For more than a decade, astronomers have been puzzled by GJ 504b — a faintly pink world orbiting a sun-like star 57 light-years from Earth. Now the James Webb Space Telescope has done what no earlier instrument could: it has looked into the planet's atmosphere and pulled out a chemical inventory nobody expected.
In results published this month and highlighted by ScienceDaily and the wider science press, an international team reported that GJ 504b's atmosphere contains water vapour, methane, carbon dioxide, ammonia — and, for the first time in an object of its kind, direct evidence of clouds made from salt.
What is the "Pink Planet"?
GJ 504b was first imaged back in 2013, and it immediately stood out. Most exoplanets we know about are either seen indirectly (by the way they tug on their parent stars or dim their light) or, when imaged directly, appear as tiny hot dots. GJ 504b is different: it glows faintly in the near-infrared with a distinctive pinkish hue, giving it the nickname the "Pink Planet."
Its nature has been debated ever since. Depending on assumptions about its age, GJ 504b could be a very young, hot gas giant several times the mass of Jupiter — or a small, failed-star "brown dwarf" that never quite ignited. Either way, it sits far from its parent star, in an orbit wider than Neptune's around the Sun, which makes it an unusually clean target for direct study.
Salt in the sky
Using JWST's NIRSpec instrument, the team obtained a spectrum of GJ 504b in just two hours of telescope time — a remarkable turnaround for direct exoplanet spectroscopy. The spectrum revealed the fingerprints of familiar molecules like water and methane, alongside carbon dioxide and ammonia. But the biggest surprise came from features consistent with alkali salt clouds — the kind of clouds that had been theoretically predicted for cool giant planets but never directly confirmed.
"This atmosphere doesn't resemble anything we had analysed before," the lead author told reporters. "We knew salt clouds were possible in theory. Seeing the evidence directly is a completely different thing."
Why it matters
Understanding a single strange planet may sound niche, but the implications are broad. GJ 504b sits in a regime — cold, wide-orbit, gas-giant-like — that overlaps with the "sub-brown dwarf" population and with hypothetical planet-mass companions imaged around young stars. Being able to read its chemistry helps astronomers refine models that apply across a whole zoo of substellar objects, from young exoplanets in nearby star systems to older, drifting worlds no longer bound to any sun.
The salt clouds also open up questions about atmospheric circulation and weather on these worlds. On Earth, ice clouds and water droplets shape climate. On GJ 504b, the "weather" may involve alkali salt particles condensing and falling through hydrogen-rich air — a completely alien kind of storm system.
What's next
The team has already flagged GJ 504b as a target for follow-up JWST observations at longer wavelengths, which could tighten estimates of the planet's temperature, mass and cloud structure. Combined with data on other directly imaged planets, the results are expected to feed into a new generation of atmospheric models that better fit the strange, faint pink dots astronomers keep finding around neighbouring stars.
For now, one of the sky's more mysterious objects has given up a little more of its secret — and, in doing so, added something genuinely new to the catalogue of alien weather.
