightly smaller than Jupiter, it is seven times more massive.
Together, the planet’s surprising temperature and mass allowed the researchers to reconstruct its history.
They discovered that this unusual world reached its peak temperature billions of years after the star turned into a white dwarf. This timing suggests the white dwarf was not the cause; something else must have reheated it long after the stellar transformation.
One possible explanation is a nearby binary star, whose tidal forces could have both heated the exoplanet and nudged it inward from a more distant orbit to its current position.
“Our findings show that giant planets like Jupiter can get a ‘second life’ after their star dies: the planet moves closer, gets reheated, and undergoes changes in its atmospheric chemistry,” said McDonald. “The death of stars is not the end—it’s a new chapter in the lives of planets like Jupiter.”
We still don’t know what this means for Earth. Some models suggest our world will be engulfed by the Sun; others say it won’t. If it survives the engulfment, we don’t know how the redistribution of solar mass will affect Earth’s orbit.
Humanity is unlikely to live long enough to witness this, but by then, new life might emerge. Studying worlds like WD 1856b could reveal the chances of that happening.
“Today’s results show we can measure the atmospheric composition of planets closely orbiting white dwarfs, opening an exciting new area of research into post-main-sequence planetary atmospheres,” said McDonald. “Ultimately, the dream would be to find a rocky planet orbiting a white dwarf, perhaps even in its habitable zone. After all, what could be more poetic than searching for life around a dead star?”
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