Aneesh Baburaj remembers the moment the spectrum of GJ504b finally came into focus—two hours of observation from the James Webb Space Telescope revealing what years of ground-based attempts could not: a world veiled in salt. Located 57 light-years away and known affectionately as the 'Pink Planet,' this massive, faint object has puzzled astronomers since its discovery in 2013. Now, for the first time, scientists have direct evidence of salt clouds in its atmosphere—crystalline haze made of sodium chloride and potassium chloride, suspended like a shimmering curtain around a world colder than any other directly imaged planetary-mass companion.
The discovery matters not just for what it reveals about GJ504b, but for what it unlocks about the universe’s coldest, dimmest objects. For years, these faint companions eluded study because their light is drowned out by their host stars and too weak for Earth-based instruments. GJ504b, at just 550 degrees Fahrenheit (290°C), is barely warmer than a kitchen oven—far cooler than most directly imaged exoplanets, which typically blaze at 1,000 to 2,000°F. Its age, estimated between 2.5 billion and 4 billion years, explains its chill: like aging embers, giant planets cool over time. But without JWST, its secrets remained locked away.
Baburaj, a postdoctoral associate at Northwestern University’s CIERA, led the study that finally cracked the code. Using JWST’s unmatched sensitivity, his team isolated the planet’s faint glow and extracted its spectrum—a fingerprint of light broken into colors that reveal chemical composition. What they found was a rich cocktail of water vapor, methane, carbon dioxide and ammonia. But models couldn’t make sense of the data—until they added clouds. Only salt clouds, the team realized, could explain the muted molecular signatures. "When we accounted for salt clouds, it subdued the signature of molecules hidden deeper in the companion's atmosphere," Baburaj said. "Then the results became physically possible."
This is the first direct evidence of salt clouds in a cold astronomical object, confirming a theory first proposed over 15 years ago. The work, conducted with Marshall Perrin of the Space Telescope Science Institute and published in The Astronomical Journal, marks a turning point in how we study cold, faint worlds. JWST didn’t just see GJ504b—it revealed an entirely new kind of atmosphere, one shaped by exotic chemistry and ancient cooling.
As astronomers turn their gaze to even colder targets, the Pink Planet stands as a beacon of what’s now possible. What once took a full night with the world’s largest telescopes and yielded nothing, JWST accomplished in two hours. The universe’s faintest secrets are no longer out of reach.