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Transcript

The Ghost

The final mystery in this region is the biggest one. It suggests that we are still missing a massive piece of the solar system. The evidence comes from a dwarf planet named Sedna. When astronomers found Sedna in 2003, they realized immediately that it didn’t fit any known category. It is a large red world, roughly half the size of Pluto. But the problem isn’t its size. It is in its orbit. Sedna has an orbit that is incredibly stretched out. Most planets circle the sun in a path that is close to a circle. Sedna travels in a massive elongated oval. It takes about 11,400 years to complete one trip around the sun. At its furthest point, it travels nearly 900 astronomical units away. That is deep into the darkness, far beyond the second belt we just discussed. But the strange part is where it is when it comes closest to the sun. Sedna never gets closer than 76 astronomical units.

This is the smoking gun. To understand why this number matters, you have to look at gravity. Objects in the Kuiper belt are usually controlled by Neptune. If they get too close to Neptune, its gravity kicks them out into scattered orbits. But Sedna never gets close to Neptune. It stays far away from the giant planets. At the same time, it isn’t far enough out to be pulled by the gravity of other stars or the galaxy itself. Sedna is in a zone that astronomers call detached. It is in no man’s land. It is too far to be pushed by Neptune, but too close to be pulled by the galaxy. This means Sedna should not be there. It could not have formed in that location. There isn’t enough material out there to build a world that size and it couldn’t have been scattered there by Neptune because it never gets close enough for Neptune to touch it.

For years, astronomers thought Sedna was just a freak accident. Maybe a passing star billions of years ago dragged it into this strange path. But then they found more of them. In the last decade, we discovered a handful of other objects with these same detached orbits. One of them is nicknamed the Goblin. When you look at these objects on a map, you see a pattern. If these were just random rocks, their orbits should be pointing in all different directions. They should be scattered randomly around the sun like petals on a flower. But they aren’t. Sedna, the Goblin, and several other extreme objects are all clustered together. Their orbits all swing out into space in the exact same direction. They are also tilted at similar angles. The odds of this happening by accident are tiny. It is less than 1 in 10,000. This clustering tells us that something is controlling them.

The only thing that can hurt planets like this is gravity. And to have enough gravity to control objects this far out, you need something massive. The math points to a single culprit. It suggests there is a massive planet hiding in the deep solar system. Astronomers call it Planet 9. Based on the way Sedna and the others move, this planet would need to be about 5 to 10 times the mass of Earth. It would be a super Earth or a mini Neptune made of gas and ice. It likely orbits hundreds of times further out than Earth. Because it is so far away, it would be incredibly faint, which explains why we haven’t seen it yet. It moves so slowly that it would look like a fixed star in our telescopes. But its gravity is the only thing that explains why Sedna exists. It is the invisible shepherd that keeps these distant objects in line. So when we look at the Kuiper belt, we are seeing the footprints of a giant ghost.