Atmospheric hazes can explain Pluto’s cool temperature, reports a paper published in this week’s Nature.

Recent observations show that Pluto’s temperature is much cooler than previously thought, suggesting the existence of unknown cooling processes. One possible coolant is water vapour, although this would require the vapour to be many orders of magnitude out of equilibrium.

Xi Zhang and colleagues find, through modelling, that Pluto’s atmospheric temperature is regulated by hydrocarbon haze particles rather than gas molecules, making it unique in the Solar System. They observe that haze particles have much larger solar heating and thermal cooling rates than gas molecules, which are commonly found in the atmospheres of other planets.

Haze particles dominate the radiative balance of Pluto’s atmosphere from the ground to an altitude of 700 kilometres, above which heat conduction maintains an isothermal atmosphere. The haze affects Pluto’s radiative energy equilibrium - the balance between incoming solar energy and the energy emitted by the planet - leading the authors to predict that Pluto is brighter at mid-infrared wavelengths than currently predicted. This prediction should be testable next year with the launch of the James Webb Space Telescope.