Is there a Ninth Planet beyond our solar system?

A theory suggesting a massive undiscovered planet lurking at the far edges of our solar system has resurfaced. This hypothetical object, called "Planet Nine," could explain a long-standing mystery in astronomy.

The discussion dates back to the 1930s with the idea of "Planet X," proposed after noticing irregularities in Uranus's orbit, thought to be caused by a massive unseen planet. Although this mystery was resolved in the 1990s by recalculating Neptune's mass, a similar theory was revived in 2016 by Caltech astronomers Konstantin Batygin and Mike Brown.

They argue that certain objects in the Kuiper Belt—made up of dwarf planets, asteroids, and other bodies beyond Neptune—have unusual orbits influenced by a strong gravitational source besides the Sun. This source could be Planet Nine, estimated to be several times larger than Earth.

Is there evidence?
Initially met with skepticism, growing observations confirm the Kuiper Belt objects' orbits are indeed irregular. In 2024, Mike Brown stated it is highly unlikely Planet Nine does not exist.

However, critics ask why, if it exists, no one has yet found it. Some believe there isn't enough orbital data, while others suggest the anomalies might be caused by a debris ring or a small black hole.

A major challenge is the limited observation time. For instance, an object named 2017 OF201 has an orbital period of about 24,000 years, requiring much longer observations to detect subtle changes.

Most recently, the discovery of 2023 KQ14, which has a more stable orbit than expected, challenges the theory further, implying that if Planet Nine exists, it might be located even farther away—around 500 astronomical units from the Sun.