SANTA CRUZ — Xi Zhang, a professor of Earth and planetary science at UC Santa Cruz, has discovered that an exoplanet classed as a “super-Jupiter” has substantial differences from our solar system’s largest planet — and in fact, has much in common with Mars.
Exoplanet VHS 1256b, located 40 light years away from Earth, was identified in 2015. The planet has a similar volume as Jupiter but is 10 to 20 times its mass, earning it the super-Jupiter or brown dwarf title — smaller than a star, and similar to gas giant planets. This particular exoplanet quickly captured astronomers’ attention with its extreme variations in brightness.
Most objects in space appear to blink, due either to physical changes within the planet or star, or external factors. For super-Jupiter exoplanets, Zhang said, this change in brightness is usually minimal, hovering at 1 to 2%. But on VHS 1256b, brightness variations neared 40%, the largest ever recorded for an object of its size.
The mystery made it a target for researchers using the James Webb Space Telescope to directly image exoplanets. One of those researchers was UCSC astronomer Andrew Skemer, who is a co-principal investigator of the James Webb Space Telescope Early Release Science program.
In 2023, Skemer and his former graduate student Brittany Miles co-authored a paper revealing the chemical makeup of the planet’s atmosphere, home to silicate clouds made of sand-like crystals that are vaporized and then condensed. Still, questions remained. Most notably, scientists wondered how these clouds were distributed across the planet, and whether they could account for the huge fluctuations in brightness.
Zhang, who had been studying planetary atmospheres for years, wanted to find out. He used a modified version of a general circulation model — a computer program commonly used to model Earth’s atmosphere and climate dynamics — to create a simulation of VHS 1256b’s atmosphere. He and his research team experimented with different versions of their model, trying to create one that would replicate the observed brightness changes.
The team was working on the assumption that the distant exoplanet had key similarities to Jupiter. At most wavelengths of light, Jupiter, like its distant brown dwarf cousins, had a 1 to 2% brightness variability. But, at a certain wavelength, that variability jumped to 20%. That represented the planet’s famous Great Red Spot, a storm roughly the size of the Earth. So, Zhang thought, maybe VHS 1256b has some kind of great red spot, too. They tried to create a model with some kind of big storm, but struck out.
“We tried, but we cannot,” Zhang said. “No way, we just cannot get it right. And so we got puzzled.”
Zhang and his team eventually had to consider the possibility that the exoplanet was, in some way, fundamentally different from Jupiter. Jupiter rotates much faster than Earth, its days lasting only nine hours. Zhang had been modeling VHS 1256b in a similar way, with fast rotation and short days. This fast rotation doesn’t allow storms or clouds to grow large. Instead, the force from the speedy rotation causes clouds to form into the neat bands and spots that can be seen on Jupiter. Zhang decided to see what would happen if he slowed down the model’s rotation.
“We said, ‘OK, why not?’ We’ll just try slow rotation,” Zhang said. “And when we try slow rotation, magic happens.”
In the slow rotating simulation, Zhang saw massive clouds of dust form across the planet’s surface. The clouds were unstable, forming and dissipating over time. Dust plumes were dredged from below into the atmosphere by the planet’s heat, which is much higher than Jupiter’s, and could form small cloud patches or global dust storms.
In the end, Zhang’s simulation showed that VHS 1256b is not as similar to Jupiter as scientists expected. The planet rotates once every 22 hours, compared to Jupiter’s nine. It is also much hotter — while Jupiter sits at around 128 Kelvin, or -224 degrees Fahrenheit, VHS 1256b is about 1,300 Kelvin or 1,880 degrees Fahrenheit. The planet’s turbulent clouds reminded Zhang not of Jupiter, but of Mars’ chaotic, unpredictable dust storms.
“I feel quite surprised,” Zhang said. “This has nothing to do with Mars, right? It’s super different, but its actual physical mechanism could be the same.”
These enormous and ever-evolving dust clouds explain the planet’s mysterious dips and spikes in brightness. Zhang believes the phenomenon could be present on other exoplanets, too.
“I think this is very strong evidence to show that silicate clouds cause brightness changes, at least for this object,” Zhang said. “But I believe it’s universal.”
This kind of research could revolutionize scientists’ understanding of planetary atmospheres. Before astronomers could directly observe exoplanets, they were limited to studying our own solar system’s eight planets. Now, with the James Webb Space Telescope and direct imaging exoplanet programs, scientists are able to dive into atmospheric dynamics that have never been observed.
For Zhang, the next step is to gather more information about other super-Jupiters to see if they follow similar patterns. Perhaps, he said, there will be a predictable correlation between brightness variation and rotation period that has to do with silicate dust clouds. VHS 1256b could end up as a point on a graph showing a clear relationship between the two factors.
“I would be very much happy to see that happen,” Zhang said, “but nature always surprises us.”
Zhang also thinks silicate dust clouds could play a significant role in a decades-long mystery known as the L/T Transition, a stage in brown dwarf or super-Jupiter planets when their temperatures are around 1,400 Kelvin or above, appear red. Somewhere between 1,400 and 1,200 Kelvin, there is an abrupt transition, and planets with temperatures below this threshold appear blue. This sharp change indicates a sudden change in the planets’ atmospheres at that temperature threshold, Zhang said, and nobody knows why.
VHS 1256b, sitting just above that temperature boundary at 1,300 Kelvin, is red. After discovering more about its atmosphere and its massive dust storms, Zhang thinks dust likely plays a significant role in this red to blue transition. Maybe, he hypothesized, these massive dust storms dissipate when temperatures drop to 1,200 Kelvin, causing the atmosphere to appear more clear.
“Unfortunately, no one has confirmed this hypothesis,” Zhang said. “But I think there’s a smoking gun.”
