The exoplanet GJ 504b appears pink in visible light, earning it the nickname "Pink Planet" among astronomy enthusiasts. Since its discovery in 2013, the composition of this planet's atmosphere has remained a mystery — its host star's glare was too bright for ground-based telescopes to capture the faint signal from the planet itself. New observations from the James Webb Space Telescope (JWST) have finally revealed the answer, and the results were unexpected.
A team led by Aneesh Baburaj of Northwestern University used JWST's near-infrared spectrograph to study GJ 504b. The entire observation took about two hours — a task that multiple teams using the world's largest ground telescopes had attempted for entire nights without success. "When we finally obtained its spectrum, it immediately looked interesting," Baburaj said. "But once we started digging deeper into the data, we realized it was not like anything we have analyzed before."
The spectrum first revealed absorption signatures of water vapor, carbon dioxide, methane, and ammonia. These molecules themselves weren't surprising — they're common in cold gas giant atmospheres. The problem was that no matter how the team adjusted their model parameters, simulations couldn't match the observations.
That changed when they added a component never before considered in exoplanet atmospheric models: salt clouds.
The team tested three different cloud types, and salt clouds provided the best fit. The salt clouds, buried deep in the atmosphere, suppressed the spectral signatures of molecules hidden below, making the overall spectrum physically plausible. "This is the first time we've found that salt clouds are critical to explaining the spectrum of an object," Baburaj said. "It's a good reminder to account for clouds in our models."
GJ 504b is a gas giant orbiting a sun-like star about 57 light-years from Earth, with a mass roughly 25 times that of Jupiter and a surface temperature of about 290 degrees Celsius. Despite its name, some astronomers believe it's more likely a brown dwarf — a "failed star" too light to sustain hydrogen fusion at its core. Such objects are collectively called "planetary-mass companions."
The team estimates GJ 504b's age at between 2.5 and 4 billion years. Gas giants and brown dwarfs are born scorching hot and gradually cool with age, which explains its relatively low surface temperature.
Another notable finding: GJ 504b's atmosphere is unusually enriched in elements heavier than hydrogen and helium — what astronomers call "metals." This means scientists still can't determine how it formed: did it coalesce in a protoplanetary disk like a planet, or collapse from a gas cloud like a star?
This puzzle will require further observations to resolve. The study was published June 18 in The Astronomical Journal.