Astronomers unveil secrets of the famous ‘Pink Planet’: “Not like anything we have analyzed before”

Located 57 light-years from Earth and orbiting a star similar to our Sun, one of the most unusual and perplexing worlds ever discovered lies hidden in deep space. Its name is GJ 504b, although the scientific community commonly refers to it as the “Pink Planet” because of the distinctive magenta hue seen in the first images captured in 2013. For more than a decade, this object has been a genuine puzzle for astronomers, who still debate whether it is truly a giant planet or a brown dwarf, a type of failed star.

A salty discovery

Now, new observations from the James Webb Space Telescope have revealed that its atmosphere appears to be shrouded in dense clouds of metallic salt, a phenomenon never before observed so clearly in such a cool and distant object. The discovery, published in The Astronomical Journal by a team led by Northwestern University, marks a major advance in the study of extreme exoplanets and opens a new window into understanding how giant worlds evolve beyond our solar system.

GJ 504b was discovered in 2013 through direct imaging using the Subaru Telescope in Hawaii. From the beginning, it attracted attention because of two highly unusual characteristics. The first was its striking pink coloration, produced by the residual heat from its formation, which still emits a faint thermal glow visible in infrared light. The second was its temperature. While most directly imaged exoplanets easily exceed 1,800°F, this world reaches only about 550°F.

Although that temperature would be scorching by Earth standards, in planetary astronomy it makes GJ 504b one of the coolest objects ever directly observed. Researchers believe this relative coolness is related to its age. According to the latest estimates, the planet is between 2.5 and 4 billion years old, enough time to have gradually cooled since its birth, much like gas giants such as Jupiter.

An in-depth study

Analyzing this strange world has never been easy. Its apparent proximity to its much brighter host star made it nearly impossible to isolate the planet’s light using ground-based telescopes. Scientists compare the challenge to trying to observe a firefly sitting next to the floodlights of a football stadium. For years, some of the world’s most advanced observatories devoted entire nights to obtaining its spectrum without success.

James Webb completely changed the situation. In just two hours of observations, it successfully captured GJ 504b’s faint light and separated it into a high-resolution spectrum, a kind of chemical barcode that allows scientists to identify the elements and molecules present in an atmosphere. The analysis revealed a complex mixture of water vapor, methane, carbon dioxide, ammonia, and other compounds.

However, computer models designed to simulate the planet’s atmosphere initially produced physically impossible results. The solution to the mystery emerged when researchers added clouds composed of vaporized mineral salts, an unexpected ingredient. Suddenly, the simulations matched the observations almost perfectly.

“Once we started digging deeper into the data, we realized it was not like anything we have analyzed before,” said Aneesh Baburaj, an expert in exoplanets at Northwestern University.

According to the research team, these salt clouds act like an atmospheric veil, obscuring deeper layers of the atmosphere and altering the light that escapes into space. This is the first time scientists have obtained such strong evidence that clouds of this type play a crucial role in explaining the atmospheric chemistry of a cool exoplanet.

On Earth, salt is a solid material associated with oceans and salt flats. On worlds like GJ 504b, however, conditions are so extreme that these minerals can evaporate, rise into the atmosphere, and then condense again to form floating clouds. This and other unusual characteristics make GJ 504b an exceptional natural laboratory for scientists.

Studying the planet allows researchers to test new observational and atmospheric modeling techniques that could eventually be used to analyze even smaller, cooler, and more complex worlds, including potentially habitable planets.

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