Pluto’s Expanding Atmosphere

Pluto is the only dwarf planet known to have an atmosphere. Its nitrogen-methane atmosphere, previously 60 km thick, has now swelled to 3000 km (1864 miles). It also contains some carbon monoxide.

Pluto made its closest approach to the sun in 1989. Astronomers expected that its atmosphere would diminish as it cooled when Pluto gained distance from the sun. Since the opposite occurred, it is possible that other factors have caused Pluto to absorb heat more efficiently, perhaps a darkening of the surface ice. Solar energy absorbed by the dark frozen surface may be the heat source now causing the atmospheric gases to expand. Another proposed explanation for the rapid expansion is interaction with the sun’s 11-year solar cycle.

These atmospheric changes likely represent a seasonal variation, but with a 248-year orbit, Pluto’s seasons will last for decades. NASA’s New Horizons spacecraft should reach Pluto by 2015 in time to make more observations of Pluto’s atmosphere before the nitrogen gas freezes for the 120-year winter.

Pluto was considered our ninth planet until a tenth one was discovered. Hundreds of other trans-Neptunian objects (TNOs) have since been found, including a few large enough to be called dwarf planets. Pluto was reclassified as a dwarf planet in 2006 because it is much smaller than any planet and shares its region of the solar system with many other similar sized TNOs.