A study published in The Astronomical Journal on June 18 presents spectroscopic evidence from the James Webb Space Telescope (JWST) suggesting the presence of salt clouds in the atmosphere of the object GJ504b.
The research was conducted by astronomers led by Northwestern University and was supported by NASA award 80NSSC20K0586.
Object Description
GJ504b, first directly imaged in 2013, orbits a sun-like star located 57 light-years from Earth. Its mass is approximately 25 times that of Jupiter, and its estimated temperature is roughly 564 K (approximately 290°C, or 550°F). This makes it one of the coldest directly imaged companions.
The object's estimated age is between 2.5 billion and 4 billion years.
Its classification is unresolved. It sits near the boundary between giant planets and brown dwarfs, and it is referred to by astronomers as a "planetary-mass companion." Its formation process—whether similar to a planet or a small star—remains undetermined.
Observations and Data
Previous attempts to obtain a clear spectrum of GJ504b using ground-based telescopes were unsuccessful due to the object's faintness. JWST's NIRSpec instrument captured its spectrum across the 2.9–5.3 micron wavelength range in approximately two hours. The data were processed to remove glare from the host star.
The spectrum revealed signatures of water vapor, carbon monoxide, methane, carbon dioxide, ammonia, hydrogen sulfide, and isotope-bearing carbon monoxide.
The object appears unusually rich in elements heavier than hydrogen and helium (referred to as "metals"). The data also indicate disequilibrium chemistry, suggesting atmospheric mixing processes.
Evidence for Salt Clouds
Atmospheric modeling that did not include clouds required implausible physical conditions to fit the data. Models that included clouds provided a better fit, with simulations incorporating salt clouds providing the strongest match among the types tested.
The inferred salt clouds are reported to be at altitudes that mute molecular signatures below, which is consistent with the observed spectrum.
Methodology and Uncertainties
The salt cloud interpretation is based on atmospheric model fitting, not on direct imaging of clouds. The study's title describes the finding as "possible."
The object's heavy element enrichment tentatively supports a planet-like formation, but its classification remains unresolved.
The research represents the first time salt clouds have been invoked to explain a spectrum of such a cold object, a phenomenon theorized over 15 years ago.