The case for this elusive world keeps mounting

Ella Alderson
Oct 10, 2018 · 6 min read

The race to find Planet Nine — sometimes known as Planet X out of respect for the now demoted Pluto — began early a couple of years ago when two astronomers noticed several objects in the Kuiper Belt behaving in a very strange way. The objects should have had random orbits, yet instead they traveled in the same direction, their oval paths stretching out into the same quadrant of the universe. The chances of this being a coincidence? 1 in 14,000. The solution to this, according to planetary scientists Konstantin Batygin and Michael E. Brown, was to suppose that far out beyond Pluto there loomed a huge and elusive planet shepherding these objects into their identical orbits. This planet would be 4 times the size of Earth and 10 times its mass, classifying it as a super-Earth. And while there are many planetary systems with several super-Earths in orbit around their stars, we have none. Our solar system ranges from small, cratered bodies like Mercury to gas giants like Jupiter and the very dense Neptune, with nothing in between despite how common super-Earths are (super-Earths are, in fact, the most common category of mass for planets in the universe, making it very strange that our solar system doesn’t posses any).

Size comparison between Earth and Planet Nine.

Sedna, Caju, and 2012 VP113 are a few of the Kuiper Belt objects sharing similar orbital paths. They’re classified as trans-Neptunian objects, bodies too far from the eight inner planets to be affected by them gravitationally. Just last week it was announced that a new dwarf planet would be joining this list — 2015 TG387, also called “The Goblin”. The Goblin is small; it’s much smaller even than our icy Pluto. Not only that but when it was first spotted in 2015, it was about 80 AU from the sun (one AU, or astronomical unit, is the average distance between the Earth and the sun which is about 93 million miles or 8 light minutes), meaning that it was 7.44 billion miles away. At its farthest, The Goblin reaches 2,300 AU from the sun. Add to that its tiny diameter of 186 miles and that makes it a feat to capture with our telescopes. For 99% of its 40,000 year orbit, the dwarf planet is too far away to see.

But there’s something special about The Goblin.

It’s tilted 54 degrees above the plane of the solar system, while all other eight planets orbit within a few degrees of each other. It was actually found during the Dark Energy Survey where researchers were observing a region far above our usual plane, looking to find out more about the rapid expansion of our universe. Scientists were surprised to find it. But the fact that it was discovered during such a routine survey of the sky is a great sign. It means that the observation wasn’t biased and the objects weren’t seen as clusters orbiting in the same direction simply because it’s what we wanted to see. 2012 VP113 and Sedna were also low-bias observations, making them very valuable as part of Planet Nine’s evidence.

Batygin and Brown weren’t surprised at The Goblin’s discovery. Their simulations of Planet Nine had perfectly predicted objects orbiting somewhere between our normal plane and a perpendicular one, a phenomenon that could only be explained if a massive planet was causing a gravitational slope. And The Goblin isn’t the only one; Caju also has a 54 degree incline. Simulations showed that these orbits would be stable given the age of the solar system and, similar to the relationship between Neptune and Pluto, Planet Nine’s gravity would keep the smaller objects from crashing into the presumed super-Earth. Introducing Planet Nine into the simulations did not at all disrupt currently observed orbits.

Illustration detailing the orbit of Inner Oort Cloud objects, including The Goblin, and their distance from our sun. From the Carnegie Institution for Science.

The assumption of a ninth planet solves so many problems that removing it leaves more questions than answers. At the moment, Planet Nine resolves at least 5 different questions about objects in our solar system.

Unfortunately for us, the elusive planet is predicted to be hundreds if not thousands of AU from the sun, making it difficult to observe directly even if it were much bigger than what we predict. At it’s closest point during orbit, the planet would be 200 AU or 18.6 billion miles away. Not to mention that its orbit lasts a whopping 10,000–20,000 Earth years.

If it may still seem unlikely for us to have missed something so large in our solar system, keep in mind that we’ve never had a survey looking for something so faint and so far away in our night skies. All objects in the solar system — whether it be galaxies, stars, or planets — look like bright little points against the black backdrop. They only way to distinguish one from the other is through motion. Galaxies never move, stars move very little, and planets move much more. These images of The Goblin were taken 3 hours apart in October of 2015:

Images by Scott Sheppard showing the movement of TG387 in the sky.

Scott Sheppard, the leader of a study at the Carnegie Institution for Science, believes there’s an 85% chance Planet Nine exists. But while the evidence is tempting, it’s important to remember that there could be other explanations for the orbits of these objects. Bias has a way of making us see things that aren’t necessarily there. Perhaps we need a bigger scope of the area; further observation of trans-Neptunian objects (TNOs) might reveal that these patterns are common and more evenly distributed across our solar system than we thought. Early 2017 revealed 22 other TNOs also in a strange pattern of orbit.

Some scientists assume a different explanation — that the sun was born as part of a cluster of stars and the interactions between those stars and the planets could have given them the unlikely orbits we notice today. Even Caju’s steep tilt could be explained by the gravity of thousands of dwarf planets possibly present during the younger days of our solar system.

Still, it’s exciting to think of a super-Earth somewhere not too far from home. We predicted Neptune long before we ever saw it, a story that should inspire us not to dismiss Planet Nine but to remain hopeful. Scientists today are part of an ongoing search to find more dwarf planets that could lead us to this discovery. It will be the largest and most in-depth survey of distant solar system objects to date.

Planet Nine is a mystery and a solution both at once. It could help us better understand the formation of the solar system, odd tilts, and odd orbits in the cold and rocky Oort Cloud and right here as we study the axis of our burning sun. For now it remains somewhere between imagination and reality, leaving us a trail of clues for where to find this exciting new world.

Predict

where the future is written

Ella Alderson

Written by

Physics student. A passion for language and the mysteries of our universe, our future, and our human condition. I can be reached at ella.aldrsn@gmail.com

Predict

Predict

where the future is written

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