Pluto completes a full orbit once every 248 Earth years. And due to the tilt of its axis, more and more of Pluto’s surface is shrouded in darkness every year.
That means that in around 2020, Pluto will reach a point in its orbit where its atmosphere could freeze.
From the TED-Ed Lesson The journey to Pluto, the farthest world ever explored – Alan Stern
Animation by Eoin Duffy