The Coldest Planet Ever Seen Is Circling a Stellar Corpse

Astronomers using the James Webb Space Telescope have directly detected the faint glow of a planet that’s colder than any world whose light has been directly observed—an astonishing detection that reveals the extreme conditions of some worlds in our universe.

The exoplanet, WD 1856+534 b, was first spotted in 2020 and is twice as old as our solar system. The world is about the size of Jupiter but about six times more massive and much chillier, clocking in at an average temperature of just -125° Fahrenheit (-87° Celsius). That makes it the coldest exoplanet ever directly observed by its own emitted light. The team’s research describing the exoplanet and its thermal emission is currently hosted on the preprint server arXiv.

The exoplanet is orbiting a white dwarf, the ghostly ember of a dead star. In fact, that’s what made the object’s detection possible; typically, stars are so bright that they drown out the much duller glow of the planets orbiting them. WD 1856 b’s star is so dim that the exoplanet itself was visible to Webb’s gaze. Astronomer Mary Anne Limbach from the Department of Astronomy at the University of Michigan contributed to the study.

What’s arguably weirder is where this planet is hanging out. WD 1856 b orbits just 0.02 astronomical units from its white dwarf star—closer than Mercury is to our Sun. “WD 1856+534 b is now the first intact exoplanet confirmed within a white dwarf’s ‘forbidden zone,’ a region where planets would have been engulfed during the star’s red giant phase,” the team explained in the paper, adding that its “presence provides direct evidence that planetary migration into close orbits—including the habitable zone—around white dwarfs is possible.”

The team’s work also puts WD 1856 at the top of the cold world pecking order. Just behind it is Epsilon Indi Ab, a planet studied by Webb just last year, which at the time made it the coldest imaged planet beyond our solar system at an estimated temperature of 35 degrees Fahrenheit (2 degrees C). Suffice to say, WD 1856 b is much colder.

The findings also settle a lingering identity crisis. Until now, WD 1856 b could have been a low-mass brown dwarf. But with its faint temperature and revised mass estimates (no more than 5.9 Jupiter masses), it’s officially part of the catalog of thousands of exoplanets scientists have compiled over the last few decades.

The research is also a major proof of concept for JWST’s ability to study cold, mature planets—and a reminder that even worlds orbiting the burnt-out cores of dead stars can still glow, however faintly.