Planet Nine: What Lies Beyond Neptune?

We all know about the planets in our Solar System: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Many of us remember when, in 2006, NASA declared that Pluto didn’t classify as a planet under their newly defined parameters for a planet. But some of you might not know that there is actually a hypothesized ninth planet in our system, beyond Neptune, and it’s not a dwarf planet like Pluto.

This planet, conveniently called Planet Nine, was first hypothesized in 2014, and, as it stands, has never been seen. However, its presence would explain the unusual clustering of the orbits of extreme trans-Neptunian objects (eTNOs). The orbits of these objects are noticeably affected something pulling them, thus leading astronomers to believe that there is a ninth planet with a gravitational pull strong enough to affect them. 

It’s predicted that this planet is five to 15 times the size of Earth and is anywhere from 45 billion to 150 billion kilometers away from the sun. There are many speculations about where this hypothetical planet might have come from. Some think that it was the core of a giant planet that was sent to the edges of our system after a collision with Jupiter in the early stages of our solar system’s life. Another hypothesis is that our sun picked it up from the outskirts of the system of another star, or that it was once a rogue planet. And yet the fact remains that we haven’t been able to find any other traces of Planet Nine.

Recently, however, a pair of physicists shared that they think that, instead of a planet, it’s a primordial black hole. Though this study has not been peer-reviewed, nor has the existence of primordial black holes even been confirmed, it would explain the gravitational pull on these trans-Neptunian objects, while also explaining why we haven’t been able to see it.

Primordial black holes are only hypothetical. Hypothetically, they were created in the early moments after the Big Bang, when our universe was less than a second young. They are black holes that aren’t formed by collapsed stars, which means that they may be any size. Collapsed stars are massive and have an extremely strong gravitational pull, but primordial black holes aren’t necessarily large, and their pull obviously corresponds to their mass, so their pull isn’t necessarily always very strong either.

At the distance Planet Nine is from the sun, it would be nearly impossible for us to see. The sunlight that manages to reach all the way past Neptune has to be reflected off Planet Nine and then returned to our eyes all the way back here on Earth. This is a lot of distance for the light to travel, and by the time it gets here, it will probably be hardly detectable. And this is assuming that Planet Nine is a planet ten times the size of Earth.

If Planet Nine is not a planet and is, in fact, a primordial black hole, that would make it even harder to spot. Since black holes are so dense, a small amount of black hole is enough to have an effect on nearby objects. If the black hole were huge, we would all be sucked in, and there would be a great amount of evidence that there is in fact a black hole at the edge of our solar system. However, the only things that are really affected by this object are the nearby eTNOs, which means that this primordial black hole is pretty small. In terms of mass, a planet the size of ten Earths is equivalent to a black hole about the size of a bowling ball.

It’s not hard to see that finding a bowling ball-sized mass of darkness in an endless sea of darkness is a lot harder than finding a planet ten times the size of Earth. Which might explain why we still haven’t been able to find Planet Nine, even though mathematically we know it must exist. But until the existence of primordial black holes is confirmed, or until Planet Nine is actually discovered, we won’t know what is out there.