Could there be a giant, undiscovered planet in the outer reaches of our solar system? This idea existed even before Pluto was discovered in the 1930s. Prominent astronomers have proposed the planet, which they have named Planet The gravitational pull of an undiscovered planet several times larger than Earth was considered a possible reason for this discrepancy.
The mystery was finally explained in the 1990s by recalculating Neptune’s mass, but then in 2016 Caltech astronomers Konstantin Batygin and Mike Brown proposed a new theory about the possibility of Planet 9.
Their theory involves the Kuiper Belt, a huge belt of dwarf planets, asteroids, and other material beyond Neptune (including Pluto). Many Kuiper Belt objects (also called trans-Neptunian objects) have been found orbiting the Sun, but like Uranus, they do not move continuously in the expected direction. Batygin and Brown argued that something with significant gravity must be influencing the orbit, and proposed Planet 9 as a potential explanation.
This is comparable to what is happening on our moon. It orbits the Sun every 365.25 days, consistent with what would be expected from their distance. But the Earth’s gravitational pull is so strong that the Moon also orbits the Earth every 27 days. As a result, from the perspective of an outside observer, the Moon appears to be in a spiral motion. Similarly, many objects in the Kuiper belt show signs that their orbits are influenced by influences other than the Sun’s gravity.
Astronomers and space scientists were initially skeptical of the Planet Nine theory, but thanks to increasingly powerful observations, there is growing evidence that the orbits of trans-Neptunian objects are indeed unstable. Brown said in 2024:
“I think it’s very unlikely that P9 doesn’t exist. There is currently no other explanation for the effects we’re seeing, or the myriad other P9-induced effects we see in our solar system.”
For example, in 2018, it was announced that there is a new candidate for a dwarf planet orbiting the sun, known as 2017 OF201. The object is about 700 km in diameter (Earth is about 18 times larger) and has a highly elliptical orbit. The absence of a nearly circular orbit around the Sun suggests either that it was placed in this orbit by an impact early in the Sun’s life, or that it is a gravitational influence from Planet 9.
Problems with theory
On the other hand, if Planet 9 exists, why hasn’t anyone discovered it yet? Some astronomers have questioned whether enough orbital data has been obtained from the Kuiper object to justify a conclusion about its existence, while other explanations for the Kuiper object’s motion have been proposed, such as the influence of debris rings or the more fanciful idea of a small black hole.
However, the biggest problem is that the outer reaches of the solar system have not been observed long enough. For example, object 2017 OF201 has an orbital period of approximately 24,000 years. The orbit of an object around the sun can be found within a few years, but it probably takes four or five orbits to notice subtle changes due to gravitational effects.
New discoveries of objects in the Kuiper belt also pose challenges to the Planet Nine theory. The latest one is known as 2023 KQ14, an object discovered by the Subaru telescope in Hawaii.
It is known as a “sedonoid”. This means that we spend most of our time far away from the Sun, although within a vast region where the Sun’s gravitational pull acts (this region is approximately 5,000 AU, or astronomical units, where 1 AU is the distance from Earth to the Sun). The object’s classification as a sedonoid also means that Neptune’s gravitational influence has little or no effect on it.
2023 KQ14’s closest approach to the Sun is about 71 AU away, and its farthest point is about 433 AU. By comparison, Neptune is about 30 AU from the Sun. This new object is another object with a highly elliptical orbit, but it is more stable than 2017 OF201, suggesting that the large planets, including the hypothetical Planet 9, are not significantly influencing its orbit. Therefore, if Planet 9 exists, it would probably need to be farther than 500 AU from the Sun.
To make matters worse for the Planet Nine theory, this is the fourth Cedonoid discovered. The other three also show stable orbits, similarly suggesting that Planet 9 must actually be very far away.
Nevertheless, it is still possible that there are giant planets that influence the orbits of objects in the Kuiper belt. However, astronomers’ ability to discover such planets is still somewhat limited, as even unmanned space travel is limited. Extrapolating from the speed of NASA’s New Horizons spacecraft, it would take the spacecraft 118 years to travel far enough to discover it.
This means we must continue to rely on ground-based and space-based telescopes to detect anything. As our observational capabilities become more detailed, new asteroids and distant objects are being discovered all the time, and we should gradually learn more about what’s out there. So let’s keep an eye on this (very large) space and see what emerges over the next few years.
