Scientists think there is a giant, invisible planet hiding in our solar system

Our solar system is a pretty busy place. There are millions of objects moving around – from planets to moons, to comets and asteroids. And every year we discover more and more objects (usually small asteroids or fast comets) calling the solar system home.

Astronomers had found all eight of the major planets by 1846. But that doesn’t stop us from looking for more. Over the past 100 years, we’ve found smaller, distant celestial bodies called 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 might be lurking on the outskirts of the solar system.

Could there be a ninth planet?

There’s a good reason why astronomers spend many hundreds of hours locating a ninth planet, sometimes called “Planet Nine” or “Planet X.” And that’s because the solar system as we know it makes no sense without a solar system.

Every object in our solar system revolves around the sun. Some move fast and some move slowly, but they 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 strong that it affects the way things move around it. We call that ‘gravity’. The Earth’s gravity keeps everything on the ground.

Furthermore, our sun has the greatest gravity of all objects in the solar system, and this is actually why the planets orbit around it.

It is through our understanding of gravity that we get our biggest clue to a possible Planet Nine.

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Unexpected behavior

When we look at very distant objects, such as dwarf planets beyond Pluto, we find that their orbits are a bit unexpected. They move in very large elliptical (oval) orbits, are grouped together, and are on an incline compared to the rest of the solar system.

When astronomers use a computer to model what gravitational forces are needed to make these objects move the way they do, they find that it would have taken a planet with at least ten times the mass of Earth 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 that 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 years.

Based on the computer models, we think Planet Nine is at least twenty times further away from the sun than Neptune. We try to detect it by looking for sunlight to reflect it, just as the moon shines at night through reflected sunlight.

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. We can’t just look for it at any time of the year either.

We only have small windows for nights when conditions need to be just right. Specifically, we must wait for a night without a moon, and when the location from which we observe points to the right part of the sky.

But don’t give up hope yet. Over the next decade, new telescopes will be built and new surveys of the sky will begin. They might just give us the chance to prove or disprove whether Planet Nine exists.The conversation

Sara WebbPostdoctoral 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 appeared in February 2023.

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