‘Crazy idea’ about cooling effects of Pluto’s haze confirmed by new Webb data

The first observations of Pluto by NASA’s James Webb Space Telescope (JWST) reveal dramatic phenomena on its surface, like seasonal cycles of volatile ice redistribution across its surface, and material being pulled from its very atmosphere onto its main satellite, Charon—an eerie interaction that happens nowhere else in our solar system.

These exotic conditions are detailed in a series of studies published this spring by an international team of researchers. But while the image of molecules from one globe’s atmosphere drifting through space and settling on its celestial sidekick’s north and south poles seems strange, one UC Santa Cruz researcher on the team is smiling.

The most recent paper, published in Nature Astronomy on June 2, confirms hypotheses first made by UC Santa Cruz’s Xi Zhang about Pluto’s atmosphere based on the historic flyby of NASA’s New Horizons spacecraft in 2015 that gave researchers the closest look yet at the curious orb at the edge of the solar system. Less than a decade earlier, Pluto’s long-held status as a full-fledged planet was downgraded to “dwarf planet” due to various cosmic criteria it didn’t meet.

In the wake of New Horizons’ observations of Pluto, Zhang published a paper in 2017 that hypothesized that Pluto’s atmosphere was dominated by haze particles, which would’ve made it completely different from other atmospheres in the solar system. Zhang, a professor of Earth and planetary sciences, posited that these haze particles heat up and cool down, controlling the whole energy balance in Pluto’s atmosphere.

“It was a crazy idea,” said Zhang, adding that many of his peers at the time expressed skepticism. But he and his co-authors also made a clear prediction in their 2017 paper: If the haze is cooling Pluto, it should be emitting strong mid-infrared radiation, and that should be observable once a big and powerful enough telescope was available to astronomers.

That moment arrived on Christmas Day 2021, when NASA launched JWST into space to enable observations that would far surpass those made by its ground-based predecessors over the last several decades. Zhang said the current JWST study was motivated by his 2017 hypothesis. “We were really proud, because it confirmed our prediction,” he said. “In planetary science, it’s not common to have a hypothesis confirmed so quickly, within just a few years. So we feel pretty lucky and very excited.”

Hazy conditions

The Pluto flyby in 2015 revealed a world with surprising landscapes, marked by complex topography—basins, mountains, and valleys—ongoing geological activity like nitrogen (N₂) and methane (CH₄) glaciers, and a chemically rich atmosphere containing volatile compounds such as N₂, CH₄, and carbon monoxide. Pluto’s hazy atmosphere formed from coupled methane and nitrogen photochemistry, similar to the haze around Saturn’s moon Titan.

In contrast, Charon was shown to lack an atmosphere and have a more uniform surface dominated by water ice mixed with ammonia-based compounds. Its darker, reddish polar regions are thought to result from the capture and chemical transformation of CH₄ molecules escaping from Pluto’s atmosphere.

The recent observations with JWST provide a fresh look at this distant system. As reported in the series of papers published this spring, for the first time, the telescope’s MIRI instrument enabled separate measurements of the mid-infrared thermal emission from Pluto and Charon in the form of light curves at 18, 21, and 25 µm.

Then, in May 2023, the instrument captured a high-quality mid-infrared spectrum (4.9–27 μm) of Pluto and its atmosphere. This spectral range, previously unexplored due to the insufficient sensitivity of earlier instruments, revealed unexpected chemical richness that led to a better understanding of atmospheric processes and the origin of Pluto’s ices.

The JWST light curves also revealed variations in surface thermal radiation by Pluto and Charon during their rotation. By comparing these data with thermal models, the researchers were able to place strong constraints on the thermal inertia, emissivity, and temperature of different regions of Pluto and Charon. These properties are what drive the global ice distributions on Pluto and the exodus of atmospheric molecules to Charon.

The new JWST data also confirmed a second prediction, made by Zhang’s former Ph.D. student Linfeng Wan, another co-author of the Nature Astronomy paper. The new observations agree well with the central prediction in their 2023 study of Charon’s rotational light-curve amplitude.

“Pluto sits in a really unique spot in the range of how planetary atmospheres behave. So this gives us a chance to expand our understanding of how haze behaves in extreme environments,” Zhang explained. “And it’s not just Pluto—we know that Neptune’s moon Triton and Saturn’s moon Titan also have similar nitrogen and hydrocarbon atmospheres full of haze particles. So we need to rethink their roles, too.”

And, Zhang added, there’s an even deeper connection. “Before oxygen built up in Earth’s atmosphere, about 2.4 billion years ago, life already existed. But back then, Earth’s atmosphere was totally different—no oxygen, mostly nitrogen, and a lot of hydrocarbon chemistry,” he said. “So by studying Pluto’s haze and chemistry, we might get new insights into the conditions that made early Earth habitable.”

For the Nature Astronomy paper, Zhang and his former Ph.D. student Linfeng Wan contributed theoretical modeling to interpret the JWST data, calculating the thermal spectra and re-evaluating the cooling rates of Pluto’s atmosphere. The team behind the series of papers was led by researchers from the Laboratory for Instrumentation and Research in Astrophysics, at the Paris Observatory, and the University of Reims Champagne-Ardenne.