What's a Planet and Why Is Pluto Not in the Planet Club Anymore? by Andrew Fraknoi

Although Seeing in the Dark doesn't directly discuss Pluto, it does celebrate the joy of observing planets. Viewers who watch the show may wonder what happened to Pluto in 2006 and whether any of the planets discussed in the show might lose their status next. So we thought it might be useful to tell the story of Pluto briefly and put it in context.

To understand what happened to Pluto, we must actually go back to the late 1700's, when a number of astronomers thought that there was something odd to the spacing of the planets orbiting the Sun. They thought Mars and Jupiter were too far apart and that there should be a planet between them. A group of German astronomers organized themselves into a Missing Planet Bureau and undertook a detailed search of likely locations for such a planet.

Giuseppe Piazzi

In 1801, an Italian astronomer named Giuseppe Piazzi beat them to the punch when he found a dim object between Mars and Jupiter. He named it Ceres, after the patron goddess of Sicily, where he lived. The disappointed Germans kept looking and in 1802 found another one, which was soon called Pallas. A third dim object was found in 1804 (called Juno) and a fourth in 1807 (called Vesta). By 1890 more than 300 of these objects had been discovered and it was clear that they were not planets in the traditional sense.

When astronomers realized that Ceres had many "relatives" in roughly the same region of the solar system, they began calling them asteroids or minor planets. But as you can see in the table at the bottom, for a while, schoolkids had to memorize Ceres, Juno, Pallas, and Vesta as full-fledged members of our planetary system. Today, when tens of thousands of minor planets have been discovered (most residing in an "asteroid belt" between Mars and Pluto), not even the meanest of teachers would require students to learn all their names.

We bring up this history because it is directly related to the story of Pluto. In 1846, Neptune, the fourth of the giant planets beyond the asteroid belt, was discovered—thirty times farther from the Sun than our Earth. With 8 planets now known, the thoughts of astronomers naturally turned to the region beyond Neptune. Were any more distant worlds out there, waiting to be found?

Two Sides of Pluto from the Hubble telescope

In 1930, a systematic search revealed what seemed to be a 9th planet, although measurements eventually showed it to be much smaller than Jupiter, Saturn, Uranus, and Neptune. In many ways it more nearly resembled Triton, a moon of Neptune's, than any of the planets in the outer solar system. Nevertheless, because it orbited the Sun like a planet, it was classified as a planet and named Pluto, after the god of outer darkness in Roman mythology.

So things stood for 65 years. Although Pluto's orbit was more elongated than the other planets' (and even crossed the orbit of Neptune—not the kind of thing you would expect a decent planet to do), it seemed alone in its region of space, much as Ceres had seemed before the other asteroids were discovered. But then, starting in 1995, astronomers with better instruments began to find Pluto's nearby relatives. Soon, they had discovered several worlds almost as big as Pluto and in 2003 they identified an object which has turned out to be bigger than Pluto. (The object is now called Eris.) Over a thousand objects are now known to orbit the Sun beyond Neptune, and once again we can't imagine torturing school children by adding even a dozen of the biggest ones to the list of planets. The region where these bodies orbit is now called the Kuiper (pronounced Chi-per) Belt, after one of the astronomers who hypothesized its existence.

Eris, artist's concept

To honor and distinguish the largest members of the Kuiper Belt—objects such as Pluto and Eris—the International Astronomical Union in 2006 recommended that they be called "dwarf planets"—much as asteroids are sometimes called "minor planets." While some people mourned the passing of Pluto from mainstream planet to this new classification, other Pluto fans thought it was pretty cool to be the first and defining member of a whole new category.