In 2006, scientists enraged space fans by taking away Pluto's status as a planet. Now, in 2015, debate still rages on about that decision.
However, the scientists who made the decision about Pluto's status had good reasons for Pluto being bumped down from planet to dwarf planet. And as we learn more about our solar system and what lies outside of it, those reasons make a lot of sense.
Of course, that doesn't mean that Pluto isn't any less important in our solar system. NASA's New Horizon mission began before scientists voted Pluto off of the planet list, but is still set to reach Pluto this summer. We've never seen Pluto up close before, so we're still going to learn a lot about it: things we did not know before. Pluto could even hold some secrets of how our solar system formed.
Pluto is still one of the largest bodies in the Kuiper belt, that area that lies on the outer edges of our solar system. Scientists believe that Pluto, along with the other objects in the Kuiper belt, are the remains of formation of the solar system.
How did Pluto get demoted from being a planet?
First, it wasn't just one scientist that made the decision to take Pluto off of the planet list. After studying over a group of 40 bodies in our solar system similar to Pluto, researchers attended a meeting of the International Astronomical Union where they overwhelmingly voted that Pluto fit the description of dwarf planet better than planet. This was after they spent a year figuring out the definition of what constituted a planet.
So what is a planet anyway?
These researchers defined a new planet as an object that orbits the sun and is large enough that it became round because of its own gravity. That planet is the dominant celestial body in its orbital neighborhood, meaning that these bodies generally knock debris, asteroids and comets, out of their paths, making their orbits clear. This also means that a planet is significantly larger than its moons.
Scientists wanted to redefine what a planet is, because such a body is more unique, different from other celestial bodies that are much more common. And now that we're searching for planets outside our solar system, having a more solid definition for a planet means that we'll now recognize a planet when we discover one.
"It's going to be hard to find a new planet," said astronomer Mike Brown, of the California Institute of Technology shortly after Pluto's demotion. "You'd have to find something the size of Mars. Finding a new planet will really mean something."
So why isn't Pluto a planet?
By this new definition, Pluto fails the full-fledged planet test, mostly because it isn't the most dominant object in its orbit around the sun. Pluto's "moon," Charon is also fairly big, compared to Pluto, about half its size. Finally, Pluto's orbit contains a lot of other objects, so it hasn't cleared away any other objects in its path.
In other words, by the 2006 definition of planet, Pluto just doesn't fit in.
So why is Pluto a dwarf planet?
It's only been over the last 20 years that our telescopes have been powerful enough to observe objects in the outer solar system. Although we knew that Pluto was out there, we had no idea that there were a large collection of other objects in that area, the Kuiper belt.
Astronomers believe that at least 70,000 icy objects with the same composition as Pluto, span the area in the belt. And as we learn more about some of these objects, we've realized that many are similar in size to Pluto, too. This means that Pluto isn't like a planet at all, but more like these objects.
Just before the meeting that resulted in Pluto's demotion, scientists discovered an object in the Kuiper belt that was the same size as Pluto: Eris. If Pluto stayed a planet, Eris, too, would be a planet. But it isn't. So therefore, neither is Pluto.
So scientists designated that a dwarf planet is an object that meets the first two criteria of being a planet, meaning that a dwarf planet orbits around the sun and has a rounded shape because of gravity. However, a dwarf planet has not cleared its neighborhood of debris and other bodies.
Because there are many other objects in Pluto's orbit, Pluto is a dwarf planet, as is its twin, Eris.
And that's why we only have eight planets in our solar system.
[Photo Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Goddard Space Flight Center]