Our solar system is a pretty busy place. There are millions of objects moving around everything from planets, moons, comets and asteroids. And every year we’re discovering more and more objects (usually small asteroids or fast comets) that call the Solar System home.
Astronomers had found all eight major planets by 1846. But that doesn’t stop us from looking for more. In the last 100 years, we have found smaller distant bodies that we call dwarf planets, which is what we now classify Pluto as.
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 hundreds of hours trying to find a ninth planet, known as “Planet Nine” or “Planet X.” And that’s because the Solar System as we know it makes no sense without it.
Every object in our Solar System revolves around the Sun. Some move fast and 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 the way things move around it. This is what we call “gravitational pull”. Earth’s gravitational pull is what holds everything on earth.
Also, our Sun has the greatest gravitational pull of any object in the Solar System, and this is essentially why the planets revolve around it.
It is through our understanding of gravitational pull that we get the most clues about a possible Planet Nine.
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Unexpected behavior
When we look at really distant objects, such as the dwarf planets beyond Pluto, we find that their orbits are a bit unexpected. They move in very large elliptical (oval-shaped) orbits, are grouped together, and exist at an inclination compared to the rest of the Solar System.
When astronomers use a computer to model what gravitational forces are needed for these objects to move in this way, they find that a planet at least ten times the mass of Earth would have been required to cause this.
It’s super exciting stuff! But then the question is: where is this planet?
The problem we have now is trying to confirm whether these predictions and models are correct. The only way to do this is to find Planet Nine, which is obviously easier said than done.
The hunt continues
Scientists all over the world have been searching for visible evidence of Planet Nine for many years now.
Based on computer models, we think Planet Nine is at least 20 times farther from the Sun than Neptune. We try to find it out by looking for sunlight that can reflect just as the Moon shines from reflected sunlight at night.
However, because Planet Nine 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 have to be right. Specifically, we should wait for a night without a Moon, and where the location from which we are observing is facing the right part of the sky.
But don’t lose 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 the opportunity to prove or disprove whether Planet Nine exists.
Sarah WebbPostdoctoral Researcher, 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.
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