If, like most people alive today, you were born between 1930 and 2006, you probably learned in school that there are nine planets in our Solar System: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Perhaps you even learned one or more mnemonic devices to keep their order straight, such as “My Very Excellent Mother Just Served Us Nine Pizzas,” or “My Very Easy Method Just Speeds Up Naming Planets.”
So, you may have been surprised to hear, over the last few years, that there are now only eight planets in our Solar System. What happened to Pluto? Did it blow up, or go hurtling out of its orbit?
Pluto is still very much a part of our Solar System, it’s just no longer considered a planet. In 2006, the International Astronomical Union created a new category for classifying bodies in space: the dwarf planet.
Dwarf planets are defined as “a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity, but has not cleared its neighboring region of planetesimals, and is not a satellite.”
So, what does that mean, exactly? A celestial body is any natural object in space. Stars, planets, comets, asteroids and other things we can see in the sky are all celestial bodies. Some celestial bodies, such as planets, are so large that their gravity rounds them off. They look more or less like balls floating in space. Others, such as asteroids, are smaller and irregularly shaped. Prior to 2006, any celestial body that orbited the Sun, and was large enough to be more or less round was considered a planet.
The new category added an additional condition to planets. A planet must also be large enough to clear its orbit of “planetesimals,” small objects in space that form into a ring or disk around the Sun. They’re called planetesimals because they are believed to be the building blocks of planets. This condition is where Pluto failed to measure up. Its “neighborhood,” as the IAU calls it, is filled with countless icy chunks of matter, some of which are almost as large, or larger, than it.
Of course, several Moons in our Solar System, including our own, are large enough to be rounded. That’s why the definition also excludes “satellites.” Moons aren’t planets or dwarf planets, they’re satellites, because they revolve around planets, not the Sun.
Along with Pluto, there are currently four other officially recognized dwarf planets in our Solar System: Ceres, Eris, Haumea, and Makemake. One of these, Ceres, was discovered in 1801, 45 years before Neptune, and was considered a planet for about 50 years, before being reclassified as an asteroid. It is the largest object in the asteroid belt between Mars and Jupiter.
The other dwarf planets, Eris, Haumea, and Makemake, were discovered in 2005, and are located beyond Pluto, in the scattered disc beyond Neptune. Eris was described as “the tenth planet” by NASA and the media, though it was never actually classified as a planet. Because it is larger than Pluto, Eris’ discovery led, in part, to the creation of the dwarf planet classification.
Astronomers believe there could be thousands of dwarf planets beyond Neptune, including at least 40 known objects. Several bodies are currently under consideration for dwarf planet status.
Now we just need to get to work on a new mnemonic. “My Very Excellent Mother Just Served Us Nachos?” “Many Very Elderly Men Just Snooze Under Newspapers?” Hmm …