Five of the planets in our solar system were known from antiquity, because they can easily be seen by naked eye. Once the telescope had been discovered, astronomers found Uranus by chance, in 1781. Neptune was then found when scientists accurately calculated the motion of Uranus and realized that its orbit was being perturbed by some mass in addition to the ones already known. Neptune's discovery in 1846 marked a turning point in planetary research; astronomers became confident of the presence of other planets, and the search for additional planets became entwined with the search for eternal glory in the scientific world.

After Neptune's discovery, astronomers repeated the calculation of the small perturbations in the planets' motion induced by the other planets' gravitational pulls: discrepancies could indicate an extra planet, as they had with Neptune. But with the exception of Mercury, it was soon found that all the planets followed the orbits predicted for them by Newtonian mechanics, with no need of extra planets to explain their paths. Mercury's behavior remained puzzling until its orbit was fully explained by the theory of General Relativity, in 1916.

The blind search for planets, which was done with increased sophistication and better instruments after 1920, led finally to Pluto's somewhat fortuitous discovery by the young astronomer Clyde Tombaugh in 1930. Pluto is so small and so distant that even modern telescopes cannot recognize any surface feature smaller than a quarter of the planet's size. No space probe has been sent to Pluto; although the National Aeronautics and Space Administration (NASA) is currently designing a mission called Pluto Express, the project has not yet received funding.