If you think it’s hard to find a needle in a haystack, just consider the problem of “finding” the planet Pluto. It is the outermost planet of our solar system, almost 40 times as far from the sun as the earth! It is so faint that a good-sized telescope is necessary to even see it. Yet, somehow it was discovered. How was it done?
There are two sets of laws that help man obtain knowledge of the sizes of planets and their distances. Keller’s law of planetary motion made it clear that the orbits of the planets around the sun were not quite circles. Newton’s law of gravitation helped Astronomers estimate the weights, sizes, and masses of the planets.
This law states that two objects attract each other with a force that depends on how much material there is in each body (its mass) and how far apart they are. The greater their mass, the stronger the pull; the closer they are together, the stronger the pull.
Now, with these two laws in mind, two men figured out, in 1846, that there was something peculiar about the planet Uranus, which was then the outermost planet known. It was not moving in its orbit as it should when allowances were made for all the known planets.
From the way it behaved, it was possible to figure out that another planet must be affecting it and just where the unknown planet would be. One of these men asked the berlin observatory to look for the new planet in a certain part of the sky and it was found there. This new planet was called Neptune.
An American astronomer, Percival Lowell, believed that the emotion of Uranus was being affected by still another planet beyond Neptune. This was in 1915. Other astronomers felt that the motion of Neptune was being affected by some planet beyond it. So a systematic search began with the telescope and by studying photographers for still another planet.
On February 18, 1930, an astronomer named Tombaugh was studying photographers looking for the new planet and he found it near the position that had been predicted by Lowell! This is the planet, Pluto.