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takyon writes:

Pluto is much colder than what would normally be expected due to a haze of hydrocarbon particles suspended in Pluto's tenuous atmosphere:

One of the more bizarre things [New Horizons] found was that the haze in Pluto's atmosphere was much thicker than our previous peeks indicated. The icy hunk of rock also had an atmosphere much cooler than earlier estimates, topping out at -333.4 ºF (more than 50 degrees colder than expected, even for something about 40 times further from the Sun than Earth is).

Now, a study [DOI: 10.1038/nature24465] [DX] published in Nature links those two atmospheric observations. A computer model developed by University of California Santa Cruz planetary scientist Xi Zhang and colleagues shows the haze of tiny droplets in the upper atmosphere is likely scattering light from the Sun, preventing heat from reaching the planet below.

"It's been a mystery since we first got the temperature data from New Horizons," Zhang, said in a statement. "Pluto is the first planetary body we know of where the atmospheric energy budget is dominated by solid-phase haze particles instead of by gases."

[...] This haze appears to be made up of large hydrocarbon droplets, created high in the atmosphere when ultraviolet light from the Sun strips electrons from particles of methane and nitrogen gas. The reaction helps form solid bits of hydrocarbon. But what gets created up there must still come down. Pulled back to the surface by gravity, the hydrocarbons start to bond together, eventually creating a thick haze. It doesn't completely block sunlight, but rather absorbs and re-scatters it, theoretically warming up part of the atmosphere while keeping most of Pluto frigid below.

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