What we know about the search for “Planet Nine” in our solar system

Is there a massive, undiscovered planet on the outer ranges of the solar system? The idea has existed since Pluto’s discovery in the 1930s. Prominent astronomers had referred to as Planet X and had brought it up as an explanation for the Uranus orbit, which drifts from the path of the orbital movement to which physics would expect.

The gravity of an undiscovered planet, several times larger than the earth, was considered a possible reason for the discrepancy.

This secret was ultimately declared by a recalculation of Neptuns Messe in the nineties, but then a new theory of a potential planet nine by astronomer Konstantin Batygin and Mike Brown to Caltech (California Institute of Technology) was brought up in 2016.

Her theory refers to the Kuiper belt, a huge belt of dwarf planets, asteroids and other matter that is beyond Neptune (and also Pluto).

Many Kuiper belt objects, which are also known as Transneptunic objects, have been discovered that they circle the sun, but like Uranus they do not do this in a continuous expected direction. Batygin and Brown argued that something with a large gravitational suit has to influence their orbit and proposed the planet Nine as a potential explanation.

This would be comparable to what happens to our own moon. It circles the sun every 365.25 days, according to what they would expect from their distance. However, the earth’s gravitational train is such that the moon also circles the planet every 27 days. From the perspective of an external observer, the moon moves in a spiral movement. Similarly, many objects in the Kuiper belt show signs that their orbits are influenced by more than just the gravity of the sun.

While astronomers and space scientists were initially skeptical about the planet Nine theory, thanks to increasingly powerful observations, there were indeed the orbits of trans -elunic objects are indeed irregular.

As Brown said in 2024: “I think it is very unlikely that P9 does not exist. There are currently no further explanations for the effects we see, nor for the countless other P9-induced effects that we see on the solar system.”

For example, it was announced in 2018 that there was a new candidate for a dwarf planet who creeps up the sun, which is known as 2017 in 2017. This object measures around 700 km (the earth is about 18 times larger) and has a highly elliptical orbit. This lack of an approximately circular orbit around the sun indicated this way either at the beginning of its lifespan or a gravitational influence of Planet Nine.

Problems with theory

On the other hand, if Planet Nine exists, why hasn’t anyone found it yet? Some astronomers are wondering whether there are enough orbital data from Kuiper objects to justify conclusions about its existence, while alternative explanations for their movement are presented, such as:

The biggest problem, however, is that the outer solar system has not been observed long enough. For example, the 20101 property has an orbital period of around 24,000 years. While the orbital path of an object can be found in a short number of years, all gravitational effects probably need four to five orbits to notice subtle changes.

New discoveries of objects in the Kuiper belt have also made challenges for the Planet Nine Theory. The latest is known as 2023 KQ14, an object discovered by the Subaru telescope in Hawaii.

It is known as “Sednoid”, which means that it spends most of the sun far from the sun, although in the huge area in which the sun has a focus (this area is around 5,000au or astronomical units, where 1au is the distance from the earth to the sun). The classification of the object as a sednoid also means that Neptune’s gravitational influence has little to no effect on it.

2023 KQ14S Engest approach to the sun is around 71au, while its most widely around 433au. In comparison, Neptune is about 30au from the sun. This new object is another with a very elliptical orbit, but is more stable than in 2017 from 2017, which indicates that no large planet, including a hypothetical planet, has a significant influence on its way. If Planet Nine exists, it might have to be further than 500au from the sun.

In order to worsen the matter for the Planet Nine Theory, this is the fourth sednoid that has to be discovered. The other three also show stable orbits, which also indicates that every planet nine should indeed have to be very far away.

Nevertheless, there is the possibility that there could still be a massive planet that affects the orbits of bodies within the Kuiper belt. However, the ability of the astronomers to find such a planet remains somewhat limited by the restrictions of an unmanned space travel. It would take 118 years for a spaceship to be far enough to find it, based on the estimates from the speed of the New Horizon Explorer of NASA.

This means that we still have to rely on floor and room-based telescopes to recognize something. New asteroids and distant objects are constantly being discovered because our observation functions become more detailed, which should gradually give more light on what could be out there. So look at this (very large) room and let us see what appears in the coming years.

Ian Whittaker is a lecturer in physics at Nottingham Trent University.

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