Beyond Pluto: The search for the hidden planets

Could one – or even two - undiscovered planets lurk at the edges of our Solar System? Nicky Jenner explores how close we are to finding the elusive 'Planet 9'

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How many planets are there in the Solar System? It sounds like an easy question, but it's more complex than you might think - and actually it's one we still can't answer. With scientists finding evidence that there's another one, or even two, just waiting to be discovered beyond Neptune, let's take an in-depth look at the century-old search for the elusive 'Planet 9'.

It all started back in the early 1900s, when astronomer Percival Lowell was hunting for a hypothetical ninth planet - Planet X - outside the orbits of Uranus and Neptune. Uranus's orbit was odd - it didn’t seem to match our predictions - but if there was a large and distant planet out there disturbing it, it could all make sense. On 18 February 1930, Planet X revealed itself. Clyde Tombaugh spotted a small dot shifting in position across two astronomical plates taken earlier that year: a planet, subsequently named Pluto, moving along its orbit.

Astronomers had predicted that such a planet would be massive, at least as hefty as Earth, in order to push Uranus around the way it did. But Pluto was small, dark and ringless. Its path around the Sun was highly elliptical, or oval-shaped, unlike the other planets of the Solar System. Doubts immediately arose: could this tiny body really be a planet, or was it more like an asteroid, comet, or even a moon?