(Note: Since August 2006, Pluto is no longer considered a major planet. See text below)
Pluto is the most unusual planet in the solar system, in terms of its orbital inclination, spin axis and rotational behavior. It was discovered by accident by Clyde Tombaugh (1906-1997) in 18 February 1930.
The story began with a belief that irregularities of the planets Uranus and Neptune was due to some strong gravitational effects of 'Planet X' that lies beyond Neptune. The search for this
mysterious planet began in the early twentieth century by Percival Lowell (1885-1916) but without success. The search continued which led to the discovery of Pluto in 1930. However, Pluto is not the 'Planet X', as it is too small to have any
influence on the much larger members of Neptune and Uranus.
Pluto is the smallest planet (well, it if is still considered as a planet), with the size only about two-third of the Moon, and is the most distant planet from the Sun. However, due to the high eccentricity of its orbit and its 17° orbital inclination to the ecliptic,
there are about 20 Earth years out of 248.5 years it is closer to the Sun than that of Neptune. The last time this occurred was between
1979 and 1999 and will only occur again until September 2226. In addition, Pluto is the only planet that rotates synchronously with the orbit of its satellite, Charon. That is, it has the same rotational period to that of
its satellite's orbital period, which is 6.387 days. Similar to Uranus but like other Planet, Pluto rotates with its pole incline almost along its orbital plane.
With an apparent magnitude of only 14, a telescope of at least 30 cm aperture is required to see Pluto. Even then, it will only show as a dot like a star, but appears light brown with a very slight tint of yellow.
Pluto is the only planet that has not been visited by a spacecraft. However, the New Horizon space probe, lauched in January 2006, is scheduled to arrive Pluto in 2015. Until then, even the Hubble Space Telescope
can barely resolve even the largest features on its surface. The planet as a whole is thought to be made of 70% rocks and 30% ice water. Its surface is thought to consist of frozen nitrogen, methane and carbon monoxide. The thin atmosphere may have frozen when
it is far from the Sun. However, the overall planetary properties are very similar to that of Triton, a satellite of Neptune, suggesting a possible historical link between the two bodies.
Pluto's planetary status has been challenged from time to time. This is because of its small size and its unusual behavior. Some astronomers regard it to be one of the largest Kuiper Belt object (KBO) ever discovered.
This viewed was further underscored when large and very distant Kuiper Belt asteroids were eventually discovered, such as Sedna (with a radius of 900 km). However, the reason Pluto obtained (and retained) the planetary status is because it was discovered long before other KBOs was discovered, in 1992. Furthermore, it is large
and bright (with a very high reflective surface), compare with other KBOs that contribute to its early discovery.
However, in August 2006, the planetary status of Pluto was once again being challenged. About 2500 scientists was congregated in Prague for the International Astronomical Union's conference and relegate Pluto as a 'dwarf planet' by a majority vote. This means that Pluto is categorised
along with Ceres, the largest asteroid located at the Asteroid Belt region. Since then, the Solar System will now officially has only eight major planetary bodies.
The scientists agreed that the criteria for a planet are as follows:
(a) it must be in orbit around the Sun,
(b) it must be large enough that it takes on a nearly round shape, and
(c) it has cleared its orbit of other object.
This automatically disqualifies Pluto because its highly elliptical orbit overlaps with that of Neptune.