Science post alert! Planet Nine is a hypothetical planet beyond Neptune in our solar system. It is believed to be a super-Earth-sized planetary body with a mass about 5 to 10 times that of Earth.
The existence of Planet Nine was proposed to explain peculiarities observed in the orbits of some trans-Neptunian objects (TNOs), which are objects that orbit the Sun beyond Neptune. These TNOs exhibit unexpected clustering and alignment in their orbits, suggesting the gravitational influence of a large, distant planetary body.
While Planet Nine has not been directly observed yet, its potential existence is supported by computer simulations and modeling of the outer solar system’s dynamics. Astronomers continue to search for this elusive planetary body through telescopic surveys and analysis of orbital data from TNOs.
Pluto, which we formerly considered planet 9, has an orbital period of nearly a quarter millennium. This means that it travels through an incredibly long distance each orbit – and it does so because of its relative distance from the Sun.
So, a planet that’s further out than Pluto is going to have an even longer orbital period – which means that there’s an absolutely enormous amount of space that needs to be searched in order to find it, and its apparent movement speed may be so slow that we can’t easily detect its motion in the sky.
But it gets worse than that. The planets we already know about don’t orbit in the same plane – their orbits are angular to each other. So it’s reasonable to expect a degree of angularity to the orbit of planet 9… which expands the area in need of searching geometrically. And that’s if it’s angularity is within a similar range as that of the other planets in system – if its orbit is eccentric, that search area expands geometrically all over again.
And then there’s the fact that we only see other planets in our system because the light of our sun is reflected off them. As far out as this planet would have to be, there’s likely not enough light hitting it for us to see it in that way. So the most likely way we’d find it would be from its shadow passing across other objects and removing them from view. And with the way light is lensed by our atmosphere, such a blanking out might not even be visible planet-side.
Which means that we’d need to search roughly the entirety of local space for a tiny darkness moving amongst the rest of the darkness, at an unspecifed distance from and angle to us, blanking out a chain of objects in a straight line in succession while travelling at an unspecified speed. And we’d need no other interfering bodies between here and there, such as asteroids or comets, fudging the chain of darkening bodies by interjecting themselves along the path of its orbit.
And this why we have yet to locate planet 9, even though its gravitational effects tell us it’s there somewhere. We’re looking for a specific invisible needle in a mobile, haystack-sized pile of needles, in the dark, with only a sense of its approximate size by which to find it. Yeah; not so easy.