For a decade, scientists have been locked in a heated debate about the existence of 'Planet 9'. Some argue that the evidence is mounting in its favor.

Our solar system once boasted nine planets. Astronomer Mike Brown, dubbed ‘the man who demoted Pluto,’ revealed that his discovery led to years of hate mail and late-night calls from upset fans of the former ninth planet.

Brown, a planetary astronomy professor at Caltech, uncovered a distant object called Eris in the Kuiper Belt — a vast, icy region beyond Neptune. This discovery, made in 2005, sparked the chain of events that led to Pluto’s controversial reclassification as a dwarf planet the following year.

Yet now, while the Kuiper Belt took away Pluto's planetary status, Brown and other researchers believe it might just give us a new planet — potentially 'Planet 9.'

The Kuiper Belt, thought to be composed of remnants from the solar system's formation, stretches 50 times farther from the Sun than Earth. A secondary region extends even farther, nearly 20 times the distance. Pluto, now considered a dwarf planet along with Eris, is one of the largest objects in the belt. It doesn't clear its orbit of other objects, which is why it no longer holds the same planetary status as the other eight planets, according to the International Astronomical Union's guidelines.

Objects in the distant Kuiper Belt are hard to detect due to their great distance from the Sun. For over a decade, astronomers have been hunting for a hidden planet in this region. While it has never been seen, its existence is suggested by the unusual orbits of nearby objects. It’s often referred to as Planet X or Planet Nine.

“Finding a new planet would be a monumental discovery,” said Malena Rice, an assistant professor of astronomy at Yale University. “It could radically change our understanding of our solar system, other planetary systems, and our place in the universe. It’s incredibly exciting — the potential to learn so much about the cosmos is enormous.”

This search has sparked considerable debate, with conflicting theories about the nature of the potential planet. Some scientists even question whether it exists at all.

“There are certainly strong skeptics when it comes to the idea of Planet Nine — it’s a hotly debated subject,” said Rice. “Some people are firm believers in its existence, while others passionately argue that it’s a myth. The debate over its nature and whether it even exists is intense, but that’s what makes it such an intriguing topic — after all, people wouldn’t be so animated about it if it weren’t interesting.”

The debate may soon reach a conclusion, with a new telescope scheduled to begin surveying the entire sky every few nights in late 2025. In the meantime, a team of scientists believes they’ve gathered the strongest evidence yet that the elusive planet exists.

A 'smoking gun'

Although the search for Planet Nine has only gained momentum in recent years, the debate over its possible existence stretches back more than 175 years.

“Ever since Neptune was discovered in 1846, at least 30 astronomers have proposed the existence of various types of planets beyond Neptune, but all have been proven wrong,” said Konstantin Batygin, a planetary science professor at Caltech and Brown’s collaborator. In astronomy, any object orbiting the Sun beyond Neptune is considered 'trans-Neptunian.'

“I never imagined I’d be discussing evidence for a trans-Neptunian planet, but unlike all the previous claims, I’m convinced that this time, we’re actually onto something,” Batygin added.

Batygin and Brown are among the leading advocates for the existence of Planet Nine. Together, they’ve been researching its potential existence since 2014, inspired by a groundbreaking study conducted by astronomers Scott Sheppard from the Carnegie Institution for Science, and Chadwick Trujillo from Northern Arizona University.

Sheppard and Trujillo were the first to notice that the orbits of several known trans-Neptunian objects appeared to be unusually grouped together. They suggested that an unseen planet, much larger than Earth and located over 200 times farther from the Sun than our planet, could be influencing the movement of these objects.

“The most compelling visual evidence remains the earliest: the fact that the farthest objects beyond Neptune all have orbits that point in the same direction,” Brown wrote in an email.

Since then, Batygin has coauthored multiple studies on Planet Nine, each presenting new evidence supporting its existence. He believes the strongest case is made in his most recent paper, coauthored with Brown and two other researchers, which was published in April in *The Astrophysical Journal Letters*.

