It is a question that feels like it should have a straightforward answer: how many planets are there in our solar system?

Since Pluto was relegated back down to dwarf planet status, almost everyone has agreed that the answer is eight.

However, scientists say that it might be time to rewrite the textbooks.

A new study has proposed that there could be a secret world lurking on the edge of our solar system.

Dubbed ‘Planet Y’ by reseachers from Princeton University, this planet is said to be Earth–sized and rocky. 

Researchers were alerted to a potential new planet after observing that 50 objects in the Kuiper Belt—a region filled with icy bodies beyond Neptune—were tilted at an unusual angle.

Lead author Dr. Amir Siraj stated, “We explored several explanations other than a planet that might account for the tilt, but what we realized is that a planet must indeed be present,” as reported by CNN.

‘This paper is not a discovery of a planet. But it’s certainly the discovery of a puzzle for which a planet is a likely solution.’

Scientists propose the existence of a ninth planet, called Planet Y, which might be concealed in the remote areas of the Solar System beyond Neptune’s orbit (stock image).

Since Pluto was axed from the list of planets, astronomers looking for a ninth planet have focused on the Kuiper Belt.

The Kuiper Belt is a doughnut-shaped ring consisting of icy bodies, asteroids, and dwarf planets beyond Neptune, thought to be remnants from the formation of the eight planets.

Potential planets in this far-off area would receive minimal sunlight, rendering them extremely difficult to detect with a standard telescope.

Nonetheless, scientists believe they could identify the presence of these elusive worlds by observing their effects on other objects in the Kuiper Belt.

In 2016, Caltech astronomers Mike Brown and Konstantin Batygin proposed the ‘ninth planet hypothesis’, arguing that the movement of a dozen Kuiper Belt objects could only be caused by an enormous and very distant ‘Planet X’.

However, Dr Siraj thinks that there could be another ninth–planet contender, hiding much closer to home.

If it exists, Planet Y should be a rocky world with a mass between that of Earth and Mercury.

That makes it much smaller than Planet X – another theorised planet hiding in our solar system, believed to be a gas giant with a mass 10 times greater than Earth’s.

Orbiting 100 to 200 times farther from the sun than the Earth, Planet Y is also much closer than Planet X – which is believed to orbit 400 times further from the sun than Earth.

However, at that distance, Planet Y would still be extremely dim and difficult to detect.

Likewise, the researchers predict that Planet Y’s orbit is likely tilted by about 10 degrees from the orbital plane, which would make it much harder to find.

Importantly, the Planet Y theory does not mean that Planet X does not exist – meaning there could be as many as 10 planets in our solar system.

In fact, Dr Siraj’s previous research suggests that there could be room for up to five more Earth–like planets to hide in the Solar System’s very outermost regions.

That means our stellar neighbourhood might be much more crowded than we had previously thought.

However, not every astronomer is convinced by Dr Siraj’s calculations.

Dr Samantha Lawler, associate professor of astronomy at the University of Regina in Canada, told CNN that these findings were ‘not definitive’.

Planet X and Planet Y are likely located in the Kuiper Belt, a doughnut–shaped ring of icy objects beyond Neptune. However, Planet Y could be two to four times closer to Earth 

Although Planet Y is a plausible explanation for the tilted orbits of Kuiper Belt objects, without observational evidence, there is no hard proof.

However, that could change very soon thanks to the Vera Rubin Observatory, which has just started taking photos of the sky with the world’s largest digital camera.

Over the next decade, the Vera Rubin Observatory will take a photo every 40 seconds for eight to 12 hours a night, repeatedly scanning the entire sky.

Scientists anticipate that this observatory will discover thousands of new objects – including Planet X and Planet Y – if they really are there.

Dr Siraj predicts: ‘I think within the first two to three years [of the observatory’s mission], it’ll become definitive. If Planet Y is in the field of view of the telescope, it will be able to find it directly.’

PLANET NINE: ORBITS OF OBJECTS BEYOND NEPTUNE SUGGEST ‘SOMETHING LARGE’ IS THERE

Astronomers believe that the orbits of a number of bodies in the distant reaches of the solar system have been disrupted by the pull of an as yet unidentified planet.

First proposed by a group at CalTech in the US, this alien world was theorised to explain the distorted paths seen in distant icy bodies.

In order to fit in with the data they have, this alien world – popularly called Planet Nine – would need to be roughly four times the size of Earth and 10 times the mass.

Researchers say a body of this size and mass would explain the clustered paths of a number of icy minor planets beyond Neptune.

First proposed by a group at CalTech in the US, this alien world was theorised to explain the distorted paths seen in distant icy bodies.

Its huge orbit would mean it takes between 10,000 and 20,000 years to make a single pass around the sun. 

The theoretical Planet Nine is based on the gravitational pull it exerts on these bodies, with astronomers confident it will be found in the coming years.

Those hoping for theoretical Earth-sized planets proposed by astrologers or science fiction writers – which are ‘hiding behind the sun’ and linked with Doomsday scenarios – may have to keep searching.