Salt clouds veiled the atmosphere's deeper layers, shaping the light that reached JWST
Fifty-seven light-years from Earth, a cold and faintly glowing world known as the Pink Planet has finally yielded one of its secrets. Using the James Webb Space Telescope, astronomers at Northwestern University have detected salt clouds in the atmosphere of GJ504b — a phenomenon long theorized but never before observed. The discovery arrives after more than a decade of frustrated attempts with ground-based instruments, and it speaks to a quiet truth in science: some worlds only reveal themselves when we build the eyes worthy of seeing them.
- For over ten years, GJ504b resisted every attempt at atmospheric study — ground telescopes spent entire nights staring and came away with nothing.
- When JWST finally captured the companion's spectrum, the data refused to make sense: atmospheric models kept producing physically impossible results.
- Adding salt clouds to the simulations resolved every inconsistency, marking the first time such clouds have been identified as the key to unlocking an object's spectral signature.
- The Pink Planet also turns out to be unusually rich in heavy elements, deepening the mystery of whether it formed as a planet or as a failed star.
- The techniques developed here now open a path toward studying an entire class of cold, faint worlds that have long been too dim for science to reach.
For more than a decade, astronomers puzzled over a distant world they nicknamed the Pink Planet. Formally designated GJ504b, it orbits a sun-like star fifty-seven light-years away and occupies an awkward place in cosmic taxonomy — at roughly twenty-five times Jupiter's mass, it straddles the boundary between giant planet and brown dwarf. What makes it especially unusual is its temperature: just 550 degrees Fahrenheit, the result of two and a half to four billion years of cooling since its fiery birth. Most directly imaged exoplanets burn far hotter, making GJ504b one of the coldest such objects ever photographed from Earth.
Ground-based telescopes could never gather enough of its faint light to reveal what lay within its atmosphere. Astronomers would spend entire nights at some of the world's largest instruments and come away empty-handed. That changed when Aneesh Baburaj and his team at Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics turned the James Webb Space Telescope toward it. The resulting spectrum showed water vapor, methane, carbon dioxide, and ammonia — but when researchers fed the data into atmospheric models, the numbers refused to align. The simulated world only made sense if it contained physically implausible features.
The breakthrough came when the team added clouds. The inconsistencies vanished. Testing three different cloud compositions, they found that salt clouds fit best — veiling the atmosphere's deeper layers and shaping the light JWST detected. It was the first time salt clouds had been identified as essential to explaining any object's spectrum. The companion also proved unusually rich in heavy elements, though whether that points to a planetary or stellar origin remains an open question.
The significance reaches beyond this one peculiar world. The methods Baburaj's team refined could help astronomers probe other cold, faint companions that have long resisted scrutiny — a class of objects that, until now, has largely hidden in the dark. For the first time, the Pink Planet is coming into focus.
For more than a decade, astronomers have puzzled over a distant world they nicknamed the Pink Planet. Now, after pointing the James Webb Space Telescope at this cold, faint object, a team led by Northwestern University has finally seen what hides in its atmosphere: salt clouds, a phenomenon scientists had theorized about for years but never directly observed.
The Pink Planet, formally known as GJ504b, orbits a sun-like star fifty-seven light-years away. Discovered in 2013, it sits in an awkward place in the cosmic taxonomy—at roughly twenty-five times Jupiter's mass, it could be a giant planet or a brown dwarf, so astronomers call it a planetary-mass companion. What makes it truly unusual is its temperature. While most directly imaged exoplanets burn at between one thousand and two thousand degrees Fahrenheit, GJ504b is merely five hundred fifty degrees—about the heat of an oven mid-bake. The reason is age. Born scorching hot like all giant planets, GJ504b has spent between two and a half and four billion years cooling down, making it one of the coldest such objects ever directly photographed from Earth.
For years, ground-based telescopes could not capture enough of its faint light to reveal what lay in its atmosphere. Astronomers would spend entire nights with some of the world's largest instruments and come away empty-handed. Aneesh Baburaj, a postdoctoral researcher at Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics, knew JWST might change that. When his team finally obtained the companion's spectrum—the breakdown of its light into component colors that reveal which elements and molecules are present—something unexpected emerged. The data showed water vapor, methane, carbon dioxide, ammonia, and other compounds. Yet when researchers fed this information into computer models of the companion's atmosphere, the numbers did not align. The simulated world matched the observations only if it contained physically implausible features.
Then the team tried adding clouds to their models. The inconsistencies vanished. When they tested three different cloud types, salt clouds fit best. The salt clouds, the researchers realized, were veiling the deeper layers of the atmosphere, filtering and shaping the light that JWST detected. This was the first time salt clouds had been identified as critical to explaining an object's spectrum. The discovery also revealed that GJ504b is unusually rich in heavy elements—metals, in astronomical parlance—though whether this means it formed like a planet or like a small star remains unclear.
What makes this finding significant extends beyond the Pink Planet itself. The techniques Baburaj's team developed could help astronomers study other cold, faint worlds that are too dim for ground-based observation. Jupiter, for instance, hosts clouds made of ammonia ice, but those remain beyond current reach. The detection of salt clouds around GJ504b suggests that astronomers are developing the tools to peer into increasingly exotic and distant atmospheres. The work, published in The Astronomical Journal, marks a step toward understanding not just this one peculiar world, but an entire class of cold companions that have long resisted scrutiny. For the first time, the Pink Planet is coming into focus.
Citas Notables
The Pink Planet is the coldest companion ever discovered using ground-based instruments. 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 had analyzed before.— Aneesh Baburaj, Northwestern University
This is the first time we've found that salt clouds are critical to explaining the spectrum of an object. It's a good reminder to account for clouds in our models.— Aneesh Baburaj
La Conversación del Hearth Otra perspectiva de la historia
Why does it matter that we found salt clouds specifically? Couldn't there be other explanations for what JWST saw?
There could have been, but salt clouds fit the data in a way nothing else did. The spectrum didn't make sense until the researchers added them. It's like a puzzle piece that suddenly makes the whole picture coherent.
How cold is five hundred fifty degrees, really? That sounds hot to me.
It is hot by Earth standards, but for a giant planet or brown dwarf, it's ancient and exhausted. Most of these objects are thousands of degrees. This one has been cooling for billions of years.
Why couldn't ground-based telescopes see it before?
The object is too faint. Its light is so dim that even the biggest telescopes on Earth couldn't gather enough of it in a single night. JWST did it in two hours.
Is GJ504b actually a planet, or not?
That's still an open question. It's too massive to be a typical planet but sits right on the boundary with brown dwarfs. The new data doesn't settle it, but they're getting closer.
What happens next? Do we look at other cold objects the same way?
Exactly. The techniques they developed here—the data processing, the modeling with clouds—those can be applied to other faint, cold worlds. It's a template for studying things we couldn't see before.