Astronomers using the James Webb Space Telescope (JWST) have taken a fresh look at the distant edges of our solar system — and found that, once again, Pluto is defying expectations.
When NASA‘s New Horizons spacecraft flew past Pluto in 2015, it shattered the notion that the dwarf planet was a dormant ball of ice, instead revealing it to be rich with icy plains and jagged mountains. But one of the biggest surprises floated above it all: a bluish, multi-layered haze blanketing the world’s sky, stretching more than 185 miles (300 kilometers) above the surface — far higher and more intricate than scientists had predicted.
Now, nearly a decade later, new data from JWST confirm that Pluto’s haze isn’t just a visual oddity, it also controls the dwarf planet’s climate.
“This is unique in the solar system,” Tanguy Bertrand, an astronomer at the Paris Observatory in France who led the analysis, told Live Science. “It’s a new kind of climate, let’s say.”
The findings, described in a study published June 2 in the journal Nature Astronomy, suggest similar dynamics may be at play on other haze-shrouded worlds in our solar system, and even offer clues about our own planet’s early climate.
Lifting the haze
Pluto’s high-altitude haze is made of complex organic molecules from sunlight-driven reactions of methane and nitrogen. The idea that this haze could control Pluto’s climate was first proposed in 2017. Computer models suggested these particles absorb sunlight during the day and release it back into space as infrared energy at night, cooling the atmosphere much more efficiently than gases alone. This could also explain why Pluto’s upper atmosphere is roughly -333 degrees Fahrenheit (-203 degrees Celsius) — 30 degrees cooler than expected.
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For years, however, testing that theory proved difficult. One major challenge was Pluto’s large moon, Charon, which orbits the frigid planet so closely that their thermal signals often overlap in telescope data. “Basically, we couldn’t know what part of the signal is due to Charon and what part is due to Pluto’s haze,” Bertrand said.
The researchers behind the 2017 study predicted that Pluto’s haze would make the world unusually bright in mid-infrared wavelengths — a prediction that, at the time, could only be tested with future instruments. That opportunity arrived in 2022, when JWST’s powerful infrared instruments were finally able to separate the two worlds’ signals. Sure enough, the faint infrared glow of Pluto’s haze matched the predictions.
“In planetary science, it’s not common to have a hypothesis confirmed so quickly, within just a few years,” Xi Zhang, a planetary scientist at the University of California, Santa Cruz who led the 2017 team, said in a statement. “So we feel pretty lucky and very excited.”
These findings also open up the possibility that similar haze-driven climates might exist on other hazy worlds, such as Neptune’s moon Triton or Saturn’s moon Titan, Bertrand said.
Even Earth’s distant past might bear a resemblance, the researchers said. Before oxygen transformed our planet’s skies, it’s possible that Earth was veiled in a haze of organic particles — a blanket that may have helped stabilize temperatures and foster early life.
“By studying Pluto’s haze and chemistry, we might get new insights into the conditions that made early Earth habitable,” Zhang said in the statement.