The Pink Planet’s salty secret: JWST solves a decade-long mystery

Since its discovery in 2013, a peculiar rosy-hued world known as the Pink Planet has kept astronomers in the dark. Too faint for any ground-based telescope to properly analyze, the object lingered as one of the most tantalizing unsolved puzzles in direct-imaging science. Now, thanks to the James Webb Space Telescope, the mystery finally has an answer — and it comes with a salty twist.

A team led by astrophysicist Aneesh Baburaj at Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) has obtained the first direct spectrum of GJ504b, the object’s formal designation, and the findings are unlike anything previously seen in a cold substellar companion. The study appears in The Astronomical Journal.

An object that defies easy labels

GJ504b orbits a sun-like star roughly 57 light-years from Earth and carries a mass approximately 25 times that of Jupiter. That puts it in an ambiguous region of parameter space, hovering at the boundary between giant planets and brown dwarfs — the class of objects sometimes called failed stars. For that reason, astronomers classify it as a planetary-mass companion rather than committing to either category.

What makes it visually distinctive is its temperature. While most directly imaged companions sit between roughly 550 and 1,100 degrees Celsius, GJ504b measures only about 290 degrees Celsius — close to what a kitchen oven reaches when baking bread. Its cool temperature is a consequence of age: the companion is estimated to be between 2.5 and 4 billion years old, and giant planets cool steadily as they age, slowly trading their primordial heat for that characteristic rosy glow.

Salt clouds where no one expected them

Using JWST and advanced data-processing techniques to strip away glare from the host star, Baburaj’s team captured GJ504b’s spectrum in roughly two hours — a task that had defeated entire nights of observation at the world’s largest ground-based facilities. The spectrum revealed a rich atmospheric inventory: water vapor, methane, carbon dioxide, and ammonia, among other molecules.

When the team tried to model that atmosphere, the results only made physical sense after they incorporated clouds. Of three cloud types tested, salt clouds provided the best match. These clouds, made of chloride compounds, suppress the spectral signatures of deeper atmospheric layers in exactly the way the observations required. It is the first time salt clouds have been shown to be essential for correctly interpreting any object’s observed spectrum — confirming a theoretical prediction that had gone unverified for over fifteen years.

The data also hint that GJ504b may be unusually enriched in heavy elements, though the question of whether it formed like a planet or like a small star remains open.

A new toolkit for cold worlds

The methodological achievement may matter as much as the atmospheric result itself. Baburaj notes that the same techniques could eventually be turned toward even colder and fainter objects — including potential analogs of Jupiter, whose ammonia ice clouds currently remain out of reach. Each step toward colder targets expands what direct spectroscopy can tell us about planetary formation, atmospheric chemistry, and the increasingly blurry line between planets and brown dwarfs.

The research was supported by NASA.

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