The study tracks icy bodies that are being perturbed, pushing them into Neptune’s orbit before they ultimately escape the solar system. “These objects have very short lifespans compared to the age of the solar system,” Batygin explained. “That means something must be influencing them. So what could it be?”

One possibility is the galactic tide, a gravitational pull exerted by distant stars in the Milky Way. However, Batygin and his team ran simulations comparing this scenario to the idea of Planet Nine’s presence. They concluded that the absence of the hidden planet is “strongly contradicted by the data.”

“It’s a remarkable piece of evidence — a real smoking gun. And looking back, it seems so obvious that I feel a bit embarrassed it took us nearly a decade to figure it out. But, better late than never,” Batygin admitted.

According to Batygin, Planet Nine is a 'super-Earth' type of planet, roughly five to seven times the mass of Earth, with an orbital period ranging between 10,000 and 20,000 years. “What I can’t determine through simulations is its exact location in its orbit or its composition,” Batygin explained. “The simplest explanation is that it’s a smaller version of Uranus or Neptune, likely a remnant of the building blocks that formed those planets.”

The super-Earth theory is among the most widely supported explanations for Planet Nine’s existence, although alternative theories also provide intriguing possibilities.

Super-Pluto? Competing Theories from the Kuiper Belt

A study published in August 2023 suggests that the hidden planet could actually be much smaller, with a mass between 1.5 and 3 times that of Earth. “It might be an icy, rocky version of Earth, or even a super-Pluto,” said Patryk Sofia Lykawka, an associate professor of planetary sciences at Kindai University in Japan and coauthor of the study.

“Due to its significant mass, it could generate a lot of internal energy, potentially supporting subsurface oceans. Its orbit would be far beyond Neptune, and much more inclined compared to the other known planets — even more so than Pluto’s orbit, which is inclined by about 17 degrees,” Lykawka explained. (In astronomy, a planet’s orbit is considered inclined when it differs from the plane of Earth’s orbit.)

The existence of Planet Nine is inferred from computational models designed to explain the unusual behaviors of trans-Neptunian objects, which share similarities with Batygin’s research. However, Lykawka noted that his model does not focus on the same orbital alignments and differs significantly from Batygin’s approach. For this reason, he avoids calling the object Planet Nine, referring to it instead as the 'Kuiper Belt planet' to clarify that these are different hypothetical objects,” he explained.

Alternative theories suggest that the anomalies being investigated could be caused by something entirely different, such as a primordial black hole — one formed shortly after the Big Bang — that was captured by our solar system as it traveled through the galaxy. Another theory posits that there may be flaws in our current understanding of gravity.

However, according to Rice from Yale University, these theories would be challenging to test. “There are many competing ideas, but I tend to favor Occam’s razor when deciding what to focus on,” she said, referring to the philosophical principle that the simplest explanation is usually the correct one. “Given that we already know there are eight planets, it’s not unreasonable to consider the possibility of another planet in our solar system.”

The most promising approach moving forward, she suggested, is to find more trans-Neptunian objects like those Batygin is using to support his hypothesis, and to demonstrate that the clustering of their orbits is statistically significant.

The Call for More Evidence

Some scientists argue that there are currently too few observed trans-Neptunian objects to draw any reliable conclusions about their orbits.

“We have approximately a dozen of these objects,” said Renu Malhotra, regents professor of planetary sciences at the University of Arizona. “However, we only observe the brightest ones, and even then, we only track a tiny fraction, primarily when they’re at their closest approach to the Sun.”

Malhotra points out that the data is hampered by observational bias, which is why many researchers remain skeptical about it. Sheppard from the Carnegie Institution for Science, a coauthor of the 2014 study that sparked Batygin’s research, is among the skeptics.

“By now, we had hoped to discover many more of these distant trans-Neptunian objects,” Sheppard wrote in an email. “If we had several dozen, we could definitively determine whether they are genuinely clustered in space. Unfortunately, we’re still dealing with a small sample size, as these objects are rarer than initially thought. While it’s still possible that a super-Earth planet exists in the distant solar system, we can’t say so with much certainty at this point.”

