Astronomers led by Northwestern University have discovered an unusual feature around the famous “pink planet”: a sky filled with salty clouds.
The ancient world, known for its pinkish haze, has remained one of astronomy’s enduring mysteries for more than a decade. One of the coldest planetary-mass companions ever imaged directly, the object is so faint that scientists have struggled to analyze the light from Earth. Now, observations from the James Webb Space Telescope (JWST) reveal an atmosphere filled with exotic chemicals and clouds unlike anything seen before.
The discovery provides some of the first direct evidence that salt clouds may exist in the cold planet’s atmosphere, confirming predictions scientists first made more than 15 years ago. The results also highlight JWST’s ability to study the frigid, low-light world that is inaccessible to ground-based observatories.
The study was published in the journal June 18th. astronomy magazine.
“The pink planet is the coldest star ever discovered using ground-based instruments,” said study leader Aneesh Babraj of Northwestern University. “Many teams around the world made follow-up observations to study its light, but it was too small for ground-based instruments, so it was a perfect target for JWST. When we finally got the spectrum, it immediately seemed interesting. But as we started digging deeper into the data, we realized it was different from anything we had analyzed before.”
Babraj is an exoplanet researcher and postdoctoral fellow at Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). The project also involved scientists from the Space Telescope Science Institute (STScI), including Marshall Perrin, who designed the astronomical observation program. Mr. Perrin is part of the JWST telescope scientist team that assists in the development of the observatory and supports its continued operations.
An unidentified cold world
First discovered in 2013, the pink planet, officially known as GJ 504 b, orbits a sun-like star about 57 light-years from Earth. Despite its nickname, researchers aren’t sure if it’s actually a planet.
GJ 504 b, which has approximately 25 times the mass of Jupiter, lies near the boundary between the giant planet and brown dwarf. Because of this ambiguity, astronomers classify it as a “planetary mass companion,” an object with the mass of a planet orbiting a star.
Its low temperature makes it even more appealing. Most directly imaged exoplanets have temperatures in the range of about 1,000 to 2,000 degrees Fahrenheit. By comparison, the GJ 504 b is only about 550 degrees (290 degrees Celsius), similar to the temperature inside a bread baking oven.
The object’s age helps explain its relative coldness, Babraj said. Giant planets start their lives very hot and gradually cool down over billions of years. A new study estimates that GJ 504 b is between 2.5 billion and 4 billion years old.
James Webb reveals the planet’s spectrum
To study the object, Baburaj and his colleagues used JWST to collect its faint light. They then applied advanced processing techniques to remove glare from the much brighter host star.
This approach allowed the team to obtain a companion spectrum that separates light into its component colors. Different elements and molecules leave unique signatures in the spectrum, and scientists can use this information to determine the composition of the atmosphere.
“In the past, other astronomers have used some of the world’s largest telescopes to observe companion stars overnight to obtain spectra,” Babraj said. “And they couldn’t see the object. Our entire observation took about two hours using JWST, but it was successful.”
Salt cloud solves age-old mystery
Observations have revealed that the atmosphere contains water vapor, methane, carbon dioxide, ammonia, and other molecules.
When researchers tried to recreate the atmosphere using computer models, they encountered a problem. The observations were matched only by atmospheric conditions that seemed physically impractical.
This solution emerged when the team added cloud to the model. When I added clouds, the strange feeling disappeared. This result suggests that salt clouds obscure the deep layers of the atmosphere, influencing the light that ultimately reaches JWST.
“We ran simulations using clouds, and the results matched what we know about cold planets,” Babraj said. “We tried three different types of clouds, and salt clouds were the best fit. Considering the salt clouds suppressed molecular signatures hidden deep in the companion star’s atmosphere. The result then became physically possible.”
The spectrum also indicates that GJ 504 b may contain unusually large amounts of heavy elements, often referred to as metals by astronomers. Still, questions remain about how the object formed. Current evidence suggests that it may have arisen either through the process of forming planets or through the process of forming small stars.
A new way to study cold alien worlds
Baburaj believes the methods developed for this study could help scientists study the celestial bodies of other cool, dark planets.
For example, Jupiter contains clouds of ammonia ice. Although current instruments still cannot directly study these cloud layers in the same detail, the detection of salt clouds around GJ 504 b suggests that astronomers are getting closer to that goal.
“This is the first time we have discovered that salt clouds are important in explaining the spectrum of an object,” Babraj said. “This is a good reminder to consider clouds in your model.”
This study, “JWST-TST High Contrast: First Direct Spectroscopy of GJ 504 b Reveals Possible Cloud and Metal Enrichment,” was supported by NASA (Award Number 80NSSC20K0586).
