9.20: Pluto
Pluto enters the solar system’s stage. The then 9th planet was discovered in 1930 by Clyde Tombaugh at Lowell Observatory. The first two letters of PL uto are also for P ercival L owell, who mathematically predicted the location. In Roman mythology, Pluto (Greek: Hades) is the god of the underworld. This seemed like a fitting name for a planet so distant from the sun. Pluto has been controversial in recent years. Is Pluto a planet? Is it a comet or a centaur? Pluto appears to outgas like a comet and it appears to have more of a carbon monoxide, CO, atmosphere. Why does it have a moon, Charon , which is nearly the same size as Pluto, and also has four smaller moons, making five moons total? Pluto and Charon orbit around each other; a true double-dwarf planet system.
Pluto has a highly-elliptical orbit, more like a comet or asteroid than a planet. It is so elliptical that Pluto’s orbit crosses inside the orbit of Neptune for a short period. And the orbit of Pluto is highly tilted against the plane of the solar system’s planets.
Many of these questions will be answered in 2015 as the NASA spacecraft New Horizons arrives at Pluto after a nearly-decade long trip from Earth.