Astronomers led by Northwestern University have uncovered an unusual feature surrounding the famous “Pink Planet”: skies filled with salty clouds.
For more than a decade, the ancient world, known for its pinkish haze, has remained one of astronomy’s enduring mysteries. As one of the coldest planetary-mass companions ever directly imaged, it is so faint that scientists have struggled to analyze its light from Earth. Now, observations from the James Webb Space Telescope (JWST) have revealed an atmosphere packed with exotic chemistry and clouds unlike any previously observed.
The findings provide some of the first direct evidence that salt clouds can exist in the atmosphere of a cold planetary object, confirming a prediction scientists first made more than 15 years ago. The results also highlight JWST’s ability to study extremely cold and faint worlds that are beyond the reach of ground-based observatories.
The study was published on June 18 in the Astronomical Journal.
“The Pink Planet is the coldest companion ever discovered using ground-based instruments,” said Northwestern’s Aneesh Baburaj, who led the study. “Many teams all around the world performed follow-up observations to study its light, but it was too faint for ground-based instruments. That made it a perfect target for JWST. When we finally obtained its spectrum, it immediately looked interesting. But once we started digging deeper into the data, we realized it was not like anything we have analyzed before.”
Baburaj, an exoplanet researcher, is a postdoctoral associate at Northwestern’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 observing program for the object. Perrin serves on the JWST Telescope Scientist Team, which helped develop the observatory and supports its ongoing operations.
A Chilly World With an Uncertain Identity
First discovered in 2013, the Pink Planet, formally known as GJ 504 b, orbits a Sun-like star about 57 light-years from Earth. Despite its nickname, researchers are not certain that it is actually a planet.
With a mass roughly 25 times that of Jupiter, GJ 504 b lies near the boundary between giant planets and brown dwarfs. Because of this ambiguity, astronomers classify it as a “planetary-mass companion,” an object with planet-like mass that orbits a star.
Its low temperature has added to its intrigue. Most directly imaged exoplanets have temperatures ranging from about 1,000 to 2,000 degrees Fahrenheit. By comparison, GJ 504 b is only about 550 degrees Fahrenheit (290 degrees Celsius), similar to the temperature inside a bread-baking oven.
According to Baburaj, the object’s age helps explain its relative coolness. Giant planets begin their lives extremely hot and gradually cool over billions of years. The new research estimates that GJ 504 b is between 2.5 billion and 4 billion years old.
James Webb Reveals the Planet’s Spectrum
To investigate 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 the companion’s spectrum, which separates light into its component colors. Because different elements and molecules leave unique signatures in a spectrum, scientists can use this information to determine the composition of an atmosphere.
“In the past, other astronomers observed the companion for an entire night with some of the biggest telescopes in the world to obtain a spectrum,” Baburaj said. “And they could not see the object. With JWST, our entire observation took around two hours, and we were successful.”
Salt Clouds Solve a Longstanding Mystery
The observations revealed an atmosphere containing water vapor, methane, carbon dioxide, ammonia and other molecules.
When researchers attempted to recreate the atmosphere using computer models, they encountered a problem. The observations could only be matched by atmospheric conditions that did not seem physically realistic.
The solution emerged when the team added clouds to the models. Once clouds were included, the strange inconsistencies disappeared. The results suggest that salt clouds are obscuring deeper layers of the atmosphere and influencing the light that ultimately reaches JWST.
“We ran simulations with clouds, and the results aligned with what we know about cold planets,” Baburaj said. “We tried three different types of clouds, and salt clouds fit best. When we accounted for salt clouds, it subdued the signature of molecules hidden deeper in the companion’s atmosphere. Then, the results became physically possible.”
The spectrum also indicates that GJ 504 b may contain an unusually large amount of heavy elements, often referred to by astronomers as metals. Even so, questions remain about how the object formed. Current evidence suggests it may have originated either through processes that create planets or through processes that form small stars.
A New Way to Study Cold Alien Worlds
Baburaj believes the methods developed for this study could help scientists investigate other cold and faint planetary objects.
Jupiter, for example, contains clouds made of ammonia ice. Although current instruments cannot yet directly study those cloud layers in the same detail, the detection of salt clouds around GJ 504 b suggests astronomers are moving closer to that goal.
“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.”
The study, “JWST-TST High Contrast: First Direct Spectroscopy of GJ 504 b Reveals Clouds and Possible Metal Enrichment,” was supported by NASA (award number 80NSSC20K0586).
