Pluto was once classified as the ninth major planet of the solar system but was reclassified in 2006 by the IAU as a dwarf planet before the new category of 'plutoid' was established in 2008. The main reason why Pluto was demoted from a major planet was that it has not cleared the neighbourhood around its orbit. Pluto orbits the Sun in the inner Kuiper belt in a region where many other objects also orbit.

Pluto is much smaller than any of the official planets, and is even smaller than seven of the moons in the solar system. It is so small and distant that we cannot see any surface detail on the planet through Earth-based telescopes. It has a diameter of 2320 km and takes 248 years to travel once around the Sun. The orbit of this body is on a different plane from those of the true planets.

Pluto orbits the Sun at an average distance of about 5913 million kilometres, making it about 40 times more distant than Earth from the

Figure 13.3 Photos of Pluto are difficult to obtain because it is so far away and no space probe has yet visited it. This image has been made up from a mosaic of photos. (Photo: NASA)

Sun. It is so distant that Pluto's brightest daylight is less than moonlight on Earth. Pluto is always further from the Sun than is Uranus, but every 248 years it moves inside Neptune's orbit for about a 20-year period, during which time Pluto is closer to the Sun than is Neptune. Pluto crossed Neptune's orbit on 23 January 1979 and remained within it until 11 February 1999.

Early views about Pluto

In Roman mythology Pluto (Greek: Hades) was the god of the underworld. The planet received this name in part because it was so far from the Sun and was in perpetual darkness.

Early astronomers did not know about Pluto because it could not be seen from Earth by the unaided eye. It is even difficult to locate using Earth-based telescopes.

The discovery of Pluto is an interesting story. Irregularities in the orbits of Uranus and Neptune led to the suggestion by US astronomers Percival Lowell and William Pickering that there might be another body (planet X) orbiting beyond Neptune. Lowell died in 1916, but he initiated the construction of a special wide-field camera to search for planet X. In 1930, Clyde W. Tombaugh at Lowell Observatory in Arizona found planet X, which was later named Pluto. As it turned out, Pluto was too small and too distant to influence the orbits of Uranus and Neptune, and the search for another planet continued. The name 'Pluto' also honours Percival Lowell, whose initials PL are the first two letters of the name.

At one time it was thought that Pluto may have once been a moon of Neptune, but this now seems unlikely. A more popular idea is that Triton was once a Kuiper belt object like Pluto, moving in an independent orbit around the Sun, and was later captured by Neptune.

Table 13.1 Details of Pluto

Distance from Sun

5 913 520 000 km (39.5 AU)


2320 km


1.27 x 1022 kg (0.002 times Earth's mass)


2.03 g/cm3 or 2030 kg/m3

Orbital eccentricity

0 0

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