7 Things We've Learnt About Pluto Since New Horizons

4. It Has Blue Skies

When New Horizons looked back after passing by the little dwarf planet, we learnt something that made it seem just that little be closer to home: The sky is blue on Pluto. The blue sky here on Earth is caused by the nitrogen in the atmosphere scattering the blue part of the visible light spectrum. But, as we've just learnt, much of the nitrogen is on the surface of Pluto is solid, so why the blue skies? The haze that surrounds Pluto is made up of something called tholins. These form when the ultraviolet rays from the sun break down and ionize nitrogen and methane. These particles are actually red, giving Pluto its characteristic red hue, but they still scatter blue light in the same way. The way the sun breaks down methane molecules in the atmosphere at different altitudes creates those odd "layers" that are visible in the picture. Pluto's atmosphere extends way off into space, much much further than Earth's, and it even forms a tail behind it as it is blown by solar winds. Sadly, scientists reckon that, if a human were to stand on the surface, Pluto's skies would be mainly dark, but for a small blue circle around the sun.