For more than ten years, a mysterious pink world has puzzled astronomers. Discovered in 2013, GJ504b, nicknamed the Pink Planet, orbits a Sun‑like star about 57 light‑years away. Despite its name, scientists aren’t even sure it’s truly a planet. At about 25 times Jupiter’s mass, it sits right on the blurry line between giant planets and brown dwarfs. So, they call it a “planetary‑mass companion”, a planet‑sized body circling a star.
It’s one of the coldest such companions ever directly imaged, but from Earth it’s too faint to study in detail. Now, the James Webb Space Telescope (JWST) has revealed something extraordinary: an atmosphere rich in exotic chemistry, with salty clouds unlike anything seen before.
This marks the first direct evidence of salt clouds in the atmosphere of a cold world, an idea scientists proposed more than 15 years ago. The discovery opens the door to studying even colder, dimmer objects that ground‑based telescopes can’t reach.
Northwestern’s Aneesh Baburaj, who led the study, said, “The Pink Planet is the coldest companion ever discovered using ground-based instruments. 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.”
Most exoplanets that astronomers have directly imaged are scorching hot, around 1,000 to 2,000 °F. By contrast, GJ504b is much cooler at just 550 °F (290 °C), about the heat of a bread‑baking oven.
Its age explains this chill, says Baburaj. Giant planets start out blazing hot but cool as they grow older. The new study estimates GJ504b is 2.5 to 4 billion years old.
The team used JWST to capture the planet’s faint glow. Later, using some advanced techniques, they were able to block out the glare from its brighter star, finally revealing the planet’s spectrum, a rainbow‑like breakdown of its light. Each color corresponds to a different element or molecule, allowing scientists to identify what’s in its atmosphere.
Baburaj said, “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. And they could not see the object. With JWST, our entire observation took around two hours, and we were successful.”
The data showed a mix of chemicals, water vapor, methane, carbon dioxide, ammonia, and more. To understand the companion, researchers built a model of its atmosphere. At first, the results didn’t make sense: the simulated atmosphere matched the observations only if it included strange, unrealistic features. But when the team added clouds to the model, everything clicked. The best fit came from salt clouds, which likely hide deeper layers and shape the light JWST detected.
“We ran simulations with clouds, and salt clouds fit best,” Baburaj explained. “They muted the signals from molecules deeper down, making the results physically possible.”
The spectrum also hinted that GJ504b is unusually rich in heavy elements. Yet its origin remains a mystery; it may have formed like a planet or like a small star.
Baburaj says the same techniques could help solve puzzles about other cold, faint worlds. Jupiter, for instance, has ammonia ice clouds. While those remain beyond current reach, the detection of salt clouds in GJ504b shows astronomers are getting closer.
“This is the first time we’ve found salt clouds are key to explaining an object’s spectrum,” Baburaj said. “It’s a reminder that clouds matter.”
Journal Reference:
- Aneesh Baburaj, Jean-Baptiste Ruffio, Marshall Perrin et al. JWST-TST High Contrast: First Direct Spectroscopy of GJ 504 b Reveals Clouds and Possible Metal Enrichment. The Astronomical Journal. DOI 10.3847/1538-3881/ae6919
