For nearly a decade, astronomers have been hunting for a massive, unseen planet lurking on the fringes of our solar system. This hypothetical world, known as Planet Nine, could be ten times the size of Earth, but no one has been able to find it — yet. Now, with the launch of the Vera C. Rubin Observatory in Chile next year, the elusive Planet Nine may finally be on the verge of discovery.

A Telescope That Could End the Mystery

The search for Planet Nine — sometimes called Planet X — has been one of the most compelling mysteries in modern astronomy. First proposed by Caltech astronomers Mike Brown and Konstantin Batygin in 2016, the planet is believed to explain the strange, elongated orbits of distant objects beyond Neptune. These trans-Neptunian objects (TNOs) move in highly unusual patterns, suggesting that something massive is tugging on them — a giant planet that we’ve never seen.

Well, if it’s so large, why haven’t we found this Planet X yet then? Good question. Using the Oort Cloud as an approximate boundary and considering it extends somewhere between 1,000 Astronomical Units (AU) and 100,000 AU (or about 9.3 trillion miles) would mean that our solar system is two to three light-years wide.

Spotting a planet that far out is no easy task. Planet Nine, if it exists, is thought to be around 500 times farther from the Sun than Earth. So, it would be incredibly faint. Even the best telescopes have struggled to detect it. But the Vera C. Rubin Observatory is different.

Set to begin operations in late 2025, this groundbreaking telescope will be equipped with an 8.4-meter mirror and the world’s largest digital camera, capable of capturing the faint light reflected from objects deep in the solar system. According to Brown, the Vera C. Rubin Observatory will be able to peer farther into space than any ground-based telescope before it, giving astronomers their best chance yet to locate Planet Nine. “It’s very possible that Planet Nine will be found within the first year of the observatory,” Brown said.

Why Planet Nine Matters

Finding Planet Nine would mark one of the most significant astronomical discoveries of the 21st century. Beyond simply identifying a new planet, it would rewrite much of what we know about the solar system’s history and evolution.

Astronomers believe that Planet Nine could help explain how the outer reaches of our solar system took shape and why some of its most distant objects behave so strangely. It could also shed light on the formation of similar giant planets orbiting other stars in the galaxy.

“It is plausible that a primordial planetary body could survive in the distant Kuiper Belt as a Kuiper Belt planet (KBP), as many such bodies existed in the early solar system,” said Patryk Sofia Lykawka of Kindai University and Takashi Ito of the National Astronomical Observatory of Japan.

But despite years of searching, this mysterious planet has eluded astronomers. Still, indirect evidence for its existence continues to build. Brown and his team have discovered more trans-Neptunian objects (TNOs)—small icy bodies that orbit far beyond Neptune—with orbits that seem to suggest the presence of a large, unseen gravitational force. In a recent study, Brown calculated that the odds of these orbits being random were just one in a million.

“I don’t see how we can have a Solar System without Planet Nine,” Professor Brown says.

The Search Narrows

Over the years, Brown and his colleagues have combed through data from multiple telescopes, including Hawaii’s Pan-STARRS, ruling out large portions of the sky where the planet could be hiding. The search is now focused on just 22% of Planet Nine’s potential orbital pathway. If it’s there, it’s hiding in one of the hardest-to-reach corners of the solar system.

But there are also skeptics. Surveys like the Outer Solar System Origins Survey (OSSOS) and the Dark Energy Survey (DES) have tracked hundreds of Kuiper Belt objects (KBOs), many on extremely elliptical orbits. While early discoveries hinted at clustering, these more recent surveys have found no such pattern.

Critics also point out observational biases — the limitations in how, when, and where we can detect these distant objects. For instance, KBOs are much easier to spot when they are closer to the sun and brighter, which means surveys might miss those farther away. When factoring in these biases, the clustering effect that sparked the Planet Nine hypothesis starts to fade.

So, if Planet Nine isn’t there, what explains the unusual orbits of some KBOs? Astronomers point to other possible causes, such as the migration of Neptune, which could have scattered these small bodies into their current positions. Others propose even wilder ideas: could a small black hole or even a flaw in our understanding of gravity be causing the strange orbits?

Still, most researchers agree that Planet Nine, if it exists, holds the potential to unlock many secrets about our cosmic neighborhood. Its discovery would likely spark a new era of exploration, motivating space agencies like NASA to send probes to study this distant world. With the new power of the Vera C. Rubin Observatory, many scientists are optimistic that this will soon happen.