Northwestern University-led astronomers have discovered salty skies surrounding the universe's famous "Pink Planet."
For more than a decade, the ancient, rosy hazed world kept astronomers guessing. One of the coldest known planetary-mass companions ever directly imaged, the elusive object is too faint for astronomers to dissect its light from Earth. But new observations from the James Webb Space Telescope (JWST) reveal an atmosphere filled with exotic chemistry - and salty clouds unlike anything seen before.
The observations provide some of the first direct evidence for salt clouds in a cold object's atmosphere, a phenomenon scientists theorized more than 15 years ago. The discovery also marks an important step toward studying increasingly cold objects, which are too dim to examine with ground-based telescopes.
The study will be published on Thursday (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."
An expert on exoplanets, Baburaj is a postdoctoral associate at Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). This work was conducted in collaboration with scientists at the Space Telescope Science Institute (STScI), including Marshall Perrin, who devised the observing program for this object. Perrin is a member of the JWST Telescope Scientist Team, which contributed to the telescope's design and is responsible for its current day-to-day operations.
Old and cold
Discovered in 2013, the Pink Planet (dubbed GJ504b) orbits a sun-like star located 57 light-years from Earth. Despite its nickname, astronomers are unsure if it's a planet at all. At roughly 25 times the mass of Jupiter, GJ504b sits near the fuzzy boundary between giant planets and brown dwarfs. So, astronomers refer to it as a "planetary-mass companion," meaning that it's a planet-sized object orbiting a star.
Further complicating the mystery, repeated attempts to study it with ground-based telescopes have fallen short. While most directly imaged exoplanets are closer to 1,000 to 2,000 degrees Fahrenheit, GJ504b is just 550 degrees Fahrenheit (290 degrees Celsius) - roughly the temperature of a bread-baking oven.
The companion's age is responsible for its chilly temperature, Baburaj said. Although they are born blistering hot, giant planets cool as they age. And the new study estimates GJ504b is between 2.5 billion and 4 billion years old.
Using the JWST, Baburaj and his team captured the GJ504b's faint light. Then, they used advanced data-processing techniques to strip away glare from its much brighter host star. This combination finally revealed the companion's spectrum, a graph that breaks down dispersed light into component colors. Each color represents a different element. So, by analyzing an object's spectrum, scientists can uncover the presence of specific elements and molecules.
"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."
Famous world comes into focus
The data revealed a rich mix of chemicals, including water vapor, methane, carbon dioxide, ammonia and other molecules. To reconstruct the companion, the researchers fed these data into an astrophysical model. But something didn't add up. The companion's simulated atmosphere only matched the observations if it contained unusual, physically implausible features. When the researchers added clouds to the model, the unusual characteristics vanished. Salt clouds likely veiled the atmosphere's deeper layers, shaping the light that reached 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 suggested that GJ504b is unusually rich in heavy elements, or metals. However, the mystery of the object's formation persists, with current data suggesting it could have formed either like a planet or a small star.
Baburaj says the techniques used in the study could help unravel other mysteries surrounding cold, faint planets. Jupiter, for example, hosts clouds made of ammonia ice. While those cloud types remain beyond the reach of current observations, the detection of GJ504b's salt clouds suggests astronomers are getting closer.
"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 GJ504b reveals clouds and possible metal enrichment," was supported by NASA (award number 80NSSC20K0586).
