EMH
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Pluto: Facts - NASA Science
Why is Pluto no longer a planet? Pluto was reclassified as a dwarf planet in 2006 by the IAU because other objects might cross its orbit.

Pluto has a thin, tenuous atmosphere that expands when it comes closer to the Sun and collapses as it moves farther away – similar to a comet. The main constituent is molecular nitrogen, though molecules of methane and carbon monoxide have also been detected.
When Pluto is close to the Sun, its surface ices sublimate (changing directly from solid to gas) and rise to temporarily form a thin atmosphere. Pluto's low gravity (about 6% of Earth's) causes the atmosphere to be much more extended in altitude than our planet's atmosphere. Pluto becomes much colder during the part of each year when it is traveling far away from the Sun. During this time, the bulk of the planet's atmosphere may freeze and fall as snow to the surface
Pluto's SURFACE AIR PRESSURE RISES WHEN PLUTO WARMS, and shrinks when Pluto cools....
Warming the atmosphere = higher surface air pressure
Cooling the atmosphere = lower surface air pressure
Wake up and smell the frozen nitrogen...