Erin Wilson, of Feilding Intermediate School, asks :-

How was Pluto, the smallest of our planets, discovered?

Frank Andrews, an astronomer at the Carter Observatory, Wellington, responded.

While observing Uranus and Neptune astronomers began to think that there were tiny differences between their observed and predicted positions which could only be accounted for by the prescence of a ninth planet.

Using the methods of Adams and Leverrier two well known American astronomers, William Pickering and Percival Lowell, came up with approximately the same position in the sky for the new planet. In 1929 a special survey was started at Lowell Observatory using a special wide field photographic telescope. Clyde Tombaugh discovered the new planet on 18 February 1930 by comparing photographic plates taken on the same area of sky on different nights. The new planet showed up because of its movement against the background stars.

Pluto is a very small world, only 2,245 kilometres in diameter which is is about two thirds the size of our moon. At an average distance from the sun of 5,963,000,000 kilometres (40 times the Sun-Earth distance) Pluto is extremely cold and composed mostly of ice.

In fact it can now be shown that the calculations of Pickering and Lowell were in error and did not assist Pluto's discovery. The discovery of the 9th planet was lucky chance combined with dedicated effort on the part of Clyde Tombaugh.