The universe has a gift for the improbable. In 2020, astronomers discovered something puzzling: a gas giant planet called WD 1856borbiting the burned core of a dead sun-like star.
And in a new study published today in Natureresearchers report an even crazier discovery: not only is WD 1856b orbiting the bright corpse of its star, but it also has an atmosphere, and it’s still hot.
“It was unlike any other exoplanet spectrum we’ve observed,” says Ryan MacDonald, an astrophysicist at the University of St. Andrews and lead author of the study. “Which caused a lot of headaches in our team.”
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To understand why this planet is so mysterious, we must first explain how our sun will die.
Stars like the Sun are not massive enough to explode in spectacular supernovae. Instead, they begin 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 still contain much of its mass.
This disappearance is more of a whimper than a crash. 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 kicked out of the system entirely. And yet, somehow, WD 1856b survived.
It is the first close-orbiting white dwarf planet with a confirmed atmosphere, opening new possibilities for what might happen to planetary systems after their host star dies. A year on WD 1856b lasts about 34 hours, an orbit so narrow that it begs the question of how it got there in the first place. Could it have survived being swallowed up by the red giant, then spat out by collapsing into a red dwarf?
“There are two theories,” Christopher O’Connor, a professor at Northwestern University and one of the study’s co-authors, said in a paper. statement. “One is that the planet was swallowed by the host star as it was dying and managed to survive inside. The other is that the migration took place due to the gravitational effect of other objects in the system.”
The answer was hidden in the heat of the planet.
By combining their temperature measurements with the planet’s mass and patterns of giant planet cooling over time, astronomers have effectively rewound its thermal history.
According to O’Conner’s first theory, the planet would have retained much more heat when the star became a red dwarf than researchers observed. This suggests that the second theory may be correct and that the planet was likely at a safer distance for over a billion years before migrating inward.
“Every time it passes near the white dwarf, it would lose a little bit of orbital energy into thermal energy, which would bring the farthest part of the orbit a little bit closer,” MacDonald says. If you looked with an infrared camera, you would even be able to see the planet glowing at the height of this process.
The study has interesting implications for our own cosmic future. In about five billion years, the Sun will follow the same path as this star, transforming into a red giant before shrinking into a white dwarf. Earth almost certainly will not survive. The gas giants of the solar system, on the other hand, are a different story.
“Jupiter has a long life ahead of it, even after the sun’s remaining core is nothing more than a smoldering ember,” says MacDonald.
If future astronomers are around to study it, they might one day read Jupiter’s atmosphere the same way researchers analyzed that of WD 1856b: as a fossil record of a planetary system that refused to die.
“Exoplanet science is a never-ending story about reinventing what is possible,” says MacDonald. “Whether it’s a science fiction fan, a scientist, or even a kid at school, who doesn’t want to know what will happen when the sun dies? »
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