In 1988, when Pluto occulted, or passed in front of a star, astronomers were able to detect an atmosphere.
If there were no atmosphere, the star would have blinked out like a light as soon as it was behind the planet’s surface.
But first, the star’s light dimmed, as if passing behind a film.
That film was Pluto’s atmosphere.
It measures 180 miles (288 km) in depth, topping a dense surface haze some 28 miles (45 km) thick.
The atmosphere may be composed of methane or methane and nitrogen.