suspected, not yet found · beyond the Kuiper Belt (hypothetical)
Planet Nine (Planet X)
Hypothesized undiscovered planet, 5-10 times Earth's mass, inferred from clustered orbits of distant icy objects.
What it is
Suspected, not yet found: Predicted by the maths; not yet seen. Every kind of thing there is →
Can you see it?
Not as one thing, no — Planet Nine is a place rather than a sight, and you are already inside it. What you go out and look at are the things in it, one at a time.
Worth knowing about Planet Nine
In weight, some scientists think there might be an undiscovered planet maybe 5 to 10 times heavier than Earth, orbiting way out in the darkness beyond the Kuiper Belt. So far away it would take 10,000+ years to circle the Sun once.
- What makes it worth knowing
- Not confirmed -- still just a hypothesis; some scientists think the clustering could be a coincidence or a trick of which objects we've happened to find. A brand-new telescope in Chile, the Vera C. Rubin Observatory, started scanning the whole southern sky in 2026 and is considered the best chance yet to finally spot Planet Nine -- or rule it out.
- How we know: A small group of very distant icy objects have oddly similar, clustered orbits, as if something big and unseen is tugging on them with its gravity -- first noticed in 2016. Rung 3 — the shape of the solar system
The numbers
| Distance from the Sun, on average | possibly 400-800 AU |
|---|
What it belongs to
Then go outside
Too big to look at all at once — but the things inside it are not. The Planetarium will show you which are up.
