Located some 5 billion kilometres away from the Sun, Pluto is one of the coldest members of our solar system. The fact that the tiny planet is chilled at -203 degrees Celsius was discovered when Nasa's New Horizons space probe made its flyby close to Pluto in 2015, and this temperature is far lower than what was earlier noted through instruments on Earth.

Now, a group of researchers from the University of California might have finally decoded the mystery behind these super-low temperatures.

According to a study published in the journal Nature, Xi Zhang and his colleagues posit that Pluto's extremely low temperatures could be due to the smog that envelops the planet.

Pluto's atmosphere mainly contains nitrogen, along with compounds like methane. At an altitude of about 500 to 1,000km, some of these gases react with the Sun's ultraviolet rays, transforming them into solid hydrocarbon particles in the process.

These particles then fall down and get chained together in the form of chemical clusters, ultimately transforming into a thick layer of haze at an altitude of around 200km, which covers the entire planet. This layer was detected by New Horizons in its 2015 flyby.

By modelling the transfer of heat from the upper to lower atmosphere, researchers were able to determine that the haze particles cool down more efficiently as they come down, in comparison to smaller gas molecules. The haze also absorbs solar energy and radiates it into space.

Previous studies had suggested that presence of hydrogen cyanide could also be the reason behind the fall in temperature, but Zhang and his colleagues said that the model which took into account the thick layer of haze matched with the temperature data taken by New Horizons. Also, the amount of hydrogen cyanide on the planet wasn't enough to bring temperatures down.

Moving ahead, Zhang's team aims to reconcile the results of their study with observations from the James Webb Space Telescope, which is slated to launch in 2019.

If the thick layer of smog is indeed bringing down temperatures, it would also be radiating solar energy into space in the infrared spectrum as well. This would mean that the planet would appear relatively bright when observed with the advanced telescope.

pluto surface new horizons
This image of haze layers on Pluto was taken by the Ralph/Multispectral Visible Imaging Camera (MVIC) on Nasa’s New Horizons spacecraft Nasa/JHUAPL/SwRI/Gladstone et al./Science