Scientists Think They May Be Able to Find the Missing Planet Nine
Planet Nine, the hypothetical ninth planet, may be orbiting out past Neptune, in or just past the Kuiper Belt.
A new preprint suggests it’s possible that—if it exists—Planet Nine may have collected some moons.
While the dark planet gives off no signals we can detect, observing the heat from those moons may be a way to finally spot Planet Nine.
There’s been a hypothesis floating around for a few years that there might be a ninth planet in our Solar System—and it’s not Pluto.
Planet Nine is unnamed, unconfirmed, and unknown. We haven’t been able to detect it, and we don’t even know for sure that if we did spot it, it would even be a planet. It might be a special kind of black hole, or be made entirely of dark matter.
But whatever it is—if it exists—we want to find it. It would explain a lot of the odd behavior astronomers detect in objects out past Neptune in the Kuiper belt. And according to a new preprint, the way to find it might be through the moons it could have scooped up over time.
The idea that this hypothetical planet may have caught a few moons isn’t particularly out there. The region of space where we think Planet Nine might be located is partially populated by roughly Pluto-sized bodies that researchers call Trans-Neptunian Objects, or TNOs. Given that Planet Nine is estimated to be about five to 10 times the mass of Earth, it’s very likely it could have trapped a few TNOs in its gravitational field.
Now, it may seem like the moons a planet collects would be even harder to detect than the planet itself. After all, not only are they far away, but they’re incredibly small.
But according to the preprint, the key lies in something called tidal heating.
Tides are a little more complex than just the rise and fall of the ocean. They’re the result of gravity pulling unevenly on a celestial body. When two bodies are very close to each other—like a planet and a moon—each object’s gravity pulls on the other object, and it pulls hardest on the nearest point.
This uneven pull deforms the body into a bit of a football shape. As the bodies move around each other, the orientation of that deformation moves, too. That means the whole body is shifting shape all the time, which generates friction inside the object and produces heat.
And that’s how we would find Planet Nine’s moons. Even though Planet Nine gives off no signals that we can detect—no light, no heat, no sound—the tidally-produced heat from those moons would be detectable. The detection would be difficult, because they probably wouldn’t get very hot, but it would be doable.
So far, this is just an idea. But if there really is a Planet Nine, it might be a promising one to chase. Using light from matter around dark objects is a big part of how we observe black holes, after all.
Why not a dark planet?
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