Even smaller than our Moon, Pluto is by far the smallest planet. It was the last planet to be discovered, by US astronomer Clyde Tombaugh in 1930. Pluto is usually the most distant planet, except for the 20 years in each orbit when it slips inside Neptune’s orbit. It has just one moon, Charon, which is, remarkably, half the size of Pluto. Both are deep-frozen worlds of ice and rock.
Table 18. ESSENTIAL DATA
|Diameter at equator||2,274 km (1,413 miles)|
|Average distance from Sun||5,900 million km (3,666 million miles)|
|Orbital period||247.7 years|
|Rotation period||6.39 days|
|Surface temperature||-223°C (-370°F)|
|Number of moons||1 (Charon)|
Pluto is the only planet that spacecraft have not visited, so there are no photographs showing what its surface looks like. Some astronomers believe Pluto might look like Triton, the largest of Neptune’s moons, which has a dimpled surface of rock and ice.
Pluto has a most unusual orbit, which takes it much further above and below the orbits of the other planets. Pluto’s orbit is also much more elliptical (oval) than those of the other planets, which all orbit in roughly the same plane (level).
Charon is Pluto’s only moon. It circles Pluto in the same time that Pluto spins round (just over 6 days), so it appears fixed in the sky. It orbits only about 20,000 km (12,500 miles) from its parent planet.
Tombaugh joined the staff of the Lowell Observatory at Flagstaff, Arizona, in 1929. There he began a systematic photographic survey of the heavens, taking pictures of the same area of the sky some nights apart, and then seeing which objects had moved. After just a few months, on 18 February, 1930, he discovered Pluto.