First spectroscopic observation of the cold, dim object GJ 504b revealed water vapor, methane, carbon dioxide, and ammonia. Atmospheric models only matched the data after researchers added a layer of salty clouds
The James Webb Space Telescope has for the first time been able to break down the light of GJ 504b – a distant, cold, and dim object sometimes called the “pink planet” – and reveal a rich mix of molecules in its atmosphere, along with evidence of clouds made of salts.
This is one of the first direct evidences that salt clouds play a major role in the atmosphere of a cold planetary body. The researchers believe that the clouds obscure the deeper layers of the atmosphere and change the way the light emitted from it reaches the telescope.
The study, led by Anish Baburaj of Northwestern University, was published in the journal The Astronomical Journal.
Pink bone, cold and difficult to classify
GJ 504b was discovered in 2013 using direct infrared imaging. It orbits a Sun-like star about 57 light-years from Earth, in a system in the direction of the Virgo constellation.
The nickname “the pink planet” was born from early images that described the object as having a dull magenta hue, reminiscent of dark cherry blossoms. This is not a standard color photograph of its surface, but a description based on infrared radiation measured from it and models of the atmosphere.
Although it is called a planet, its classification is still uncertain. The new analysis suggests a mass of about 25 Jupiter masses, with a wide range of uncertainty. Such a mass places it on the fuzzy border between giant planets and brown dwarfs – objects that may form like stars, but are not massive enough to sustain stable hydrogen fusion in their cores.
That's why researchers use the more cautious term "planetary-mass companion": an object of similar size to a planet, orbiting a star, but whose exact origin and classification have not yet been determined.
What ground-based telescopes couldn't see
GJ 504b is one of the coldest satellites directly imaged before James B began operating. Its effective temperature is about 564 Kelvin, which is about 291 degrees Celsius. This is a high temperature in terrestrial terms, but very low compared to most directly imaged exoplanets.
The object is also much dimmer than the star it orbits. Previous attempts to obtain a spectrum from it using some of the world's largest telescopes have failed to properly separate its light from the star's glow.
Using James Webb's NIRSpec spectrograph and processing procedures designed to remove starlight, the researchers were able to produce a high-quality spectrum after an observation that lasted only about two hours.
A spectrum breaks down light into its component wavelengths. Different molecules absorb and emit light at characteristic wavelengths, so a spectrum can be used as a kind of chemical “fingerprint” of the atmosphere.
Water, methane, ammonia and also hydrogen sulfide
The spectrum of GJ 504b contained signatures of water vapor, carbon monoxide, methane, carbon dioxide, ammonia, and hydrogen sulfide. The researchers also identified isotopologues—versions of the same molecules that contain different isotopes of carbon and oxygen.
The mixture indicates a complex atmosphere with chemistry that is not necessarily in equilibrium. This means that the movement of gases between different layers of the atmosphere may bring molecules to areas where, given the local temperature and pressure, they should not be present in the measured amounts.
However, when the researchers tried to fit the spectra to a model of a cloud-free atmosphere, they obtained implausible physical properties. Only after introducing clouds into the model were the researchers able to reproduce the observations without requiring outlier parameters.
The clouds that hide the deep layers
The research team examined three possible types of clouds. The model that best fit the data included clouds made of salt compounds, which condense at temperatures appropriate for GJ 504b's atmosphere.
Such clouds act as a partial screen: they attenuate the spectral signatures of molecules in the deeper, warmer layers. Without taking this screen into account, the model could misinterpret the strength of the chemical signatures and draw unrealistic conclusions about the structure of the atmosphere.
Salt clouds have been predicted in theoretical models of brown dwarfs and cold planets for more than a decade, but until now it has been difficult to find direct observational evidence that they are indeed necessary to explain the spectrum of such an object.
According to Baburaj, this is the first time that salt clouds have been found to be an essential component in explaining the spectrum of a directly observed object.
Formed like a planet or like a star?
The spectrum also indicates that GJ 504b's atmosphere is relatively rich in heavy elements. In astronomy, all elements heavier than hydrogen and helium are collectively referred to as "metals."
Such enrichment may support a scenario in which the object formed in a disk of gas and dust around the star, similar to planets. During their formation, planets can accumulate solid materials rich in heavy elements, so their atmospheres may be enriched relative to the star.
However, the researchers emphasize that the results do not yet rule out a formation more similar to that of stars and brown dwarfs – the direct collapse of a gas cloud. GJ 504b's great distance from its star also makes it difficult to explain its formation using simple models of planet formation.
A step towards exploring colder worlds
The research is not just about a single object. It demonstrates how James Webb can be used to study cold, dim planets and brown dwarfs, for which spectra could not be obtained using ground-based telescopes.
As astronomers move closer to studying objects with temperatures more similar to those of the solar system's gas giants, new types of clouds are expected to emerge. In Jupiter's atmosphere, for example, there are clouds containing ammonia crystals. These are still difficult to detect directly on exoplanets, but the discovery of the salt clouds shows that observations are getting closer to this realm.
The result also reminds us that identifying molecules in the spectrum is not enough. To understand the atmosphere of a distant world, one must also take into account clouds, vertical turbulence, pressure, and temperature – all of which affect the light that ultimately reaches the telescope.
FAQ
Why is GJ 504b called the “Pink Planet”?
The name is based on infrared imaging and measurements that showed it as a dull magenta object. This is not a standard color photograph of the planet.
Is GJ 504b really a planet?
This is still uncertain. The mass obtained in the new study places it in the border region between giant planets and brown dwarfs, so the researchers call it a planetary-mass companion.
What are the clouds in its atmosphere made of?
The models indicate clouds of salt compounds that condense at temperatures prevailing in the atmosphere. The clouds obscure some of the deeper layers and change the observed spectrum.
What molecules were found in the atmosphere?
The researchers identified, among other things, water vapor, carbon monoxide, methane, carbon dioxide, ammonia, and hydrogen sulfide.
How hot is GJ 504b?
The effective temperature measured is about 564 Kelvin, or about 291 degrees Celsius. It is considered very cold compared to most directly imaged exoplanets.
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