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“It should be around here.”
The story of the discovery of the outer planets
Astronomy has always been a fascinating subject.
With the development of observational techniques, astronomers started seeking new planets outside of Saturn.
In 1781, as the British astronomer Sir William Herschel was surveying the skies through his telescope, he discovered a new celestial object. At first, he reported that he a found “a new comet,” but with continued observations he was able to confirm that the object was actually a planet, moving outside of the orbit of Saturn. And so, Herschel went down in history as the discoverer of Uranus, the seventh planet of our solar system.
Incidentally, the first man to observe the sixth planet, Saturn, through a telescope was none other than Galileo Galilei, in 1610. From that point, it took over 170 years before Uranus was discovered, but that was certainly not because the astronomers were inattentive. Rather, it was the capacity of telescopes that needed to be improved.
As a matter of fact, Uranus had been observed already long before Herschel’s discovery. (The oldest recorded observations are from 1690.) Unfortunately, none of those early observers realized that it was a planet they were seeing, and so the honor of a great discovery slipped through their fingers.
Now, the search for new planets began in earnest. When analyzing the observational results, it turned out that the orbit of Uranus was slightly different than the one derived from Newton’s law of gravitation. Was the theory wrong? Or was it the observational data that were incorrect?
In 1843, the Cambridge student John Couch Adams hypothesized that some unknown planet was perturbing the orbit of Uranus. However, nobody took his theory seriously enough.
At around the same time and totally unaware of Adams’s calculations, the French mathematician Urbain Jean Joseph Le Verrier began investigating the possibility of an unknown celestial body influencing the orbit of Uranus. Le Verrier presented his own independent predictions to Johann Galle of the Berlin Observatory, and eventually in 1846, they discovered the eight planet, Neptune, in its orbit outside of Uranus. Astronomy has an image of “accidental discoveries” made during long series of tenacious observations, so it is particularly interesting to note that Neptune’s position was first predicted by mathematical calculations: “It has to be around here.” The observations were made in order to confirm that prediction.
After Uranus and Neptune followed the discovery of Pluto, counted as the ninth planet of our solar system until recently. However, at the 2006 congress of the International Astronomical Union, Pluto was demoted to a “dwarf planet.” The discovery of Pluto was also quite dramatic, but that story will have to wait for another time. Never mind how mankind chooses to classify it, Pluto still keeps orbiting around the sun, just like before.
