Our solar system is a pretty busy place. There are millions of objects moving around – everything from planets to moons to 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 found all eight major planets by 1846. But that doesn’t stop us from looking for more. Over the past 100 years, we have found 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 is a good reason why astronomers spend many hundreds of hours searching for the ninth planet, also known as “planet nine” or “planet X”. And that’s because the Solar System as we know it really doesn’t make sense without it.
Every object in our solar system orbits the Sun. Some move fast and some move slowly, but all move in accordance with 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. That’s what we call “gravitational pull”. Earth’s gravity is what holds everything on earth.
Our Sun also has the greatest gravitational pull of any object in the Solar System, which is essentially why the planets orbit it.
It is through our understanding of gravitational force that we have obtained our biggest clue to a possible planet nine.
frameborder=”0″ allow=”accelerometer; automatic playback; clipboard-write; encrypted media; gyroscope; picture in picture; web-share” referrerpolicy=”strict-origin-when-cross-origin” allowfullscreen>
Unexpected behavior
When we look at really distant objects, like the dwarf planets beyond Pluto, we find that their orbits are a bit unexpected. They move in very large elliptical (oval) orbits, are clustered, and exist tilted relative to the rest of the Solar System.
When astronomers use computers to model what gravitational forces are needed to make these objects move like this, they find that it would take a planet at least ten times the mass of Earth to do so.
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 definitely easier said than done.
The hunt continues
Scientists around the world have been searching for visible evidence of Planet Nine for many years.
Based on computer models, we believe that Planet Nine is at least 20 times farther from the Sun than Neptune. We try to detect it by looking for sunlight that it can reflect – just as the moon shines at night with reflected sunlight.
However, because Planet Nine is so far from the Sun, we expect it to be very faint and hard to see even with 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 the conditions have to be just right. Specifically, we have to wait for a moonless night, on which the place from which we observe is facing 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 surveys of the sky will begin. They may just give us an opportunity to prove or disprove whether Planet Nine exists.
Sara 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.