Social media was all aglow with the news yesterday that there may be a new planet in our Solar System, much further out than anyone expected. The new potential planet has been dubbed, with tongue firmly planted in cheek, “Planet 9.” But what exactly is this new “planet?” How have we missed it for so long? How could we know it exists now? What is Planet 9?

Here’s a little primer on the new discovery and the history of planet exploration. It’s just for the sake of granting a bit of clarity to what is sure to be generally awful reporting in mainstream circles.

What is Planet 9?

Well, for one thing, it’s not definitively a planet just yet. For now, Planet 9 is a bunch of clues that all seem – to the eyes of the researchers following it – to point in the direction of a previously-unknown planet. Scientists are extrapolating a possible conclusion from odd data they have collected. Which is definitely not the same thing as coming to a firm conclusion.

What data? And what’s so weird about it?

The study published yesterday concerns itself with objects in the Kuiper Belt, another asteroid belt like the one between Mars and Jupiter, positioned outside of the orbit of Pluto. Actually Pluto, recently demoted from planet to planetesimal, stands at the inner doorway to the Kuiper Belt.

What is Planet 9? An example of an elliptical orbit

An illustration of orbital eccentricity. Note that normal orbits are oblong, deviating from the perfect circle.

On the opposite edge of the Kuiper Belt, scientists have observed an irregularity about the way the asteroids move. You would expect that objects orbiting another object would move in oblong circular patterns like our Earth does. Such an orbit is called “elliptical” and the deviation an orbit has from a perfect circle is known as “orbital eccentricity.”

But the Kuiper Belt objects scientists are observing don’t quite behave that way. Thier orbits are much more eccentric at what’s known as the perihelion, which is where the object is closest to the sun. Many theories have been posited to explain this eccentricity, but so far a working model has not been agreed upon by scientists studying the phenomenon.

This study by two boffins of the American Astrological Society posits that the eccentricity could be accounted for by the gravity of another planet, interfering with the orbits of the asteroids and space debris.

So, we can’t see it. Have we ever discovered a planet this way?

What is Planet 9? Urbain Jean Joseph Leverrier

Urbain Jean Joseph Leverrier \

Yes! The planet Neptune was predicted by a man by the name of Urbain Jean Joseph Leverrier after astronomers puzzled at the odd eccentricities of Uranus’ orbit. Noticing that Uranus did not quite move in the path predicted by Sir Isaac Newton – whose theory of gravity we still use today to calculate the movement of heavenly bodies – Leverrier reversed the math and determined that an object of exactly the mass of Neptune would exist just beyond Uranus. And he predicted the position of Neptune within one arc degree, which is very close, indeed.

On the other hand, the very same Leverrier also predicted the planet Vulcan would exist just inside the orbit of Mercury, because again astronomers found eccentricities they could not account for. Don’t bother looking up Vulcan in a Solar System atlas, because it doesn’t exist. It turned out that, close as Mercury is to the Sun, Newtonian physics actually do break down. It would take decades and an entirely new field of physics – Einstein’s Relativity – to explain that particular mystery.

It seems doubtful that an entirely new way of calculating gravity would be required to solve the mystery of the Kuiper Belt and the putative “Planet 9.” But strange things happen in space.

How could there be a new planet? After all our astronomical exploration?

Space is big. That may seem silly to say, but it’s a really, really hard thing to quantify for those who don’t deal with it every day.

Solar system planets, relative distance. Planet 9 would be double this.

The proposed planet is about nine times the size of Earth. For perspective, Jupiter is 12 times the size of Earth and Neptune is about five times larger than Earth. This proposed planet, if it exists, would be at least 50 astronomical units (AU) away from Earth. That’s 50 times the distance from the Sun to the Earth. We didn’t know about Neptune until Leverrier, and this object is almost twice the distance from us.

Could we have missed an object that small and that far away? Absolutely. Even the Kuiper Belt itself was only just discovered in the 90’s, so there’s a lot left to understand.

How could there be another planet, way out there?

Our Solar System presents itself as a remarkably orderly affair: rocky inner worlds formed of rock and metal, followed by gas giants floating loftily in the outer solar system. But elsewhere in the galaxy, we have discovered some damned-strange stuff: “Hot Jupiters” orbiting closer to their parent stars than Mercury or planets that slingshot in wildly eccentric orbits. Most of these bizarre planets, it is thought, have had their orbits interrupted in some way, which either flings gas giants into the center of a system or else hurls a rocky world in a crazy orbit.

Even within the Solar System, there are oddities. Scientists believe Mars’ moons Phobos and Deimos are actually gravitational “abductees,” having been snatched into Mars’ orbit when they passed too close to it. Uranus rolls on it’s axis like a barrel, instead of a top as most planets do; scientists believe that this is because Uranus was collided with and had its orbit altered. Our own moon Luna is made of the same stuff as Earth because it is the result of a massive collision between Earth and some other protoplanet in Earth’s ancient history.

So while Planet 9 is by no means the only answer to the question of the Kuiper Belt’s odd orbital mystery, it is far from being an impossibility. But until we get a telescope trained in that direction – exactly in that direction – we won’t know for sure. Even then, there won’t be a lot of light from the far-distant sun, so spotting it will be tricky.

Whether Planet 9 turns out to be a new planet, a crazy loophole in Newtonian physics or some other unexpected wonder, our understanding of gravitational physics and our Solar System will be greatly enhanced. We will know something about our universe that no previous human generation has ever known. But as wonderful as that sounds,.. I think we’re all hoping for a new planet. Right?