Researchers at Northwestern University have used the James Webb Space Telescope to probe the faint light of GJ 504 b, the famous “Pink Planet,” and found evidence that its chilly atmosphere is veiled by clouds made from salts.
The object has intrigued astronomers for more than a decade. It is cold, dim and difficult to study from Earth. Webb changed that by separating the companion’s weak glow from the glare of its much brighter star.
The findings, published in The Astronomical Journal, offer some of the first direct evidence for salt clouds in a cold planetary atmosphere. They also show how Webb can open a new window on distant worlds that have long sat just beyond the reach of ground-based telescopes.
JWST brings GJ 504 b into focus
The new observations target GJ 504 b, a rosy-hazed companion orbiting a sun-like star about 57 light-years from Earth. Since its discovery in 2013, the object has been famous for its pink appearance and frustrating faintness.
Lead researcher Aneesh Baburaj, a postdoctoral associate at Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics, said the world stood out because of its low temperature. “The Pink Planet is the coldest companion ever discovered using ground-based instruments,” Baburaj said.
Webb’s sensitivity made the difference. The team captured light from the companion and used advanced processing to subtract the overwhelming glare of the host star. That revealed a spectrum, which is a kind of chemical fingerprint made by spreading the object’s light into component wavelengths.
The full observation took about two hours, according to Northwestern. Earlier attempts using major ground-based telescopes could spend an entire night on the object and still fail to recover the same kind of spectrum.
Why the Pink Planet stayed mysterious
GJ 504 b sits in a difficult category. Astronomers call it a planetary-mass companion because it has a planet-like mass and orbits a star. Its estimated mass, roughly 25 times that of Jupiter, places it near the boundary between giant planets and brown dwarfs.
That boundary matters because it shapes ideas about how the object formed. Giant planets are often thought to grow inside disks of gas and dust around young stars. Brown dwarfs can form more like small stars, from collapsing clouds of gas.
The companion’s age adds another complication. Giant planets begin hot and fade as they cool over billions of years. The new study estimates that GJ 504 b is between 2.5 billion and 4 billion years old, which helps explain why it is so dim.
Its temperature is about 550 degrees Fahrenheit, or 290 degrees Celsius. That sounds hot by Earth standards, yet it is cool for a directly imaged planetary companion. Many directly imaged exoplanets are closer to 1,000 to 2,000 degrees Fahrenheit.
That coolness made GJ 504 b a particularly promising Webb target. As Baburaj put it, “That made it a perfect target for JWST.”
A spectrum filled with exotic chemistry
Webb’s spectrum showed that the companion’s atmosphere contains a rich mix of molecules. The team identified signs of water vapor, methane, carbon dioxide, ammonia and other compounds in the faint light that reached the telescope.
Each molecule absorbs light in a distinctive way. When astronomers spread a planet’s light into a spectrum, they can look for dips and features that match known chemical signatures. It is similar to reading a barcode, with each pattern carrying information about the atmosphere.
For a cold and faint object like GJ 504 b, that measurement is difficult because the host star is so much brighter. Webb’s observing power and the team’s data-processing approach allowed researchers to isolate the companion and study its atmosphere directly.
The result gives scientists a rare look at chemistry on a cold, directly imaged world. It also shows why faint companions can hold surprises. Their atmospheres can contain molecules and clouds that behave differently from the hotter planets astronomers have studied more often.
Salt clouds solve the atmospheric puzzle
The team first fed Webb’s spectrum into atmospheric models. Those models tried to reconstruct the conditions that could create the observed light. At first, the results required unusual atmospheric features that the researchers considered physically implausible.
Clouds changed the picture. When the team added cloud layers to the simulations, the models became more consistent with what scientists know about cold planetary atmospheres. Several cloud types were tested during the analysis.
“We tried three different types of clouds and salt clouds fit best,” Baburaj said. The finding points to salt clouds as a key ingredient in the companion’s atmosphere.
These clouds likely mask deeper layers of the atmosphere. By blocking or softening light from below, they can weaken the visible signatures of molecules hidden beneath them. That effect made the spectrum easier to explain without forcing the model into unrealistic conditions.
The idea of salt clouds in cold worlds has been discussed by scientists for more than 15 years. GJ 504 b now provides direct observational support for that prediction in a cold planetary-mass object.
A cold world with a murky origin
The spectrum also suggests that GJ 504 b may be enriched in heavy elements, often called metals by astronomers. In this context, metals include elements heavier than hydrogen and helium. That enrichment could carry clues about how the object formed.
Formation remains uncertain. A planet-like pathway could involve growth inside a disk around the host star. A star-like pathway could involve collapse from gas, on a smaller scale than a normal star.
The current observations leave both possibilities open. The object’s mass, age, chemistry and orbit all matter for that question. Webb has sharpened the view, while future work may be needed to distinguish between the formation scenarios.
That uncertainty is part of the scientific value. Objects such as GJ 504 b sit between familiar categories. Studying them helps researchers test where giant planets end, where brown dwarfs begin and how diverse atmospheres can become.
What this means for future exoplanet studies
GJ 504 b shows how direct spectroscopy with Webb can reveal the atmospheres of cold, faint companions. These objects are especially important because they bridge the gap between hot young exoplanets and cooler worlds more like the giant planets in our own solar system.
Jupiter, for example, has ammonia ice clouds. Those exact cloud layers remain challenging to observe on distant worlds with current techniques. The detection of salt clouds around GJ 504 b suggests that astronomers are moving closer to reading colder atmospheres in greater detail.
The study also highlights the importance of clouds in atmospheric models. Clouds can hide molecules, alter spectra and change the interpretation of a planet’s chemistry. “It’s a good reminder to account for clouds in our models,” Baburaj said.
Future Webb observations and improved modeling could extend this approach to other faint companions. As astronomers gather more spectra from cold worlds, they can compare cloud types, chemical mixtures and heavy-element enrichment across a wider range of planetary environments.
For the Pink Planet, the surprise is especially vivid. A distant world once known mainly for its color now appears to have an atmosphere shaped by exotic chemistry and salty skies.
