 We found an object three times further away than Pluto and bigger than Pluto itself. We're calling this new object the tenth planet in the solar system. This will absolutely rewrite the history of astronomy textbooks. There's now a tenth planet out there. But that's not what happened. In fact, the discovery of this tenth planet is a reason that we don't even have Pluto anymore. Pluto used to be the ninth planet in our solar system, but it got demoted ten years ago to a dwarf planet. Pluto isn't alone out there. There are thought to be hundreds, if not thousands of dwarf planets. And astronomers are finding more of these tiny worlds all the time. So why do we even make the distinction between a planet and a dwarf planet? It all started in 2005 when a team of scientists led by Mike Brown found a new tantalizing object at the edge of the solar system. It was a rock that would eventually be named Eris. And when it was found, astronomers thought it might actually be bigger than Pluto. Don't worry, Pluto is the larger one, but Eris is more massive. Eris even has its own moon, just like most of the other planets do in our solar system. That led many in the science community, including NASA, to think Eris was the tenth planet beyond Pluto. But not everybody was on board with the title. When they found Eris, astronomers had been finding more and more tiny objects beyond Neptune. Calling Eris a planet could open the door to calling all these objects planets too. Plus, other Pluto-like objects were being discovered around the same time. Scientists everywhere were clashing over what makes a planet a planet, to the point that it became downright catty. The debate came to an emotional head in 2006 with a meeting of the International Astronomical Union. That's the group that's responsible for giving names to celestial bodies. After a meeting in Prague, the IAU came up with the first ever official definition of a planet. And it boils down to three main points. It has to orbit the Sun. It has to have enough mass so that it's rounded out by its own gravity. And then there's the last one, which people freaked out about. It needs to have cleared the neighborhood of its orbit. Okay, so what does that even mean? Again, it comes down to gravity. The idea is that the planet has to be big enough that it's swept away all the other objects out of its orbital path. And that's where Pluto doesn't fit in. Pluto orbits in a region known as the Kuiper Belt, a large cloud of small icy bodies that circle the Sun beyond Neptune. Many of the Kuiper Belt objects cross Pluto's path, making Pluto's neighborhood a little crowded. It's the same deal for Eris, which is also in the Kuiper Belt, and the Object Series, which shares its orbit with the asteroids in the Asteroid Belt. It's a criteria that didn't sit well with a lot of people, especially Alan Stern, the head of NASA's New Horizons mission to Pluto. That whole zone clearing criteria is, at its root, very anti-scientific, because it was designed to give a specific answer, and science doesn't work that way. Engineering does. So they engineered an answer because they wanted a small number of planets. Okay, and there was a concern that school kids wouldn't be able to remember the names if there were going to be a lot of planets. Now, I have to say, as a scientist, I find that kind of silly. To be fair, it is pretty easy to poke holes in the neighborhood argument. For example, Pluto's orbit also crosses Neptune, so doesn't that mean Neptune hasn't cleared its neighborhood either? And Earth isn't exactly alone in its orbit. There are more than 15,000 asteroids that orbit along with us. Wouldn't that mean Earth is a dwarf planet too? To address this, Stephen Soder of the American Museum of Natural History came up with a clarification. It's not that a planet has completely cleared its orbit, but that it dynamically dominates its orbit. Basically, a planet has to be the boss of its orbital zone. A good way of establishing this dominance is to compare the mass of a planet and the combined mass of all the other objects in a similar orbit. So Earth is 1.7 million times bigger than all of the other asteroids in its orbital zone combined. Likewise, Jupiter is 625,000 times larger than all of the Trojan asteroids in its orbit. The planet may be way more massive than ours, but it orbits with a larger swarm of objects. Now, let's take a look at Pluto. It's just 7% of the mass of all the objects that cross its path. By Soder's definition, Pluto does not dominate its orbit. But why even distinguish between planets and dwarf planets at all? For many, the classifications are tied to the roles these objects played in the formation of our solar system. The planets are the ones that help to clear everything out by jettisoning smaller bodies out into interstellar space or sending them hurtling toward the Sun. Dwarf planets, however, are leftover debris from the early days of the solar system that got kicked around by the larger planets. Pluto, for instance, is thought to be where it is now because of a great Neptune migration long ago. Ten years later, critics like Stern still think it shouldn't matter if an object's orbit has been cleared. If it's rounded out by gravity, then it's a planet. But the IAU has mostly closed the case on the matter, so the planet definition is going to stick around for the foreseeable future. But if you're bummed about only having eight planets in our solar system, it's possible there could be one more. Evidence is mounting that there's a mysterious Neptune-sized planet orbiting even farther out than Pluto and Eris. But that's for another video. To be fair, Stern also has an argument that if you were to put Earth where Pluto is now, it also wouldn't be big enough to clear its orbital zone because the orbital zone would be too big. But I'm not about labels, man. You know, we're all just floating in space together as one.