In May 2023, the James Webb Space Telescope turned toward Pluto and recorded something planetary scientists had wanted to see for years: the dwarf planet’s mid-infrared glow, separated from the light of its moon Charon. That glow carried the signature of Pluto’s haze. It also confirmed a strange idea first proposed after NASA’s New Horizons flyby in 2015. On Pluto, haze is not just a byproduct of atmospheric chemistry. It helps control the planet’s heat balance.
That makes Pluto unusual. On Earth, Mars, Titan and the giant planets, gases do most of the work when it comes to heating and cooling an atmosphere. On Pluto, according to a 2025 Nature Astronomy paper led by Tanguy Bertrand, tiny haze particles appear to dominate that job instead. The Webb data show that this haze absorbs sunlight and then radiates energy away in the mid-infrared, cooling the atmosphere in the process.
The paper, titled Evidence of haze control of Pluto’s atmospheric heat balance from JWST/MIRI thermal light curves, gives the clearest evidence yet that Pluto’s atmosphere behaves in a way not seen elsewhere in the Solar System.
Pluto already stood apart before Webb looked at it. New Horizons had shown a world with nitrogen ice plains, methane frosts, mountains of water ice and dozens of thin blue haze layers rising far above the surface. It also found a thin atmosphere made mostly of nitrogen, with smaller amounts of methane and carbon monoxide. Sunlight breaks apart methane high above Pluto, and the fragments react to form larger hydrocarbons and nitriles. Those compounds gather into haze particles.
After the flyby, some researchers suggested those particles might do far more than add color to Pluto’s sky. Xi Zhang and colleagues argued in 2017 that the haze could be the main factor setting Pluto’s atmospheric temperature. Their model predicted that the haze should emit strongly at mid-infrared wavelengths. At the time, there was no instrument sensitive enough to test the idea directly.
Webb changed that.
Using the MIRI instrument, Bertrand and colleagues measured Pluto and Charon at 15, 18, 21 and 25.5 micrometres. They also obtained a spectrum from 4.9 to 27 micrometres. That wavelength range matters because haze particles that are cold by Earth standards can still emit detectable thermal radiation there. Pluto’s upper atmosphere is only about 70 kelvin, or about minus 203 degrees Celsius. Even at those temperatures, the haze leaves a measurable fingerprint.
The team found that Pluto’s mid-infrared signal could not be explained by surface emission alone. They detected thermal emission from the haze itself. Their analysis showed that the haze likely contains Titan-like organic particles along with hydrocarbon and nitrile ices. The haze emission was also strong enough to show that these particles make up a major part of Pluto’s atmospheric energy budget.
That is the key result. The haze is central to the system’s temperature balance.
This sets Pluto apart from every other known planetary atmosphere. On Earth, greenhouse gases such as water vapor and carbon dioxide trap outgoing heat. On Titan, haze affects atmospheric chemistry and temperature, but it does not appear to take over the full heat balance in the same way. Pluto sits in a colder and thinner regime, where tiny suspended particles can cool the atmosphere by radiating energy to space very efficiently. In effect, solids floating in gas have an unusually large role in shaping climate.
Webb also helped solve a long-standing observational problem. Pluto and Charon are close together, and older thermal measurements often blended the two bodies into one unresolved source. That made it hard to tell which world was producing which signal. Webb separated them. This allowed the team to build thermal light curves for each body as they rotated.
Those light curves carry information about surface properties. The researchers used them to constrain temperature, emissivity and thermal inertia across different terrains on Pluto and Charon. Thermal inertia describes how fast a surface heats up and cools down. Low thermal inertia means a surface changes temperature quickly. High thermal inertia means it holds heat longer. These values matter because they affect how nitrogen and methane ices move around Pluto over its long seasons.
Pluto takes 248 Earth years to orbit the Sun, and its tilt is extreme. That means sunlight shifts across the surface in dramatic ways over time. The movement of volatile ices between hemisphere, pole and equator helps drive changes in surface pressure and atmospheric structure. If haze controls how efficiently Pluto cools, then haze also feeds into those long-term climate cycles.
The new Webb data also support earlier ideas about the connection between Pluto and Charon. Methane escaping from Pluto can end up at Charon’s poles, where it is altered by radiation into darker reddish material. That process had already been suggested from New Horizons observations. By tightening the thermal models and the seasonal picture of Pluto’s surface and atmosphere, Webb helps place that exchange in a broader climate context.
Pluto also matters as a test case for atmospheric physics under conditions that do not exist on Earth. It is cold, distant and weakly heated by the Sun. Its atmosphere is thin and chemically active. If haze can control heat flow there, then researchers need to think more carefully about how suspended particles behave in other nitrogen and methane atmospheres, including those of Titan and Triton.
There is a second reason scientists care about this result. Pluto’s haze chemistry may resemble processes that were more common on the early Earth, before oxygen became abundant in the atmosphere around 2.4 billion years ago. Back then, Earth’s air was very different from today’s. Nitrogen was still a major component, and hydrocarbon chemistry likely played a larger role. Pluto is no stand-in for ancient Earth, but it offers a way to study haze-rich chemistry in a real planetary setting.
The Webb observations did not answer every question. Researchers still need to pin down the exact composition, size and vertical distribution of the haze particles. They also need to track how Pluto’s atmosphere changes over time as the dwarf planet moves along its orbit. Even so, the main result is now hard to ignore.
Pluto’s atmosphere does not behave like a scaled-down version of Earth’s or Titan’s. A veil of haze, built from sunlight breaking apart methane, appears to govern how the system gains and loses heat.
That idea once sounded speculative. Webb has now turned it into evidence.
References
Bertrand, T., Lellouch, E., Holler, B., Stansberry, J., Wong, I., Zhang, X., Lavvas, P., Dufaux, E., Merlin, F., Villanueva, G., Wan, L., Pinilla-Alonso, N., de Souza Feliciano, A. C., & Murray, K. (2025). Evidence of haze control of Pluto’s atmospheric heat balance from JWST/MIRI thermal light curves. Nature Astronomy, 9, 1300–1308. https://doi.org/10.1038/s41550-025-02573-z
Stern, S. A., Bagenal, F., Ennico, K., Gladstone, G. R., Grundy, W. M., McKinnon, W. B., Moore, J. M., Olkin, C. B., Spencer, J. R., Weaver, H. A., Young, L. A., & the New Horizons Team. (2015). The Pluto system: Initial results from its exploration by New Horizons. Science, 350(6258), aad1815. https://doi.org/10.1126/science.aad1815
Zhang, X., Strobel, D. F., & Imanaka, H. (2017). Haze heats Pluto’s atmosphere yet explains its cold temperature. Nature, 551, 352–355. https://doi.org/10.1038/nature24465
