Remember Pluto? For humans of a certain vintage, like me, it was a name familiar from our earliest brushes with astronomy. It was the ninth planet in our Solar System, the furthest from the Sun.

Those nine were Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. There were even some semi-famous mnemonics to help with those names. One I remember was "My Very Educated Mother Just Served Us Nine Pizzas".

Pluto spent much of its known planetary life - and mine - rounding out the count of known planets. "Rounding" being, of course, an obvious word to use, given the planets' orbits around the sun. "Pluto" was indeed a familiar name, maybe even beloved. I mean, there's Pluto the Dog. I could be wrong, but I don't think there's a cartoon character named for another planet.

In 2006, and to the consternation of many, Pluto was suddenly demoted. No longer is Pluto to be considered a planet - it is too small an object to be one. Instead, it is a "dwarf" planet like a few others in that general area of the Solar System. So now we have just eight planets, and the mnemonic has mutated: "My Very Educated Mother Just Served Us Nachos." Yet it may be a sign of how deeply rooted Pluto is that it still makes regular appearances. The morning I wrote these words, for example, the horoscope for Leo warned: "Venus advocates domestic entertaining and family harmony, but Pluto has other ideas."

More seriously than horoscopes, the 2006 demotion also gave us this definition of a planet: "A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit."

Now Pluto does orbit the Sun and it is pretty much spherical. What about clearing the neighbourhood? As they orbit, planets interact gravitationally with other nearby, smaller objects. Some may collide with and be absorbed ("accrete") into the planet. Some may be pulled into orbit around the planet itself - our Moon, and the other moons in the Solar System, are examples. Still other objects may be forced out of their orbits and sent soaring elsewhere, perhaps unpredictably. But all this requires the planet itself to reach a certain heft, and Pluto never did. So unlike its its larger planetary neighbours, it hasn't managed to sweep its neighbourhood clean.

This is why Pluto was demoted.

Beyond the orbit of Neptune is a vast collection of rocks sailing around the sun, called the Kuiper Belt. That's actually where Pluto roams too. So astronomers now think of Pluto as another of those Kuiper Belt rocks, if among the largest that we have observed there.

But here's the thing. Even if we have officially downsized from nine planets to eight, even if Pluto is banished to dwarfhood for at least the near cosmic future, many astronomers also believe there is indeed a ninth planet - a real one - out there anyway. Not Pluto, but a larger ball making its way around the Sun. They have been scouring the Kuiper Belt for several years, but nobody has seen this larger ball.

Yet.

If nobody has seen this possible Planet Nine (also called Planet X, or P9), what makes these astronomers believe it is actually out there? Aha! Now we're getting at an essential technique astronomers use to make inferences about the existence of a cosmic object: what its effect is on other cosmic objects that we have observed.

The story here starts about a decade ago, when two astronomers announced that they had observed an object well beyond Neptune, into the Kuiper Belt. They gave it the name 2012 VP113. Though it was quickly nicknamed "Biden", the name of the then-VP of the USA and, you may recall, also currently in the news. This Biden in the Kuiper has an orbit that absolutely dwarfs all the other planets' orbits, as well as Pluto's. The astronomers also soon noticed that it shares some similarities with a few other extremely distant Kuiper Belt objects. In particular, the angles that the orbits of these objects makes with the Earth's orbit are all similar.

Well, not quite: one of these otherwise similar objects, 2013 FT28, is different in one respect. A certain orbital measure, the longitude of the perihelion, is different for FT28 by almost 180 degrees from Biden and the others.

This one difference apart, why are these orbits so similar? The astronomers suggested that there is a large planet out in the Kuiper Belt - over 200 times as far from the Sun as our Earth is - that is effectively "shepherding" these objects into similar orbits. This planet, they calculated, is several times larger than the Earth; possibly near the size of Neptune, which is about 17 times as heavy as the Earth. They think it takes between 10,000 and 20,000 years to orbit the Sun.

If their observations and calculations about the shepherded flock of objects are right, this unknown shepherd planet would need to be that large. A real planet, then, not a dwarf like Pluto.

What's more, some numerical simulations showed that such a planet could also drive an object like 2013 FT28 into its "anti-aligned" orbit. So it turns out that even that outlier in this grab-bag of extremely distant outliers supports the hypothesis of a ninth planet.

All this, suggesting the existence of a planet that nobody has yet observed. Another team of astronomers - including Michael Brown, responsible for Pluto's demotion in 2006 - used these results in their own research, writing in a recent paper: "Our results reveal that the orbital architecture of this group of objects aligns closely with the predictions of the P9-inclusive model."

In fact, "in stark contrast, the P9-free scenario is statistically rejected." And the evidence "strongly favors a model of the solar system that includes P9."

Put simply, these astronomers believe there is a large ninth planet in our Solar System. We just need to find it.

And maybe devise a mnew mnemonic.

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