Webb Telescope Finds Salt Clouds on the 'Pink Planet' GJ504b

A Cold, Pink World Finally Comes Into Focus

The James Webb Space Telescope has cracked open one of astronomy's longest-standing puzzles. New observations published Wednesday in the Astronomical Journal reveal that GJ504b, the famous magenta-hued 'Pink Planet', has an atmosphere swirling with clouds made of salt. It's some of the first direct evidence for salt clouds in a cold object's atmosphere, a phenomenon scientists theorised more than 15 years ago but had never confirmed until now.

GJ504b orbits a sun-like star roughly 57 light-years from Earth in the constellation Virgo. Discovered in 2013, the object has long frustrated astronomers because it's too faint and cold for ground-based telescopes to dissect its light. At just 290 degrees Celsius, roughly the temperature of a bread-baking oven, it's far cooler than most directly imaged exoplanets, which typically burn at 500 to 1,000 degrees Celsius.

A Two-Hour Triumph

A Northwestern University-led team, headed by postdoctoral researcher Aneesh Baburaj, used Webb to capture the companion's spectrum in about two hours, a feat that had eluded ground-based observatories even after entire nights of observation. "When we finally obtained its spectrum, it immediately looked interesting," Baburaj said. "But once we started digging deeper into the data, we realised it was not like anything we have analysed before."

Salty Skies and Rich Chemistry

The spectrum revealed water vapour, methane, carbon dioxide, ammonia and other molecules. But when the team fed these data into atmospheric models, the results were physically implausible, until they added clouds. Of three cloud types tested, salt clouds composed of compounds such as potassium chloride and zinc sulphide fit best, subduing the signatures of molecules hidden in the atmosphere's deeper layers.

The study also revised GJ504b's vital statistics. Previous estimates placed its mass at roughly four times that of Jupiter, but the Webb data suggest it's approximately 25 times Jupiter's mass and between 2.5 billion and 4 billion years old, placing it near the boundary between giant planets and brown dwarfs.

Opening a Window on Colder Worlds

The techniques developed for this study could help astronomers probe even colder and fainter objects in the future. Jupiter, for instance, hosts ammonia-ice clouds that remain beyond current observational reach, but the detection of GJ504b's salt clouds suggests researchers are closing the gap. "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."