The universe has a flair for the unlikely. In 2020, astronomers found something baffling: A gas giant planet called WD 1856b, orbiting the burned-out core of a dead, sun-like star.
And in a new study published today in Nature, researchers report an even wilder discovery: Not only is WD 1856b orbiting the glowing corpse of its star, but it also has an atmosphere—and it’s still warm.
“It was unlike any other exoplanet spectrum we’ve seen,” says Ryan MacDonald, an astrophysicist at the University of St. Andrews and the lead author of the study. “Which caused a fair amount of head scratching in our team.”
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To understand why this planet is such a mystery, we first need to explain how our sun will die.
Stars like the sun aren’t massive enough to explode in spectacular supernovae. Instead, they start to swell and turn a rusty color, becoming red giants. Then, they shed their outer layers and collapse into white dwarfs—stellar embers that are significantly smaller than the original star, but which still containing much of its mass.
This demise is more of a whimper than a bang. But the corresponding upheaval is still catastrophic for any surrounding planets, which either get swallowed up by the expanding star, or drift into wider orbits as the star’s gravity weakens—some are flung out from the system entirely. Yet somehow, WD 1856b survived.
It marks the first closely orbiting white dwarf planet with a confirmed atmosphere, opening up new possibilities for what might happen to planetary systems after their host star dies. A year on WD 1856b is roughly 34 hours long, an orbit so tight that it begs the question of how it got there in the first place. Could it possibly have survived being swallowed up by the red giant and then spat back out as it collapsed into a red dwarf?
“There are two theories,” said Christopher O’Connor, a professor at Northwestern University and one of the study’s co-authors of the study, in a statement. “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 the migration took place due to the gravitational effect of other objects in the system.”
The answer was hiding in the planet’s heat.
By combining their temperature measurements with the planet’s mass and models of how giant planets cool over time, the astronomers effectively rewound its thermal history.
According to O’Conner’s first theory, the planet would have retained far more heat from when the star went red dwarf than the researchers observed. That suggests the second theory may be correct, and the planet was likely at a safer distance for more than a billion years before migrating inward.
“Each time it makes a close pass by the white dwarf it would lose a little orbital energy into heat energy, which would make the furthest part of the orbit move a little closer,” MacDonald says. If you were watching with an infrared camera, you’d even be able to see the planet glow at the height of this process.
The study has intriguing implications for our own cosmic future. Roughly five billion years from now, the sun will follow the same path as this star, expanding into a red giant before shrinking into a white dwarf. Earth almost certainly won’t survive. The solar system’s gas giants, however, are another story.
“Jupiter has a long life ahead of it, even after the leftover core of the sun is merely a smoldering ember,” MacDonald says.
If future astronomers are around to study it, they might one day read Jupiter’s atmosphere the same way researchers analyzed WD 1856b’s—as a fossil record of a planetary system that refused to die.
“Exoplanet science is a never-ending story of reimagining what is possible,” says MacDonald. “Whether a science fiction fan, a scientist, or even a kid in school, who doesn’t want to know what will happen when the sun dies?”
