Planet Nine Facts for Kids

Planet Nine is a name given to a possible planet that scientists think might be orbiting our Sun far beyond Neptune. It's a "hypothetical" planet, which means it hasn't been directly seen yet. Scientists believe it could be out there because of the strange way some icy objects in the outer Solar System move.

What is Planet Nine?
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What is Planet Nine?

Planet Nine is a theoretical planet that scientists believe could be hidden in the outer reaches of our Solar System. It's much farther away than Pluto and would be very cold and dark. This idea first gained a lot of attention in 2016.

Why Do Scientists Think It's There?

Scientists don't have a direct picture of Planet Nine. Instead, they look for clues in the movements of other objects. Imagine a bunch of small boats in a pond. If they all start moving in a strange, similar way, you might guess there's a bigger, unseen current pushing them.

Clues from Distant Objects

The main clue comes from a group of small, icy worlds beyond Neptune. These are called Trans-Neptunian Objects (TNOs). Some of these TNOs have very unusual orbits. Their paths around the Sun seem to be clustered together. They also tilt in a similar direction.

Scientists like Konstantin Batygin and Michael E. Brown from Caltech noticed this pattern. They calculated that a large, unseen planet could be pulling on these TNOs. This pull would explain their strange, grouped orbits. Without such a planet, the orbits of these objects would likely be scattered randomly.

What Might Planet Nine Be Like?

If Planet Nine exists, it would be quite different from Earth. Scientists have made predictions about its size and where it might be.

Size and Composition

Based on the gravitational pull needed to explain the TNOs' orbits, Planet Nine is thought to be a "super-Earth" or a "mini-Neptune." This means it could be about 5 to 10 times the mass of Earth. It would likely be made of a mix of rock, ice, and gas. It would be much smaller than giant planets like Jupiter or Saturn.

Its Long Journey Around the Sun

Planet Nine's orbit would be incredibly long and stretched out. It might take between 10,000 and 20,000 Earth years to complete just one trip around the Sun. At its closest point, it could be about 200 times farther from the Sun than Earth is. At its farthest, it could be 1,200 times farther away. This extreme distance makes it very hard to spot.

How Are Scientists Looking for It?

Finding a planet so far away is a huge challenge. It would be very faint and hard to see against the dark background of space.

Scientists are using powerful telescopes to search for Planet Nine. They are looking for a tiny, dim point of light that moves slowly across the sky. Telescopes like the Subaru Telescope in Hawaii and the Vera C. Rubin Observatory (currently being built) are key to this search. The search involves taking many pictures of the sky and looking for anything that moves in a way that matches the predicted orbit of Planet Nine.

Even if Planet Nine is found, it will still be a long time before we can send a spacecraft there. For now, it remains one of the biggest mysteries in our own Solar System.

Images for kids

One hypothetical path through the sky of Planet Nine near aphelion crossing Orion west to east with about 2,000 years of motion. It is derived from that employed in the artistic conception on Brown's blog.
Final frame orbits]].)
Long term evolution of ETNOs induced by Planet Nine for objects with semi-major axis of 250 AU. Blue: anti-aligned, Red: aligned, Green: metastable, Orange: circulating. Crossing orbits above black line.
A possible orbit of the 11-Jupiter-mass exoplanet HD 106906 b