Does Planet Nine Exist?

Planet Nine has been a recent topic in news, so if you’re wondering how scientists could come up with this theory, or how the heck we missed an ENTIRE PLANET, then you’ve come to the right place!

Where did the theory come from?


Contrary to what we learn in school, the majority of the solar system lies outside of Neptune’s orbit. Just the Oort Cloud itself is about 1.4 light years wide!

For comparison, Neptune’s distance from the Sun is roughly 4 light hours.

Not Years. Hours.

Beyond Neptune, there are millions of objects, from tiny asteroids to dwarf planets with moons. These Trans-Neptunian Objects (TNOs) often have highly elliptical and inclined orbits. However some of these objects have extremely bizarre paths around the Sun, and are called eTNOs. Sednoids and detatched objects are two great examples.

All of these objects have eccentricities greater than 0.83 (for perspective, Earth’s is 0.017) and some orbit perpendicular to the Solar System’s plane! Not only that, but most of them are very clustered together, something slightly unexpected.

Nine’s orbit and a few eTNOs. Notice how a lot of them are clumped into one quadrant? Credit: R. Hurt/JPL-Caltech

A significant portion of the eTNOs show little to no influence by Neptune, which brings about the question “how did these objects get this way?”

These objects don’t quite fit the standard model of the solar system, which means the nature and origin of them aren’t explained very well.

Planet Nine aims to fix this!

Using computer simulations, astronomers have modelled the Kuiper belt and Planet Nine, and it seems to solve all of the issues we have with our current model of the solar system.

The far away planet also needs a highly eccentric orbit, but has enough gravitational influence to shepherd the eTNOs into roughly the same place. This gravitational pull can also elongate the orbits of the eTNOs, thus they can have altered orbits but not from Neptune. Any objects with secular resonance with Nine will slowly increase their inclination to the ecliptic, whilst also having an increase in eccentricity!

What would the planet look like?

Here’s the most updated estimates for Planet Nine:


As for what it looks like, well we have no clue yet! The most common claim is that Nine resembles a “Mini Neptune”. The planet, having a large gravitational force, most likely would have pulled in a lot of gases such as hydrogen and helium. It would have a thick atmosphere and a core slightly larger than Earth’s.

Where did it come from?

There’s a few theories on Nine’s origin. The most simplest being that the planet was ejected from the main part of the solar system. It could have been a fairly large piece of debris from the early solar system, which was able to acquire much more mass over time until it reached its current mass.

Nine could have also orbited another star, but captured by our Sun during a close encounter. Or perhaps Nine was a captured rogue planet, however the chances of any type of planet being captured and stabilised is incredibly slim!

What do YOU think?

Personally, there’s a fair bit of evidence backing up Planet Nine which makes me want to sway in favour of its existence. Understandably, Nine is very faint, and we must remember that our technology of today still has many limitations!

Unfortunately, none of our spacecrafts (Voyager 1 &2 and New Horizons) are anywhere near Nine, and it seems like the planet will unlikely be imaged soon!

I’m interested to hear what you think, so pop a comment below and look out for next week’s post!


  1. The first I heard about Planet Nine was from someone who I knew was deep down the conspiracy theory rabbit hole, so I did not take it seriously. But the orbits of those eTNOs make for really compelling evidence. There has to be something out there.


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