Five of the planets in the solar system Mercury, Venus, Mars, Jupiter and Saturn are so bright that astronomers have known about them for thousands of years. But in the past couple of centuries, astronomers have found three more distant and fainter planets: Uranus, Neptune and Pluto. These are also indications of a 10th planet beyond Pluto. The discovery of these and new planets requires both mathematical calculations and luck.
Until 1781, no one suspected that there for planets beyond Saturn, so no one was actually looking for them. Then on March 13, the amateur astronomer William Herschel found Uranus, while looking for pairs of stars. He knew it was not a star because it had a visible disc, just like the Moon shows when it is full. As astronomers tracked its motions, they decided it had to be a planet.
After this largely accidental discovery, astronomers began to wonder if there might be another planet, even farther out. This suspicion was reinforced when they discovered that Uranus did not orbit the sun at a constant rate. It seemed that the planet was feeling the gravitational pull of a more distant and unknown planet.
Two brilliant mathematicians- John Couch Adams, in Cambridge, and Urbain Leverrier, in France calculate independently where this new planet would be. On August 31, 1846, Leverrier sent his prediction to Berlin Observatory and the astronomers there identified a star as the new planet, now called Neptune.
By the end of the last century, it was suspected that both Uranus and Neptune were being pulled by the gravity of the planet father out still. This time it was an American astronomer, Percival Lowell, who calculated where this planet X should be. In 1930, Clyde Tombaugh, working at the Observatory founded by Lowell, detected a faint speck of light that moved from night to night. It was indeed a new planet, close to Lowell’s calculated position, but much fainter than Lowell had predicted. This planet was called Pluto.
But many astronomers believe that Pluto is too small to affect the giant planets Uranus and Neptune. In 1978, astronomers and the US Naval Observatory found a moon orbiting Pluto. The motion of this moon revealed Pluto’s gravity, and it is far too weak to pull on Uranus and Neptune.
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