How many planets are there in our solar system? The answer might not be as obvious as you think. Royal Observatory astronomer Colin Stuart explains.

So officially the answer is currently eight – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Wind back the clock back ten years though and Pluto would also have appeared on the list. It has since been reclassified as a dwarf planet due to having not cleared the neighbourhood around its orbit (it sometimes crosses over with Neptune). Go back even further to 1801 when Ceres was discovered and it, too, was called a planet and Pluto had yet to be discovered.

Given that it has changed on several occasions before, it is no surprise that our list of planets may need rewriting again. That’s because there is growing evidence that we might have missed a planet in our solar system – “Planet Nine”.

Artist's impression of Planet Nine

It all started when astronomers were looking at objects in the Kuiper Belt – the region of small, icy objects in orbit around the Sun beyond Neptune. Pluto orbits in this very region. So do worlds known as Sedna, VP113, VN112, RF98, TG422 and GB174. The odd thing is that the orbits of all six bodies are similar. In particular, they are all tilted at the same angle compared to the eight confirmed planets (see diagram below).

'Planet Nine' and orbits of Kuiper belt objects

The astronomers behind the Planet Nine announcement believe that the chance of this alignment happening randomly is just 0.007%. So it is likely that something is causing them to line up in this way. Their hunch is that another planet could exist much further from the Sun than the rest, and it’s this planet’s gravity that has pulled those six objects into similarly shaped orbits. Their calculations suggest that the planet would need to be 10 times more massive than the Earth and would take up to 20,000 years to complete one orbit of the Sun.

It wouldn’t be the first time astronomers have found a planet by first spotting its effect on other objects in the solar system – it is exactly how Neptune was found. After Uranus was discovered in 1781, astronomers began to notice that its orbit was a little odd. They guessed these peculiarities could be explained if another planet existed beyond Uranus and was pulling on it. When the position of this new planet was calculated, and telescopes were pointed in that direction, Neptune was there waiting for us.

It should be said that a ninth planet isn’t the only explanation for the similar orbits of those six Kuiper Belt objects. But the excitement of potentially finding a brand new planet will inspire astronomers to scour the skies in search of this world. If it is found we’ll have to tear up all the textbooks, but then that wouldn’t be the first time.

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Find out more about the planets in our solar system