Up until recently, there were nine planets in our solar system. The ninth, Pluto, was kicked out in August of 2006. It didn’t actually go anywhere. But, we stopped calling it a planet. From then on, Pluto became a dwarf planet. As we’ll see, the difference between the two is simple but important. What’s more, there are four other dwarf planets in our solar system!
Planets Vs. Dwarf Planets
It is surprisingly easy to separate planets from dwarf planets. That’s because there is only one difference between them. That is: Dwarf planets are planets which haven’t cleared their orbit of debris. What this means is that other things orbit along with them. Planets, on the other hand, usually destroy things orbiting with them (if they’re not moons).
If planets and dwarf planets were cars, planets would have their own lane. Dwarf planets would be stuck in traffic.
Knowing this, we can separate Pluto from the planets. Pluto is part of the Kuiper Belt. This is a ring of comets and asteroids orbiting the Sun. Remember, if something orbits with other objects, it’s not a planet.
Notably, we didn’t originally know Pluto was part of the Kuiper Belt. That’s because we found Pluto in 1930 while the Kuiper Belt was found in the 90’s. As we started to find more objects like Pluto in the belt, scientists knew something would have to change. The discovery of Eris (see below) was what really ruined Pluto’s planethood.
How Many Dwarf Planets are There?
Dwarf planets are very difficult to detect, due to their size. As a result, we can only guess how many there are. Within the Solar System, we’ve found five so far. Yet some scientists think there could be up to 200 in the Kuiper Belt! Who knows how many might exist in our Universe?
For now, we’re still trying to learn more about those we’ve already found. We’ve known about Pluto for a long time, though only one mission has ever gone there. As a result, dwarf planets are kind of mysterious.
Fortunately, they’re not a total mystery. Here’s what we know about the few we’ve observed.
Dwarf Planets in Our Solar System
Ceres is a dwarf planet within the Asteroid Belt and the closest one to Earth. It’s also the smallest of them! That said, Ceres was discovered way back in 1801. The most notable thing about Ceres is its volcanic mountain, Ahuna Mons. Scientists believe that Ahuna Mons is the result of volcanic activity under Ceres’s surface. Although, this isn’t regular volcanism. Instead, it seems that Ahuna Mons is a sort of ice volcano!
Could we one day live on Ceres? Scientists think that there might be water there.
And if there’s ice on Ceres, humans might be able to use it as a source of water. Maybe, instead of Mars, we’ll live on Ceres someday!
Eris is a large, faraway dwarf planet. It’s slightly smaller than Pluto, though also slightly heavier. Notably, Eris is the furthest natural object we’ve found in our Solar System. Its wide orbit makes it so that one year there equals 558 Earth years.
Humorously, Eris was initially called ‘Xena.’ This is the name of a character from a TV show which aired in the early 2000’s. Planets and dwarf planets are normally named after Gods. Eventually, though, Eris did get a name from Greek mythology. It’s named after the goddess of discord and strife.
Haumea is probably the strangest of the dwarf planets. Rather than being a sphere like the rest, it’s stretched out, almost like a potato. This odd shape comes from its extremely fast rotation speed: Haumea spins on its axis once every four hours!
A drawing of Haumea. It certainly doesn’t look like a planet!
Makemake is another dwarf planet in the Kuiper Belt. An interesting fact about Makemake is that its surface appears almost black from outer space. But, it remains bright as its shiny ice melts due to the Sun’s heat.
Pluto is, of course, the most familiar of the dwarf planets. It’s the largest among them, too. You can read much more about Pluto on our Pluto page. But, here’s a fun fact about it. Its only 1/3 of the size of our Moon! No wonder it can’t clear its orbit.
Pluto, our favorite ex-planet.