Famous Pink Planet Mystery Decoded: JWST Says Salt Clouds are the Culprit

This Magenta world's strange atmosphere made no sense until JWST found intriguing clouds made of salt.

One of the coldest planetary-mass objects ever discovered has finally revealed its secrets. After years of failed attempts using ground-based telescopes, astronomers used the James Webb Space Telescope (JWST) to obtain the first detailed spectrum of GJ504b, a mysterious world nicknamed the Pink Planet.

“The Pink Planet is the coldest companion ever discovered using ground-based instruments. Many teams all around the world performed follow-up observations to study its light, but it was too faint for ground-based instruments,” Aneesh Baburaj, who led the JWST-backed study and is an astronomer at Northwestern University (NU), said.

The observations uncovered water vapor, methane, carbon dioxide, and ammonia in its atmosphere, along with evidence for salt clouds that appear to shape the light escaping from the object. 

These results open a new window into studying cold, faint worlds that have remained beyond the reach of previous instruments.

A world that refused to give up its secrets

When GJ504b was first discovered in 2013, orbiting a Sun-like star about 57 light-years away in the constellation Virgo, it immediately stood out. Unlike many directly imaged giant exoplanets, which are blisteringly hot and relatively young, this object appeared surprisingly cool.

The object’s nature is also uncertain. With an estimated mass roughly 25 times that of Jupiter, GJ504b sits near the boundary between giant planets and brown dwarfs—failed stars that form like stars but never become massive enough to sustain hydrogen fusion. 

Astronomers therefore classify it as a “planetary-mass companion” rather than a confirmed planet. The new JWST study estimates that GJ504b is between 2.5 billion and 4 billion years old—far older than earlier estimates that suggested an age of only around 160 million years. 

Giant planets and brown dwarfs are born extremely hot and gradually cool over billions of years. The revised age helps explain why GJ504b’s atmospheric temperature is only about 290 degrees Celsius (550 degrees Fahrenheit), making it one of the coldest planetary-mass companions ever directly studied.

However, its low temperature also created a major observational challenge. As the object emits so little light, astronomers spent years attempting to obtain a detailed spectrum, a measurement that reveals the chemical fingerprints hidden within a planet’s light. 

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Observations from powerful ground-based telescopes repeatedly came up short. In some cases, researchers spent an entire night using some of the world’s largest telescopes and still failed to obtain a useful spectrum. The signal was simply too weak and too easily overwhelmed by the glare of its host star.

JWST changed the equation

Using the James Webb Space Telescope’s exceptional infrared sensitivity, researchers collected enough light from GJ504b in just a couple of hours to build its first detailed spectrum. 

To isolate the faint companion, the team employed sophisticated processing techniques that removed the overwhelming glare of the nearby star, allowing the object’s atmospheric fingerprints to emerge. 

Once they had the spectrum, they found evidence for water vapor, methane, carbon dioxide, and ammonia. Still, something was wrong. The data refused to match standard atmospheric models. The results repeatedly implied atmospheric conditions that did not make physical sense.

“When we finally obtained its spectrum, it immediately looked interesting. But once we started digging deeper into the data, we realized it was not like anything we have analyzed before,” the study authors note.

The mystery deepened until the researchers tried a different assumption.

The missing ingredient was salt

Clouds often cause trouble in planetary science. They scatter and absorb light, hiding deeper atmospheric layers and making planets appear very different from what they really are. 

The JWST team tested several cloud scenarios in their simulations. One by one, they compared the resulting spectra against the telescope’s observations. 

Interestingly, the best match came from an unexpected source, which was clouds composed of salts.

“We ran simulations with clouds, and the results aligned with what we know about cold planets. We tried three different types of clouds, and salt clouds fit best,” the Northwestern team added.

Once salt clouds were added to the model, the apparent contradictions vanished. The clouds effectively concealed portions of the deeper atmosphere, muting chemical signatures that had been confusing the analysis. Suddenly, the atmosphere behaved in ways that matched what astronomers would expect from a cold planetary-mass object.

More than just a salty sky

The discovery marks the strongest evidence yet that salt clouds can shape the appearance of cold worlds beyond the solar system. 

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While such clouds have been predicted for years, GJ504b appears to be the first known case where they are needed to explain an observed spectrum. The finding also highlights JWST’s growing ability to probe increasingly cold and faint objects, revealing atmospheric processes that were previously out of reach.

At the same time, GJ504b continues to guard some of its secrets. The spectrum suggests the object may be enriched in heavy elements, a clue that could help scientists determine how it formed. 

However, the evidence remains inconclusive. Some characteristics point toward a giant planet, while others are consistent with a brown dwarf origin. Future JWST observations and improved atmospheric models could help resolve this mystery. 

The study is published in The Astronomical Journal.