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| Pluto |
Throughout the ages, the number of planets in our solar system has gone up and down. Since antiquity, there were five planets beyond Earth, which are the ones visible to the naked eye, Mercury, Venus, Mars, Jupiter and Saturn. In the late 1700s, Uranus was discovered, which was revolutionary at the time as no-one considered there could be more planets beyond the visible five. After this discovery, more and more planet candidates were being found, like Ceres, Vesta, Pallas and Juno, which are all found in what we now know as the asteroid belt, or the large belt of asteroids between the orbit of Mars and Jupiter. In science textbooks in the early 1800s, they were all listed as planets. In the middle of the 1800s, Neptune was also discovered, and within just a few decades, we had quite a list of planets on our hands. At that time there wasn’t a clear distinction between the eight planets we know today and the large asteroids that were being discovered. By the 1860s, over one hundred asteroids had been found, and they were finally given their own distinct classification as asteroids.
In 1930, Pluto was discovered, and with the excitement of finding something so large and distant, it was classified as a planet. Although, within a few decades, it was discovered that it didn’t follow the conventions of traditional planet. The planets we know have circular orbits than align roughly with the plane of the solar system. Pluto, on the other hand, orbits at an angle to the solar system, and its orbit is so elliptical that at some points during its year, it’s closer to the Sun than Neptune. But it was also unique, so no-one minded it being called a planet. But then, in the 1970s, Pluto was discovered to be a lot tinier than expected. It turned out that its mass is only one sixth of our moon. Trouble continued for Pluto in the 90s, when telescopic technology had greatly increased, and other worlds were being discovered in and around the orbit of Pluto. Just like the discovery of the asteroid belt, it quickly became apparent that there was another belt beyond the orbit of Neptune, now known as the Kuiper belt. These objects are still being discovered, the most famous ones being Eris, Sedna, Haumea and Makemake. In 2006, the International Astronomical Union knew that something had to be done. They finally classified a planet as: a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. So, although Pluto fits the bill for the first two categories, because it lies in the Kuiper Belt, it has not cleared its orbit and thus is not a planet. It is instead known as a dwarf planet, which is the almost the same except it’s not massive enough to have cleared its orbit. We shouldn’t see this as a sleight against Pluto, but rather because of these new discoveries, we have a whole new category of celestial objects to learn about.