What is Planet Nine?

Most people know there are 8 planets in our solar system: Mercury, Venus, Earth, Mars Jupiter, Saturn, Uranus, and Neptune. Many people are also aware of Pluto, and there is often confusion as to whether it is a classified as a planet or not. As of today, Pluto is defined as a dwarf planet — it is not one of the ‘official’ planets in our solar system. So, what on Earth (pardon the pun) is this so-called ‘Planet Nine’?

Now it must be made clear that Planet Nine has not been observed directly, but rather, its effects have. We can use Newton’s Laws of Gravitation, mainly that the gravitational force between two objects is proportional to their masses and inversely proportional to the square of the distance between them. However, this has been superseded by Einstein’s Theory of General Relativity, which allows for more accurate models to be made. Using mathematical equations from these theories, we can predict just how various bodies will move through space based on the other bodies around them. For example, by measuring the way bodies in the milky way move, it’s clear that there must be a massive object in the centre attracting everything around it: a black hole!

There are many scenarios where we have proven that the equations from General Relativity or Newton’s Laws do work with incredibly high accuracy, and so we can say it is validated. But what has this got to do with Planet Nine? Well, when astronomers were looking out to the Kuiper belt (a field of icy debris beyond the orbit of Neptune), they noticed some clustering of orbits of objects. These objects are known as extreme trans-Neptunian objects (ETNOs). The strange thing is that when considering the predicted forces that these ETNOs should experience from known bodies in the solar system, these objects have no reason to be clustering the way they are. And so, astronomers are now claiming that these unusual gravitational effects could serve as evidence for an undiscovered giant planet in the mysterious far reaches of the solar system.

In fact, Newton actually used his laws of gravitation to predict the existence of both Pluto and Neptune before they had ever been observed, and that was over 400 years ago! So, perhaps this could be a similar situation? As it’s currently just theoretical, it has not been given a name. Hence, it’s only known as Planet Nine.

Originally published at http://thephysicsfootprint.wordpress.com on January 6, 2022.