The debate can become quite intense, according to Malhotra. “Scientists, like anyone else, have different personalities. Some are more forceful with their findings, while others take a more cautious approach,” she said. “There’s a perception that the idea of a Neptune-sized Planet Nine is being pushed more forcefully than the available data would justify.”

Malhotra coauthored a paper in August 2017 proposing the existence of a Mars-sized planet in the Kuiper Belt, but she’s not entirely dismissing the Planet Nine hypothesis either.

“It’s still uncertain. It’s right at the edge of what the data can support statistically,” she explained. “But there’s nothing in the physics or observations that rules out the idea of large planets existing tens of times farther from the Sun than Neptune.”

Directly observing the planet would settle the debate once and for all, but every attempt so far has failed to find it.

Batygin coauthored a study in March that utilized data from the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) in Hawaii, enabling researchers to scan 78% of the sky where Planet Nine might be located — but the planet was nowhere to be found.

“It’s been a tough challenge,” he said, reflecting on the difficulty of working with limited telescope time and contending with equipment failures and unfavorable weather conditions.

Finding such a distant object without knowing exactly where to look is incredibly challenging, akin to using a sniper rifle to locate a target rather than binoculars, according to Batygin.

“The sky is overwhelmingly vast when you’re searching for something so faint,” he explained. “This object is roughly 100 million times dimmer than Neptune — that’s pushing the capabilities of the world’s largest telescopes to their limits.”

Other searches, like one conducted for a December 2021 study using the Atacama Cosmology Telescope in Chile, have also failed to locate the planet. “I had to test tens of thousands of possible orbits. In the end, I found nothing,” said Sigurd Naess, the study's lead author and a researcher at the University of Oslo’s Institute of Theoretical Astrophysics.

Naess noted that the instrument’s sensitivity was strong enough to detect a planet at distances between 300 and 600 times farther than Earth is from the Sun.

“That’s sufficient to provide some insight, but it’s not enough to rule out the existence of Planet Nine entirely,” Naess explained in an email.

A possible ‘new chapter’

Despite the ongoing debate and differing opinions, all researchers are united on one front: a new wide-field telescope currently under development could soon resolve the issue. Once operational in late 2025, the Vera C. Rubin Observatory, equipped with the world’s largest digital camera, will begin scientific observations from its perch atop an 8,800-foot mountain in northern Chile. This could bring clarity to the Planet Nine question once and for all.

Batygin believes the new telescope represents a significant leap forward. “This next-generation telescope will scan the entire observable sky every few days,” he said. “It could directly detect Planet Nine, which would be an incredible outcome and bring the search to a satisfying conclusion. Even if it doesn’t find the planet, it will discover many more Kuiper Belt objects. Regardless, it will validate the Planet Nine theory by testing all the statistical patterns we’ve observed through an independent survey.”

Rice shares Batygin’s optimism about the potential of the Rubin Observatory to settle the debate, particularly in terms of clarifying the statistical significance of the alignment of distant trans-Neptunian objects—one of the key pieces of evidence supporting the Planet Nine hypothesis.

Should the Rubin telescope locate a super-Earth, Rice notes that it would be a remarkable discovery. These planets, ranging between the size of Earth and Neptune, are common among exoplanets but have yet to be found within our solar system.

“It’s strange that we don’t have one in our own system, especially considering how numerous they are around other stars. It would be amazing to study one up close, as exoplanets are so far away that we can’t get a true sense of what they’re physically like,” Rice added.

Discovering a smaller planet would also generate excitement, Rice pointed out, because each new planet within our solar system helps scientists better understand the vast number of exoplanets being discovered across the galaxy.

But what if the search yields no results? Even in that case, knowing the exact number of planets in our solar system would still be a valuable discovery, Rice said. “It’s humbling to realize that we don’t even know the full count of planets right here in our own system.”

This highlights how even fundamental facts, the kind we grew up learning in school, can evolve as our understanding of the universe deepens.

“And that’s actually a beautiful thing,” she added. “Knowledge is always progressing — sometimes it’s major breakthroughs, sometimes it’s just a thoughtful back-and-forth. It’s a perfect illustration of how science works.”