The dark outskirts of our solar system could be hiding not one but 200 planets, some so strange that scientists can't figure them out. So, where did the search for Planet 9 take us, and what could be lurking at the edge of our solar system? The idea of an extra planet circling our Sun actually dates back to ancient times, with the Babylonians, who were skilled astronomers, including a celestial body called Nibiru in their cosmology. Interestingly, they described it as a star, not a planet. However, in the 1970s, an American writer named Zecharia Sitchin decided that Nibiru was a planet, claiming it swings by the Sun occasionally, causing natural disasters on Earth—a theory unsupported by scientific evidence.
In recent years, credible scientists have also speculated about a hidden planet. Astronomers Scott Sheppard and Chad Trujillo discovered numerous Trans-Neptunian Objects (TNOs), small celestial bodies beyond Neptune's orbit, chilling in the Kuiper Belt, a region of frozen remnants from the solar system's formation. In 2014, they suggested the strange trajectories of these objects could be influenced by a massive, unseen planet.
By 2016, Konstantin Batygin and Michael Brown from the California Institute of Technology examined the six most distant TNOs. These objects had incredibly elongated elliptical orbits, all clustered together in a way that seemed impossible without some gravitational influence. Statistically, the odds of such alignment happening randomly were just 0.2%. This led Brown and Batygin to propose the existence of a massive planet, possibly five to ten times the mass of Earth, with an orbit so extended it could take 10,000 to 20,000 years to circle the Sun.
Mathematical prediction like this isn't new. It was used in 1846 when Uranus's odd orbit led French mathematician Urbain Le Verrier to predict Neptune's existence, which was confirmed by German astronomer Johann Galle. Similarly, in 1906, Percival Lowell speculated about a planet beyond Neptune, calling it Planet X. Though he never found it, his work led to the discovery of Pluto by Clyde Tombaugh in 1930. However, Pluto was eventually reclassified as a dwarf planet since its mass couldn't explain the anomalies observed in TNO orbits.
Brown and Batygin's models suggest Planet 9 could be between 400 and 800 astronomical units (AU) from the Sun—one AU is the distance from Earth to the Sun, about 150 million kilometers. Such a planet would be incredibly cold and difficult to detect. Even with advanced telescopes, it would appear as a tiny, nearly invisible speck. Pluto, which is 20 times closer, still appears as a blurry spot in most telescope images.
If Planet 9 exists, it may be a dense, icy world with a frozen surface composed of methane, nitrogen, and ice, reflecting almost no light. Some theories even suggest it could have thin rings or small moons, further complicating detection. Interestingly, some astronomers have proposed that Planet 9 might not be a planet at all. In 2019, James Unwin from the University of Illinois suggested it could be a primordial black hole—an ancient black hole formed shortly after the Big Bang. Such black holes can be incredibly small but exert significant gravitational influence. A black hole with five times Earth's mass could be as small as a tennis ball, making it nearly impossible to detect.
However, the search for Planet 9 has led to many groundbreaking discoveries, such as the existence of over 200 dwarf planets in the Kuiper Belt. One of the most fascinating is Haumea, a fast-rotating dwarf planet shaped like a football, with a year lasting 285 Earth years. Surprisingly, it has rings, a feature previously thought to be limited to gas giants. Other intriguing finds include Eris and Makemake, dwarf planets potentially capable of supporting underground oceans due to geothermal activity.
Some Kuiper Belt objects defy celestial mechanics entirely. For instance, Niku orbits the Sun in the opposite direction of most planets and has a tilted orbit, suggesting it may have originated from another star system or been influenced by a massive undiscovered object.
The search for Planet 9 has also involved scanning the sky with advanced telescopes, but so far, no definitive evidence has surfaced. Some researchers, like Kevin Napier, argue that the clustering of TNOs might be due to observational bias rather than an undiscovered planet. However, Brown and Batygin maintain that their data supports the Planet 9 hypothesis.
Interestingly, German astronomer Susanne Pfalzner proposed another theory: the orbits of TNOs might have been influenced by a passing star billions of years ago, disrupting the Kuiper Belt enough to mimic the effects of a large planet's gravity. This theory is consistent with the idea that stars occasionally pass near our solar system, affecting celestial bodies in the outer regions.
While the search for Planet 9 continues, it's possible we may discover something even more extraordinary—perhaps even a rogue planet or a celestial body that could serve as a stepping stone for humanity's expansion into the galaxy. One thing is certain: the dark edges of our solar system still hold many secrets waiting to be uncovered.