Our solar system is a pretty busy place. There are millions of objects moving around – everything from planets to moons, comets and asteroids. And every year we discover more and more objects (usually small asteroids or fast comets) that call the Solar System home.
Astronomers had discovered all eight major planets by 1846. But that doesn’t stop us from looking for more. Over the past 100 years we have discovered smaller, distant bodies that we call dwarf planets, which we now classify as Pluto.
The discovery of some of these dwarf planets has given us reason to believe that something else may be lurking on the outskirts of the solar system.
Could there be a ninth planet?
There’s a good reason astronomers spend many hundreds of hours trying to find the ninth planet, also known as “Planet Nine” or “Planet X.” And that’s because the Solar System as we know it doesn’t really make sense without it.
Every object in our solar system orbits the sun. Some move fast, some move slowly, but all move according to the laws of gravity. Everything with mass has gravity, including you and me. The heavier something is, the more gravity it has.
A planet’s gravity is so great that it affects how things move around it. This is what we call its “gravitational pull”. Earth’s gravitational pull is what keeps everything on the ground.
Also, our Sun has the greatest gravitational pull of any object in the Solar System, and this is the main reason the planets orbit it.
It is through our understanding of gravitational pull that we get our greatest insight into a possible Planet Nine.
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Unexpected behavior
When we look at really distant objects, such as dwarf planets beyond Pluto, we find that their orbits are a bit unexpected. They follow very large elliptical (oval-shaped) orbits, are clustered together, and exist at an angle relative to the rest of the Solar System.
When astronomers used a computer to model what gravitational forces would be needed to make these objects move in this way, they found that a planet with a mass at least ten times that of Earth would be needed to cause it.
This is super exciting stuff! But then the question is: where is this planet?
The problem we have now is to try to confirm whether these predictions and models are correct. The only way to do this is to find Planet Nine, which is definitely easier said than done.
The hunt continues
Scientists around the world have been on the hunt for visible evidence of Planet Nine for many years.
Based on computer models, we estimate that Planet 9 is at least 20 times farther from the Sun than Neptune. We try to find it by looking for sunlight that it can reflect – just as the Moon shines from reflected sunlight at night.
However, because Planet 9 is so far from the Sun, we expect it to be very faint and difficult to see even for the best telescopes on Earth. Also, we can’t just look for it at any time of the year.
We only have small windows of nights where conditions should be right. Specifically, we must wait for a night without a moon and in which the place from which we observe faces the right part of the sky.
But don’t give up hope just yet. In the next decade, new telescopes will be built and new studies of the sky will begin. They might just give us an opportunity to prove or disprove the existence of Planet Nine.
Sarah WebbPostdoctoral Research Fellow, Center for Astrophysics and Supercomputing, Swinburne University of Technology
This article is republished from The Conversation under a Creative Commons license. Read the original article.
An earlier version of this article was published in February 2023.
