Just over a decade after GJ 504 b—colloquially known as the Pink Planet because of its dull-magenta thermal glow—was discovered, researchers have finally been able to determine the chemical composition of its atmosphere (Astron. J. 2026, DOI: 10.3847/1538-3881/ae6919). They not only discovered evidence of salty clouds but also created a new molecular inventory that may ultimately help determine how the object formed.
“GJ 504 b is a planetary-mass companion, something which we aren’t sure whether it’s a planet or a brown dwarf,” says Aneesh Baburaj, a postdoctoral scholar at Northwestern University who led the work.
To understand how planetary systems evolve across the universe, it’s vital to discern brown dwarfs from gas giants. Despite their differing formation mechanisms, the largest gas giants are similar in size and brightness to the smallest brown dwarfs. At 25 times the mass of Jupiter, GJ 504 b falls into this blurred region and resists easy identification. So, to determine if the Pink Planet is truly a planet, scientists need to characterize the isotopes in its atmosphere to trace how it formed.
For the last decade, this was easier said than done. GJ 504 b is a relatively cold object 57 light-years away from Earth that only faintly emits infrared light. Although it was detected by a ground-based telescope in 2013, scientists couldn’t collect its spectrum until they were able to point the James Webb Space Telescope (JWST) at it in 2024.
“We could see a lot of molecules in its atmosphere,” Baburaj says. The researchers detected water, carbon dioxide, ammonia, hydrogen sulfide, and more. In addition to identifying various molecules, they were also able to identify isotope ratios of oxygen and carbon.
But when they used the various spectra to model the thermal structure of the atmosphere, the result was strange. “Certain parts of the atmosphere were physically implausible,” Baburaj says. That is, until they added clouds to the equation, and the best fit came from clouds of salt particles. The researchers don’t have the spectra to identify the specific salts, but based on GJ 504 b’s temperature and pressure profiles, they believe potassium chloride or zinc sulfide to be the most likely candidates.
The Pink Planet—formally known as GJ 504 b—was discovered in 2013 through direct imaging. In the infrared image shown here, the planetary-mass object appears in the scattered starlight as a bright white dot distantly orbiting its parent star. Credit:
NASA’s Goddard Space Flight Center/National Astronomical Observatory of Japan
In recent years, scientists have considered the possibility of salt clouds in these kinds of planetary-mass objects, says Jason Dittmann, an astronomer at the University of Florida who focuses on exoplanet atmospheres. And it’s not uncommon for other types of clouds to mute spectral data from brown dwarfs and exoplanets. “That was a paper that everyone has written,” Dittmann says.
The key difference here is that the data collected by the JWST provide enough information to say something about the clouds themselves. “Now the data are good enough where the cloud isn’t just ruining your data and making you not see anything,” Dittmann says. Now, “the cloud is the signal.”
More than just identifying salty clouds, the study is also “part of this effort to start building up these molecular inventories for more and more cold planets,” says Elisabeth Matthews, an astrophysicist at the Max Planck Institute for Astronomy who studies exoplanet atmospheres. As the inventory grows, it’s possible that the data will reveal isotope patterns that distinguish large gas planets from brown dwarfs, she says.
Matthews hopes the study will inspire other researchers to target cold, planetary-mass objects with the JWST. “People have spent entire nights from the ground trying to observe this object” and were unsuccessful, she says. But “with JWST, with a couple of hours, we get this amazing data that really pushes our understanding of the object forward.”
