'Planet Nine': why scientists still believe in a giant at the edge of the solar system

The idea of a mysterious "Planet X" has been on the minds of scientists since the early twentieth century, long before the discovery of Pluto. Initially, the hypothesis explained the oddities in the orbit of Uranus, which deviated from the expected trajectory. Scientists believed that the orbit was affected by the gravity of an unknown object much larger in mass than Earth.

Later, in the 1990s, this mystery was solved by recalculating the mass of Neptune. However, the search did not end there: in 2016, astronomers at the California Institute of Technology Konstantin Batygin and Mike Brown put forward a new theory about the existence of the "planet nine". This is reported by the publication The Conversation.

The mystery of the Kuiper belt

Batygin and Brown's theory is related to the Kuiper belt - a huge region beyond the orbit of Neptune, filled with dwarf planets, asteroids and other icy "rubbish". Among these trans-Neptunian objects are bodies whose orbits, like Uranus', deviate from expected orbits. The authors of the theory claim: some massive planet must distort the motion of these objects with its gravitational field.

By analogy with the Moon: it makes a revolution around the Sun in 365.25 days, but under the influence of the Earth's gravity moves on a complex spiral trajectory. So too, orbits in the Kuiper belt are observed to indicate the influence of something other than the Sun.

Arguments for "planet nine"

Scepticism among scientists is gradually replaced by interest: more and more observational data confirm the anomalous behaviour of the orbits of distant objects. Mike Brown himself in 2024 noted:

"I consider it extremely unlikely that Planet Nine does not exist. At this point, there are no other explanations for the effects we are seeing."

For example, in 2018, a new candidate for a dwarf planet was discovered - object 2017 OF201, about 700 kilometres in diameter and with a highly elongated orbit. This orbit can be explained either by an ancient collision or by the gravitational influence of the hypothetical planet.

Problems with the hypothesis

Why has "planet nine" not yet been found? Some astronomers believe that there are still not enough observations to draw definite conclusions: perhaps the oddities in the orbits are not explained by a massive planet, but, for example, a ring of space debris or even the existence of a miniature black hole.

Another problem is the timing of observations. The orbital periods of distant objects are huge: for example, 2017 OF201 completes a revolution around the Sun in 24,000 years. It is difficult to observe gravitational effects in a short time: tens of thousands of years of observations or several revolutions are needed.

The discovery of new bodies also adds to the uncertainty. The latest example is object 2023 KQ14, discovered with the Subaru telescope in Hawaii. It belongs to a special class of "sednoids": it spends almost all of its time very far from the Sun, almost unaffected by Neptune. The closest point in its orbit to the Sun is 71 astronomical units (AU), and the furthest point is 433 AU. By comparison, Neptune is 30 AU from the Sun.

Interestingly, the orbit of 2023 KQ14 is less elongated and more stable than that of 2017 OF201, which means that the influence of the hypothetical "planet nine", if it exists, should be felt even further away - perhaps more than 500 AU away.

There are now four known sednoids, and all of them have stable orbits. This suggests that the potential "planet nine", if there is one, is very far away.

The future of the search

It still can't be ruled out that a massive planet is affecting the orbits of Kuiper Belt objects. However, the possibilities of modern science are limited: even the automatic station NASA New Horizons at its speeds will get to such distances in 118 years.

Therefore, scientists continue to use ground and space telescopes, and as technology advances, more and more distant objects are being discovered. These discoveries will gradually help shed light on the mysteries of the outskirts of our solar system. It remains to be seen if the search for Planet Nine will be successful in the coming years.