This picture was taken by the New Horizons spacecraft just after its closest point in its encounter. Around the outside of the planet you can see the thin atmosphere. Pluto’s atmosphere is made up mostly by nitrogen. [1]
The gravity of Pluto is week, but it has a thicker atmosphere than many objects bigger than it. The nitrogen must be replenished by some source inside the planet, but it is unknown how this actually happens. It could be geysers or ice volcanoes, but none have been identified.
Don’t get the wrong idea, Pluto's atmosphere is only one Pascal. That is 100,000 times less than Earth’s.[2]
