You’ve probably had the layout of the solar system drilled into you so much that at this point it’s second nature. Which is why it might surprise you to learn that those 8 planets in that neat layout weren’t always where they are today. It’s even possible there was once a 9th planet that helped shape the solar system, and now it's missing.
I am not talking about Pluto. This 9th planet researchers recently proposed is in the ballpark of Mars or Earth in terms of mass. You know, big enough to be a planet.
Pluto is almost 50 times less massive than Mars, which is why it was reclassified as a dwarf planet all the way back in 2006. It’s never rejoining the planet club, we’re all going to have to come to terms with that. I’m also not talking about the long-hypothesized Planet 9. Well, ok this would be a 9th planet, but the prevailing idea for what has been dubbed Planet 9 is something else entirely.
Scientists proposed Planet 9 to explain why many objects way out past Neptune, in what’s called the Kuiper Belt, had elliptical orbits that appeared to mostly cluster in one quadrant of the solar system. 
Computer models showed that a very distant 9th planet with a highly elliptical orbit of its own could explain this observed clustering, but in order to make that model work, Planet 9 would have to be on the scale of 10 Earth masses.
This newly proposed 9th planet is something completely different. What the researchers from the University of British Columbia and University of Arizona proposed in their recent paper is that billions of years ago the outer solar system had at least one Mars-sized planet, possibly more.
Why? Well, to them, it seemed like something was off. The outer solar system formed the massive rocky cores of the gas giants; Jupiter, Saturn, Uranus, and Neptune. And there are several dwarf planets like little Pluto and smaller. But there’s nothing in-between, and to them that seemed unlikely.
They backed up their hunch with computer models. Thanks to past models, scientists believe the outer planets once had different orbits that shifted as the planets grew and tugged on each other. Jupiter moved closer to the sun, while the other three drifted farther away. Still, no one model explains everything we observe, like how many objects in the Kuiper belt got so far out when their composition indicates they probably formed much closer to the Sun.
When the authors of this paper looked at models that inserted one or more Mars-sized planets into the outer solar system’s early mix, things fell into place much better.
Making simulated stellar objects learn the steps of our cosmic tango is a start, but there’s no substitute for cold hard evidence. I mean that literally; if there really is a rocky planet that’s out past Neptune, that thing is going to be very cold and hard and astronomers need to find it. That’s easier said than done.
Objects way out that far are notoriously hard to spot because they don’t create their own light. Astronomers would either need to catch it reflecting sunlight, or distorting the images of the stars behind it. Most Kuiper Belt Objects have an elliptical orbit, so if this planet does too, the laws of physics would mean it spends most of its time at the farthest part of said orbit. That is, if it’s still orbiting the Sun at all; in about half the simulations the Mars-sized planets in question were launched out of the solar system altogether.
So it’s very possible that if we do have a distant rocky cousin, it’s long since left our cosmic neighborhood, and we’ll never reconnect with it. It’s also possible that other early configurations of our solar system could explain what’s around us today. The only way to find out is to keep doing what we’re already doing: build better observatories, test new computer models, and keep scouring the night sky for more clues that might reveal how everything got to where it is today.
Like I said, this proposed 9th planet isn’t what’s often called Planet 9, so if you’d like to learn more about how that wild and controversial theory came about, check out Dr. Ian’s video on that here. Do you think we’ll ever find a 9th planet or are we just trying to fill that void Pluto left in our hearts when it was demoted?
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