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Is There a Ninth Planet?

September 12, 2018


If you’ve spent time on the internet over the past few years 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).


Back in April of 2015, NASA shared information on some asteroids way out past Neptune and Pluto with orbits that make some astronomers think there might be an unknown planet out there - potentially even two herding them around.


And just recently, 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 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, so they tilt the same way, they’re stretched out the same amount, and they bring the asteroids closest to the Sun all at the same place.


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. This 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 asteroids.


Initially, there were problems with these predictions. In order 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. However, if that actually happened, it should have also affected orbits in the inner Solar System - something we don’t have evidence of.


So the team from Caltech decided to take another approach: what if there were just one extra planet out there with a tilted orbit stretched out in the opposite direction, shepherding all those asteroids around? They decided that a ninth planet 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 is 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 so far from the Sun, it would take tens of thousands of years to complete just one orbit.


Plus, wherever it is, it’s bound to be extremely dim. Luckily, there is a way of testing for the planet without having to see it.


The authors of the study make a clear, testable prediction: if the planet is out there, we should be able to find asteroids that are kind of a mirror image of the ones I mentioned earlier, 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.


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, and 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. I’ll let you know if they find it!


Thank you for choosing SkyFeed! Be sure to follow my social media (Instagram: @astrolia) and subscribe to get the best of space exploration delivered to your inbox every week.


Source: This story was originally published on SciShow Space. I am republishing a lightly edited version on SkyFeed in light of interest on the subject. Green, Hank. "Planet 9 from Outer Space." SciShow Space, YouTube. 28 Jan, 2016. Web video.

Citation: Rovira, Lia N. "Is There a Ninth Planet?" SkyFeed. 12 Sept, 2018. Web article.

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