JWST Reveals Salt Clouds on the 'Pink Planet' GJ 504b

For over a decade, GJ 504b has captivated astronomers with its distinct dark magenta hue, earning it the nickname the "Pink Planet." Discovered in 2013, it was one of the coldest directly imaged exoplanets known, but its faintness made detailed study impossible from Earth. Now, the James Webb Space Telescope (JWST) has peered into its atmosphere, revealing a world far stranger than anyone imagined: a place where the clouds are made of salt.

A Rich and Exotic Atmosphere

Using its NIRSpec and MIRI instruments, JWST captured the first direct near- to mid-infrared spectrum of GJ 504b . The data revealed an unusually rich suite of molecules, including water vapor (H₂O), carbon monoxide (CO), methane (CH₄), carbon dioxide (CO₂), ammonia (NH₃), and hydrogen sulfide (H₂S) . Isotopic variants such as ¹³CO and C¹⁸O were also detected .

One of the most significant detections was ammonia (NH₃). A study by Mâlin et al. (2025) reported the first unambiguous detection of ammonia in the atmosphere of a planetary-mass companion, at a 12.5σ confidence level using JWST's MIRI coronagraphic photometry . This precise ammonia measurement opens a new window into nitrogen chemistry on cold planets, a key tracer of their formation conditions .

The star system is estimated to be about 160–250 million years old, making GJ 504b a relatively young world .

A Salty Surprise in the Skies

The biggest surprise from the JWST data was the discovery of salt clouds in GJ 504b's atmosphere. Instead of the silicate clouds seen on hotter exoplanets, the clouds here are composed of sulfides and metal salts such as potassium chloride (KCl) and zinc sulfide (ZnS) .

This is the first time such exotic cloud species have been confirmed on a directly imaged planetary-mass object. The salts form haze layers at the world's cool temperature of ~550 K, validating theoretical models that predicted Jupiter-like atmospheres at these temperatures should form condensate clouds of alkali salts, not silicate dust .

This finding, reported by Northwestern University-led astronomers in June 2026, provides some of the first direct evidence for salt clouds in a cold object's atmosphere .

A Heavier World on the Planet-Brown Dwarf Border

JWST data has also revised the mass of GJ 504b significantly upward. Initially estimated at roughly 4 Jupiter masses, the object is now thought to be ~20–28 Jupiter masses, with a best estimate of around 25 M_J .

This places it near the deuterium-fusion boundary, which blurs the line between planet and brown dwarf (a "failed star"). As a result, astronomers often refer to it as a "planetary-mass companion" rather than a planet . Its high mass makes GJ 504b a critical test object for understanding where the planet/brown dwarf boundary lies and how objects near that limit form and cool .

A New Benchmark for Cold Exoplanets

Before JWST, our understanding of directly imaged exoplanets was largely limited to hot young Jupiters with temperatures between 800 and 1800 K. With an atmospheric temperature of ~550 K, GJ 504b is the coldest directly imaged exoplanet to have its spectrum captured, providing a unique bridge between these hot worlds and our own cold Jupiter (~130 K) .

The detection of salt clouds and a complex chemistry including ammonia has transformed GJ 504b from a faint pink dot into a well-characterized benchmark. Its observations allow scientists to validate models of cloud physics, nitrogen chemistry, and the evolution of gas giant atmospheres at lower temperatures, making it a new anchor point for understanding cold, directly imaged planets .