James Webb Finds Salt Clouds on the Pink Planet GJ 504b

A world 57 light-years away has been quietly glowing a strange shade of pink for as long as we've been able to see it. We now have a good idea why: its sky appears to be full of salt.

Using the James Webb Space Telescope, astronomers have read the atmosphere of GJ 504b — nicknamed the Pink Planet — and found that clouds of salt are the missing ingredient needed to explain its colour and its light. It is the first time researchers have had to lean on salt clouds to make sense of an object's spectrum, and it nudges open a window onto a class of worlds that sit in one of the genuinely weird gaps of planetary chemistry.

A salt cloud where there should be no salt cloud

Clouds are usually a giveaway of temperature. On Earth they are water. On Jupiter they are ammonia. On scorching giant worlds, clouds can be made of vaporised rock — actual silicate droplets raining iron and minerals. GJ 504b lives in the awkward middle: too hot for water or ammonia clouds to form, too cool for rock clouds to condense.

In that no-man's-land, something stranger settles out of the air. The Webb data is best explained by chloride salts such as potassium chloride, possibly mixed with sulfide salts like manganese sulfide, hanging high in the atmosphere. These are exactly the compounds chemists predicted should appear at these temperatures, but seeing real evidence of them in a real atmosphere is new.

The lead researcher, Aneesh Baburaj of Northwestern University's astrophysics centre, summed up the surprise plainly: people had theorised salt clouds might exist at these temperatures, and here was a world that seemed to wear them.

Why the planet looks pink

The nickname is not marketing. When GJ 504b was first imaged in 2013, it stood out as a faint magenta-rose dot, an oddly warm colour for a giant world. For more than a decade that pinkish tint was a puzzle, and one of the leading guesses was that fine salt clouds were scattering and filtering the light.

Until now that was a hypothesis with no hard proof. The new Webb spectrum is the first concrete support for it. When the team modelled the atmosphere with salt clouds included, the clouds muffled the signatures of molecules buried deeper down and the whole picture finally clicked. As Baburaj put it, salt clouds simply fit best.

The coldest alien world we've ever photographed

Most exoplanets are studied indirectly — we watch a star dim as a planet crosses it, or wobble as a planet tugs on it. GJ 504b is rare because we can actually point a telescope at it and collect its own light. That makes it part of a tiny club of directly imaged worlds.

It is also remarkably cool for a giant. Its temperature sits around 290°C (roughly 550°F). That sounds blistering by human standards, yet for a young gas giant it is downright chilly, which is part of what makes it so valuable. By some measures it is the coldest world astronomers have ever directly photographed, and cold worlds are precisely where this kind of exotic cloud chemistry is expected to show up.

The object is thought to have formed somewhere between 2.5 and 4 billion years ago, old enough to have cooled to this temperature. It orbits its sun-like star, GJ 504 A, at about 43 astronomical units — a distance roughly like Pluto's orbit around our own Sun, which is itself a headache for theories of how giant worlds form so far out.

Planet, or failed star?

Here is where GJ 504b gets philosophically slippery. It weighs in at about 25 times the mass of Jupiter. That places it right on the fuzzy border between a giant planet and a brown dwarf — an object too big to be a planet but too small to ignite as a true star.

Because of that, scientists hedge and call it a planetary-mass companion. The honest answer is that we don't have a clean category for it. That ambiguity is not a footnote; it is exactly why objects like this are worth studying. They are natural laboratories for the question of where planets end and stars begin.

Two hours that ground telescopes couldn't manage

The technical feat behind the discovery is as striking as the result. Imaging a faint companion next to a bright star is like trying to photograph a firefly hovering beside a stadium floodlight. The star's glare drowns everything.

Webb's instruments are built to block and subtract that glare, isolating the dim light of the companion and spreading it into a spectrum. The atmosphere it revealed is busy: water vapour, methane, carbon dioxide and ammonia, alongside the salt clouds. What stands out is the speed. The full observation took around two hours. Ground-based attempts had wrestled with this object across whole nights and still struggled. The work was part of a Webb programme dedicated to high-contrast imaging, and it produced the first direct spectroscopy of GJ 504b, published in The Astronomical Journal in June 2026.

Why this matters beyond one pink dot

It is tempting to file this under cosmic trivia — a faraway world with salty skies. The bigger payoff is in the method.

• It confirms that salt clouds are real features of cool giant atmospheres, not just a theorist's hunch.
• It shows clouds can hide molecules sitting deeper in an atmosphere, a warning to anyone reading alien skies without accounting for them.
• It demonstrates Webb can pull a detailed chemical fingerprint from a faint, directly imaged companion in a couple of hours.

That last point is the quiet revolution. As astronomers turn Webb and future telescopes toward smaller, cooler, more Earth-like targets, knowing how clouds distort a spectrum is the difference between reading an atmosphere correctly and being fooled by it. A planet whose surface looks barren might simply be veiled in cloud; a signal that looks like one gas might be another, muted by a haze of salt.

The Pink Planet, in other words, is a practice run. It taught us that when we finally peer at a distant world hoping to spot the chemistry of life, we had better first ask what the clouds are made of. Sometimes, the answer is salt.

What comes next

GJ 504b will almost certainly be revisited, both to pin down whether it is truly planet or brown dwarf and to refine exactly which salts float in its air. Its hint of possible metal enrichment — more heavy elements than expected — also feeds into the deeper debate about how it formed so far from its star.

For now, the headline is simple and true. A world that has glowed pink in our images for over a decade has finally given up its secret, and the answer was hiding in plain sight: a sky dusted with salt, on the coldest planet we have ever managed to photograph.