18-08-2007 Pluto: out in the cold. (Article 99)

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Simon Kenny
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18-08-2007 Pluto: out in the cold. (Article 99)

Post by Simon Kenny » Wed Feb 04, 2009 11:16 am

Pluto: out in the cold. (Article 99)

It was like searching a beach for a single grain of sand, but when 24 year old Clive Tombaugh finally found it in 1930, the astronomical world joyfully greeted the long expected ninth planet of the solar system. By comparing two photos of a small area of sky in Gemini, taken six days apart in January 1930, one small point of light, insignificant from all the others, had moved a little. Subsequent tests confirmed the discovery and it was named Pluto, after the Roman god of the underworld. This was appropriate, considering the dark, cold realm through which Pluto moves at the outer edges of our solar system.

From the beginning, Pluto was disappointing. It was much smaller than expected, even though early estimates made it twice as large as it really was. This was probably due to the poor resolution of early photographs that couldn’t distinguish Pluto from its then unknown companion, Charon. This little planet, smaller than our own moon, could not explain the perturbations in neighbouring Neptune’s orbit, as was hoped. Pluto’s orbit was also unusual: the shape of its orbit was more elliptical and the plane of its orbit was different to that shared by the other planets. This irregularity in Pluto’s orbit brings it inside Neptune’s orbit for part of its 248.5 year cycle around the Sun. The two planets will never collide, as the orbital periods of both planets are linked so that Pluto orbits three times to Neptune’s two.

Apart from its orbit, little is known of Pluto, or Charon. Only recently, two more tiny moons were discovered orbiting the group: Nix and Hydra. Its tiny size and great distance make resolution of surface detail very difficult. Hubble Space Telescope can only resolve a few vague smudges on its surface.

When a larger planet, Eris, (formerly Xena) was discovered beyond Pluto in 2003, the International Astronomical Union convened in Prague last year to discuss Pluto’s planetary status. Controversial new planetary definitions were agreed, and Pluto was re-categorised as a ‘dwarf planet’, with Ceres and Eris. The IAU deemed three characteristics necessary to define a planet:
· That it orbits the Sun.
· It has enough mass to assume a spherical shape.
· It has swept other materials out of its orbit.
Pluto was considered to have failed the third requirement. However, controversy continues.

In an ironic footnote, NASA’s interplanetary probe, New Horizons, was launched to investigate the Pluto-Charon system a few months before Pluto was re-defined as a ‘dwarf planet’. It is due to reach its destination in 2015. The spacecraft will closely observe Pluto and Charon in the visible, infra-red, and ultraviolet spectra. It will measure atmospheric density and composition and interaction with the solar wind. Pluto may remain a dwarf planet, but new discoveries will at last lift the veil of mystery from that tiny spark at the edge of our solar system.

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