Is the ninth planet back? The Solar System's hidden giant is back in the spotlight

Debates have flared up once again about the existence of an undiscovered giant planet in an unknown corner of the Solar System.

According to the news in Science Alert, this hypothetical celestial body, called "Planet Nine," could be the hidden force behind the irregular orbital movements in the Kuiper Belt, according to some astronomers.

This idea was first proposed in the 1930s, when deviations in Uranus's orbit were observed. This unknown object, then known as "Planet X," was partially explained by the discovery of Pluto, but was later discredited by recalculations of Neptune's mass.

MYSTERIOUS MOVEMENTS IN THE KUPIER BELT

In 2016, astronomers Konstantin Batygin and Mike Brown of Caltech revived this old hypothesis. The unexpected alignment of orbits of objects in the Kuiper Belt beyond Neptune may indicate the existence of an unseen, large object in the system.

Much like Earth's gravitational pull on the Moon, this unknown mass is thought to disrupt the orbits of small objects. According to Batygin and Brown, this effect can only be explained by a very large planet—perhaps 5 to 10 times the mass of Earth.

WHAT DO THE OBSERVATIONS SAY?

Initially skeptical, the theory has gained increasing observational support over the years. The fact that some objects in the Kuiper Belt have similarly tilted and elongated orbits suggests these deviations are not coincidences. Mike Brown reinforced this view in a statement in 2024, stating, "The probability of Planet Nine not existing is very low."

THERE ARE OPPOSITIONS TOO

Of course, there are those in the scientific community who oppose this hypothesis. The most fundamental question is: "If such a planet exists, why hasn't it been observed yet?" While some researchers argue that the available data is not yet sufficient, others suggest that the deviations in the Kuiper Belt could be caused by other gravitational sources, such as a small black hole or a wide ring of debris.

THE BIGGEST OBSTACLE: TIME

One of the biggest challenges to the Planet Nine theory is time. Some objects in the Kuiper Belt, such as 2017 OF201, complete a full orbit around the Sun in approximately 24,000 years. Such long orbital periods make it extremely difficult to draw meaningful conclusions from observations.

A new Kuiper object, 2023 KQ14, discovered in 2023, has provided further theoretical challenges. Its more stable orbit suggests that if Planet Nine truly exists, it could be much farther away, perhaps as far as 500 astronomical units (AU) from the Sun.