Erin Wilson, of Feilding Intermediate School, asks :-
Could anyone discover a planet?
Frank Andrews, an astronomer at the Carter Observatory, Wellington, responded.
The bright planets of our Solar System: Mercury, Venus, Mars, Jupiter and Saturn, are easily seen with the naked eye. They have been known since before historical records were kept. Hence anyone could have discovered them.
The three outer planets, Uranus, Neptune and Pluto can only been seen using a telescope. William Herschell made a 16 cm diameter reflecting telescope. From the back garden of his home he carried out a careful and systematic survey of the sky. On 13 March 1781 he discovered Uranus. Hence any early telescope maker who was systematic and curious about the night sky could have discovered Uranus.
By 1790 astronomers found that Uranus was not following the orbital path that it should be. After 1830 there was increasing speculation that this was because the gravity of an unknown planet was causing the difference between the observed and computed position of Uranus. By 1846 John Adams in England and Urbain Leverrier in France had calculated where the new planet should be in the sky. Unfortunately astronomers in neither country seemed willing to carry out the search and it was left to Johann Galle at the Berlin Observatory to locate the new planet on 23 September 1846 using Leverrier's calculations. So any astronomer who wished to test predictions could have discovered Neptune.
Subsequently it was realized that Galileo had observed Neptune on 28 December 1612 and 28 January 1613 but had failed to recognize it as a planet. Even famous people can miss discoveries.
Astronomers are currently studying nearby stars other than our own to see if any other planets exist around them.