On August 24, 2006, the International Astronomical Union (IAU) demoted Pluto to the new category of "dwarf planet". To add insult to injury, the Minor Planet Center gave Pluto a number — 134340 — just like any lowly asteroid.
But before we get too emotional, let's consider the following:
In the beginning there were seven planets: the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn. Then, in 1543, Copernicus upset the apple cart and promoted the Sun to a special position. The Moon, however, became a mere satellite of the "new" planet Earth.
Alas, poor Moon! So now, we had six planets.
Herschel's discovery of Uranus in 1741 made it seven and Piazzi's discovery of Ceres in 1801 gave us a total of eight planets. Admittedly, Ceres seemed to be extremely small but it fit nicely into the empty gap between Mars and Jupiter.
But then the floodgates opened. By the mid-nineteenth century, fourteen more very small "planets" were discovered in addition to huge Neptune. This was too much for astronomers. Neptune was far beyond the orbit of Uranus and similar in size but what about all those smaller objects? They all shared the space between Mars and Jupiter and more were sure to be discovered. Thus in 1852, Ceres and her fourteen siblings were demoted to "asteroids".
Alas, poor Ceres (and 14 others)!
The solar system had now shrunk from 23 to 8 planets.
Some astronomers, however, thought there were irregularities in the motions of Uranus and Neptune that could be caused by another large planet even further away. The search began and Clyde Tombaugh found Pluto in 1930.
And now there were nine planets.
Unfortunately, Pluto turned out to be much too small to cause the calculated anomalies. The search continued for a tenth planet but to no avail. Measurements made by the Voyager spacecraft cast doubts on the original calculations and apparently no large planet is required to account for the motions of Uranus and Neptune. It seems that Pluto acquired the title of planet by false pretenses!
Image Credit: NASA/JPL=Caltech/R.Hurt (SSC-Caltech)
From the beginning, it was suspected that Pluto was an oddball. Its orbit is inclined at 17 degrees to the ecliptic; most of the rest of the planets have inclinations of less than 4 degrees. (Mercury's inclination is 7 degrees). Pluto is a small, rocky world; the other outer planets are gas giants. And Pluto is the only "planet" that crosses another planet's orbit. At certain points in its 250-year trip around the Sun, Pluto is closer to the Sun than Neptune.
It was long suspected that there were other worlds out in the depths of space dubbed the Kuiper Belt. And indeed, modern techniques allowed us to bag a number of these denizens of deep space. Some of these Kuiper Belt objects (KBO's) were a good fraction of Pluto's size and in 2003 the inevitable happened. Object 2003 UB313 turned out to be bigger than Pluto.
Yet Pluto was still called a planet. It seemed only fair that UB313, nicknamed "Xena", should also be a planet. And what about some of the other KBO's? How big did an object have to be to be called a planet? It was 1852 all over again.
After several proposals, the IAU has now finally settled on the definition of a planet and Pluto didn't make it. But being a dwarf planet isn't so bad, especially for an impostor! Ceres, too, is now a dwarf planet after 150 years in servitude as an asteroid.
And UB313? Our Xena is now officially 136199 Eris, another dwarf planet.
Alas, poor Xena!