Is there a massive undiscovered planet lurking in the cold, dark outskirts of our Solar System? The idea has been kicking around longer than Pluto’s been a planet - which, come to think of it, isn't a planet anymore. Originally called Planet X, it was trotted out to explain why Uranus wasn't following the orbital script physics had written for it. The gravitational tug of an unseen world, several times larger than Earth, seemed like a plausible culprit.
That mystery got solved in the 1990s when someone recalculated Neptune's mass and everything made sense again. But then, in 2016, Caltech astronomers Konstantin Batygin and Mike Brown revived the ghost with a new theory of Planet Nine, this time pointing fingers at the Kuiper Belt - that giant ring of dwarf planets, asteroids, and other cosmic leftovers beyond Neptune (yes, Pluto lives there now). Many Kuiper Belt objects, also known as trans-Neptunian objects, have orbits that don't follow the expected path, and Batygin and Brown argued that only something with a hefty gravitational pull could be messing with them.
Think of it like our Moon: it orbits the Sun every 365.25 days, but Earth's gravity yanks it into a monthly loop around us. From an outside view, it spirals. Similarly, many Kuiper Belt objects seem to be dancing to a tune that isn't just the Sun's. Astronomers were initially skeptical, but mounting evidence from increasingly powerful observations has shown that these orbits are indeed erratic. As Brown said in 2024, "I think it is very unlikely that P9 does not exist. There are currently no other explanations for the effects that we see, nor for the myriad other P9-induced effects we see on the Solar System."
In 2018, a candidate dwarf planet called 2017 OF201 - about 700km across (Earth is roughly 18 times bigger) - was found with a highly elliptical orbit, hinting at either an ancient impact or the gravitational nudge of Planet Nine. On the other hand, if Planet Nine exists, why hasn't anyone found it yet? Some astronomers question whether there's enough orbital data from Kuiper objects to justify any conclusions, while alternative explanations like a ring of debris or - wait for it - a small black hole have been proposed.
The biggest hiccup? We haven't been watching the outer Solar System long enough. For instance, 2017 OF201 has an orbital period of about 24,000 years. To spot subtle gravitational effects, you'd probably need to observe four to five orbits. That's a lot of coffee breaks.
New discoveries keep complicating things. The latest is 2023 KQ14, spotted by the Subaru telescope in Hawaii. It's a "sednoid," meaning it spends most of its time far from the Sun but still within the Sun's gravitational reach (about 5,000 AU away, where 1 AU is the Earth-Sun distance). As a sednoid, Neptune's gravity barely touches it. 2023 KQ14's closest approach to the Sun is about 71 AU, its farthest about 433 AU. By comparison, Neptune is about 30 AU out. This new object has a very elliptical orbit, but it's stabler than 2017 OF201, suggesting no large planet - including hypothetical Planet Nine - is significantly affecting its path. If Planet Nine exists, it would have to be farther than 500 AU from the Sun.
To make matters worse for the Planet Nine theory, this is the fourth sednoid discovered, and the other three also have stable orbits, implying any massive planet would have to be very far away indeed. Still, the possibility remains that a big planet is out there, influencing Kuiper Belt orbits. But finding it is a challenge: it would take 118 years for a spacecraft to travel far enough, based on estimates from NASA's New Horizons explorer.
So we'll have to keep relying on ground- and space-based telescopes to spot new asteroids and distant objects as our observing capabilities improve. Watch this (very big) space - something might eventually emerge. Or not. Stay tuned.