Uranus’ accidental discovery 240 years ago greatly enhanced our understanding of the solar system, doubling the size of the solar system as we knew it at the time. Here’s how a German astronomer stumbled upon the ice giant.
Before Uranus’ Discovery, Saturn Was the Farthest Known Planet
For millennia, Saturn was the farthest planet from the Sun that had been identified. The earliest known record of Saturn is from around 700 BCE, by the Assyrians.
In the same era, Uranus was thought to be a star rather than a planet. It makes sense that people thought that Saturn was the extent of the solar system for so long, since, without a telescope, it would be extremely difficult to detect that Uranus wasn’t just another star.
Uranus Was Discovered Unexpectedly in 1781
Before its discovery as a planet, Uranus had been observed. But it was so dim and small, due to its vast distance from Earth, that it was thought to be a star. Additionally, Uranus takes 84 years to orbit the sun, which makes spotting its motion against background stars very difficult.
On March 13, 1781, a German-born musician and astronomer named William Herschel, looking up at the sky with his telescope in his back garden in Bath, saw what he thought was a star or a comet. On March 17, he looked back up to where he noted this object and realized it had moved a little bit compared to the other stars, which further made him believe it to be a comet. He reported his discovery to The Royal Society, who began to investigate further.
On April 23, the Astronomer Royal, Nevil Maskelyne, let Herschel know it was just as likely to be a planet as a comet, but he couldn’t see a tail on it, which is characteristic of a comet. Further observations by additional astronomers confirmed it to be a planet—one farther from the Sun than any of the others discovered before it.
The planet was then given the name Georgium Sidus, meaning “George’s star,” as a nod to King George III. It was eventually renamed Uranus, a name suggested by Johann Bode, so that it would follow the pattern of other planets being named for mythological gods. The name Uranus, the Greek god of the sky, was also fitting because the god’s Roman counterpart, Caelus, is the father of Saturn, and Saturn is the father of Jupiter.
Uranus Is Twice as Far From the Sun as Saturn
Saturn sits at a distance of 1.4 billion kilometers from the Sun on average, or 9.5 astronomical units (AU). For reference, the distance from the Sun to Earth is one astronomical unit. So for millennia before the discovery of Uranus, the known limit of our solar system was 1.4 billion kilometers. That’s massive, but in 1781, it got a whole lot bigger.
In stark contrast, Uranus sits a whopping 2.9 billion kilometers, or 19 AU, away from the Sun. So with the discovery of Uranus, our understanding of the size of our solar system effectively doubled.
What Did the Discovery of Uranus Give Us?
After the discovery of Uranus, astronomers observed an issue with its orbit. Its orbit around the sun was slightly ahead of where it was predicted to be, leading astronomers to theorize that another planet was affecting its gravitational pull. In searching for an answer to the anomaly, researchers ended up discovering Neptune. Its position was predicted mathematically, and it was ultimately observed through a telescope in 1846.
Furthermore, Neptune and Uranus’ discoveries led to studies of their orbits, both of which had wobbles, leading astronomers to theorize another planet was impacting their gravitational pulls. This eventually led to the discovery of Pluto.
Uranus’ discovery also fit right into Bode’s law, a rough predictor of the placement of planets in a solar system. The mathematical pattern was first publicized by Johann Titius in 1766 and further popularized by Johann Bode in 1772. It’s a sequence of numbers following the pattern of 0, 3, 6, 12, and so on, where numbers after 3 are doubled. Then 4 is added to each number, and then all are divided by 10. This leaves you with a sequence of 0.4, 0.7, 1.0, 1.6, 2.8, 5.2, 10, and onward. Sure enough, this sequence does fit the distance of most of the planets in our solar system from the sun in AU, with the exception of Neptune. Since the law was theorized before the discovery of Uranus, the fact that it fit the pattern once it was discovered further reinforced the validity of the law.
While all astronomical discoveries are important in their own rites, the discovery of Uranus was one of the most consequential when it comes to our understanding of the vastness of our solar system. Our understanding of the massive, gassy ice giant opened up new horizons for the field of astronomy, pun intended.