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 |
| Mass (Earth=1) | 0.002 |
| Gravity (Earth=1) | 0.067 |
| 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.
BIOGRAPHY: CLYDE TOMBAUGH American, 1906-1997
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.
