A LAYER OF GAS
A second stroke ot luck occurred in June 1988, when
Pluto passed in front of a
12th magnitude star. As they watched the star blink out, astronomers discovered that Pluto has a thin, cold ; atmosphere consisting mainly of nitrogen and methane.
The data also hinted that the atmosphere might have hazes. But the strangest thing is that Pluto's atmosphere is both dynamic and transient. Pluto's weak gravity cannot hold the atmosphere forever, and it is escaping into space much like a comet's gases do.
The atmosphere is also episodic: it is W gaseous only when Pluto is around perihelion, the peak of its orbital "summer." Pluto reached perihelion in 1989. As the planet retreats toward aphelion (the coldest part of its orbit), it is cooling, and scientists expect the entire atmosphere will condense into snow or frost by the 2010s. And it will remain frozen until Pluto again draws near the Sun, in the twenty-second century.