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Could we have a possible 9th Planet? Hank Green tells us what we think we know with this elusive object.

Planet 9 from Outer Space poster:

Host by: Hank Green
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If you've spent any time on the internet over the last week or so, you've probably noticed a lot of people talking about one of the most exciting announcements ever, for space nerds like us: The solar system might have nine planets! And this time, the debate has nothing to do with Pluto. 

Last April, we told you about some asteroids way out past Neptune and Pluto with orbits that make some astronomers think there might be an unknown planet out there -- or even two herding them around. 

And last week, researchers at the California Institute of Technology announced that a ninth planet isn't just a possible explanation for these oddities. It's actually the most likely explanation. 

The planetary scientists were trying to explain something peculiar about the orbits of a bunch of bodies out in the Kuiper Belt, a region of icy asteroids and dwarf planets -- including our old friend Pluto -- left over from the formation of the solar system. See, some of the most distant Kuiper Belt objects we know of seem to follow a similar pattern: their orbits all kind of line up. Basically, their orbits are just differently sized copies of each other: They tilt the same way, they're stretched out the same amount and they bring the asteroids closest to the Sun at the same place. 

Now, if everything in the Kuiper Belt had a random orbit and we were finding them in a random order, the chances of finding such a high percentage of similar orbits would only be about 1 in 14,200. Those odds aren't too tiny, but they're small enough that it's probably more than just luck. Which is why over the past couple of years, different groups of researchers have been proposing different explanations -- like a ninth planet, and maybe even a tenth, affecting the orbits of these objects. 

But there were problems with these predictions: For the math to work out, our solar system would've had to have a close encounter with another star at some point in our history. Except, if that actually happened, it should have also affected orbits in the inner solar system - and astronomers haven't seen evidence of that.

So the team from Caltech decided to take another approach: what if there were just one extra planet out there, with a titled orbit stretched out in the opposite direction, shepherding all those asteroids around? They found that a ninth planet, around ten times the mass of Earth and more than 250 times farther from the Sun, would explain the weird asteroid orbits we're seeing -- without complications like our solar system brushing past other stars. And the math works out really well! That's why everyone's so excited.

So the next step is to look for more evidence of this new planet 9. But it's going to be hard to find it directly, partly because it could have many different orbits and partly because it's so far from the Sun, it would take tens of thousands of years just to complete one orbit. Wherever it is, it's bound to be almost unobservably dim. Luckily, there is a way of testing for the planet without having to see it. 

The authors of the study made a clear, testable prediction: If the planet is out there, we should be able to find asteroids that kind of a mirror image of the ones that we talked about before, but on the opposite side of the solar system. So, if astronomers find the predicted asteroids in the coming years, it would be strong evidence for the existence of a new ninth planet. And if we find more asteroids with similar orbits, it might help to narrow down just where the new planet is. 

This might all seem like a very indirect way of finding a planet, but it's exactly how we found Neptune in the 1840s. At the time, astronomers knew that something weird was happening with Uranus's orbit. So, they used these strange features to predict where Neptune had to be --- and once they knew where to look, they eventually found it using telescopes. So, with better observations of the deeper areas of the solar system, astronomers might soon know just where to look for the predicted planet. 

And we will of course let you know if they find it! In the meantime, we'll just have to gaze longingly at this sweet poster, we'll be hanging on the wall here at SciShow headquarters. If you want one for yourself and help support SciShow, you can check it out at 

And don't forget to go to and subscribe! 

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