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Did you grow up thinking there were nine planets in the solar system? You might have been right all along! Today we discuss the possibility of distant worlds in our solar system.

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Hank: There are only eight planets in the solar system, at least that we know of.  I mean, planets are pretty big, and we have all these telescopes, so you'd think we'd know if there was another one sitting around, but some scientists aren't so sure.  According to at least two teams of researchers, there might be a few more planet-sized worlds lurking out beyond Neptune.  Of course, there are lots of good scientific reasons that some astronomers think it's unlikely that there are unknown planets out there.  For one thing, according to our best current theories, there's no way that the original disk of material that formed the eight planets could have extended out that far.  We measure distances in the solar system in terms of astronomical units, or AU, each one being equal to the distance between Earth and the Sun, and our best estimates right now are that the original disk of planet-forming stuff couldn't have extended more than 100 AU, and this seems to make sense, because we haven't found any other planets beyond that.  NASA's space based Wise Telescope, which spends its time scanning the farthest reaches of the solar system, has already determined that there aren't any objects larger than Jupiter within 26,000 AU of the Sun, and nothing larger than Saturn within 10,000 AU, but Wise is only designed to detect the hotter gas giants, so there could be other smaller, rocky worlds that we just haven't detected yet.  

In March 2014, for example, scientists announced that they discovered a dwarf planet sized object orbiting the Sun called 2012 VP113 out beyond the Kuiper Belt.  Nicknamed 'Biden', it makes its closest approach to the Sun, also known as its Perihelion, about 80 AU away, and that was kind of striking to astronomers because unlike some other kinds of objects, such as comets, which swing by really close every once in a while, Biden has the most distant Perihelion of any known object in the solar system.  That means that its closest approach to the Sun is farther out there than the Perihelion of any other body around, at least that we know of, but its distance wasn't the only reason the object seemed weird.  

An orbit is usually just a flat elliptical path around the Sun, and most of the planets orbit in approximately the same plane of the solar system, but Biden's orbit is tilted slightly above and below that plane, and its Perihelion happens to occur right where its path starts to rise above it.  That in itself might not be such a big deal, except that almost every other known asteroid whose orbits are so far away do the very same thing, reaching Perihelion right where they cross the plane of the solar system, and according to researchers, the best way to explain all those Perihelia lining up is that a big distant planet is affecting those orbits.  Specifically, they say that an undiscovered planet at least 10x the size of Earth quietly orbiting the Sun at a distance of 250 AU could be affecting the orbits of asteroids between 30 and 150 AU.  Find it hard to believe that something that big could be out there that we have never seen?  Well, in May 2014, two Spanish astrophysicists concluded that there might actually be two new planets out there.  They looked at 13 known asteroids orbiting way out in the same general region, they modeled the orbits of these objects and their findings matched what the earlier study had predicted: All 13 of them behaved like there was an unknown planet orbiting the sun at 250 AU, but they also noticed that five of the asteroids had orbits that were similar in another weird way.  One pair of asteroids was taking almost exactly the same amount of time to orbit the sun, and the other three were all taking precisely 2/3 that time, and that would make sense if these asteroids' orbits were being affected by yet another unknown planet.  This still hypothetical world, the team says, would be about 200 AU from the Sun, and would have a mass bigger than Mars but less than Saturn.  

So if the math is suggesting where these planets might be, why haven't we found them yet?  Well, if they exist, they're most likely distant, dark, and slow, with such large orbits they'd definitely be hard to spot, and there could be some other reason why the Perihelia of all these asteroids line up or why those five have such funky orbits.  Astronomers will get a big boost in their search for any of Earth's long-lost planetary siblings when the new Large Synoptic Survey Telescope in Chile comes online in the 2020s.  It's designed to look for dim, faraway objects within our solar system, so the hope is that if there are any more planets out there to be found, this telescope will find them.  So for all you people who were raised thinking there were nine planets in the solar system, maybe you were right, maybe there were more than that, who knows what the future holds?

Thank you for joining me here on SciShow Space, especially to our patrons on Patreon.  If you want to help us keep exploring the universe, you can go to  There, you can also learn about all kinds of cool rewards that you can get as part of being a supporter.

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