The Ninth Planet of the Solar System: Are We Close to Unraveling Its Mystery?

Photo: Caltech/R Hurt (IPAC)

Since 2025, the Vera Rubin Observatory in the mountains of northern Chile has been working to radically change our understanding of the Universe. Among other things, scientists hope to finally explain some strange phenomena in the vicinity of our Solar System.

Since 2016, the question of Planet Nine has been a topic of great interest and disagreement among scientists. It was in that year that astronomers Konstantin Batygin and Michael Brown from the California Institute of Technology (Caltech) published an article presenting evidence that there is a planet in the outer reaches of the Solar System with a mass approximately 10 times that of Earth.

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The misalignment of the orbits of six trans-Neptunian objects led astronomers Batygin and Brown to hypothesize the existence of Planet Nine in 2016.

According to scientists, only the presence of a massive celestial body can explain the behavior of a group of six distant Trans-Neptunian Objects (TNOs) orbiting the Sun in the Kuiper Belt (their average distance to the star is greater than Neptune's). The orbits of TNOs are unusually tilted and elongated, suggesting they may be influenced by the gravitational pull of a larger "neighbor".

"If Planet Nine doesn't exist, then we have no explanation for many strange phenomena," Professor Brown told the BBC.

If you don't follow astronomy, you might not grasp the irony: Brown is one of the main proponents of the mysterious new planet hypothesis, but his work was key to stripping Pluto of its ninth-planet status 20 years ago.

Pluto was discovered in 1930 and until the early 21st century was considered the smallest and most distant planet in our Solar System.

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Pluto lost its planetary status but remains the most famous trans-Neptunian object. Photo: NASA

In 2005, Brown, along with two colleagues, discovered Eris—an object similar in size to Pluto, orbiting the Sun beyond Neptune's orbit. The discovery of Eris significantly influenced the International Astronomical Union's (IAU) decision to redefine a planet in 2006 and strip Pluto of its status, "demoting" it—along with Eris—to a dwarf planet.

Strange, Dim, and Distant

The catch with the idea of another Planet Nine is that no one (officially, at least) has seen it. Batygin and Brown, for example, used computer models for their article. This is partly because Planet Nine—if it exists—is very far from us.

According to estimates by Caltech astronomers, its average distance from the Sun is 20 times the average distance from Neptune to our star. This means that one orbit around the Sun for Planet Nine could take up to 20,000 Earth years.

An object at such a distance from the Sun reflects very little light, making it extremely dim. What complicates the situation further is that Planet Nine's orbit, according to astronomers' calculations, is very unusual. While the eight known planets of our Solar System orbit in nearly circular paths with minimal tilt, Planet Nine's orbit is likely very elongated and inclined.

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The eight planets we know orbit the Sun in nearly circular paths with minimal tilt.

But perhaps we will soon be able to see Planet Nine. Powerful telescopes of the past, such as the James Webb, were designed to observe specific objects in deep space. But the Vera Rubin Observatory scans the entire sky in the Southern Hemisphere every few nights.

The observatory, with all its technologies (including the world's largest digital camera), is expected to catalog billions of cosmic objects over 10 years, including more than 40,000 new TNOs.

"Rubin can find many objects in space that are much dimmer and much more distant than we could ever see before," says astronomer Sarah Greenstreet, who works at the observatory.

"If Planet Nine exists where it's hypothesized to be, and its size matches the hypothesis… Rubin will find it," the researcher asserts.

History Repeating with Neptune?

Brown also believes that the Rubin Observatory "will either find Planet Nine itself, or it will find irrefutable confirmation—or refutation—of its existence." If it exists, the scientist believes the planet could be discovered within the next year or two. This would be a huge breakthrough.

"Planet Nine could be the fifth largest in our Solar System and the first discovered in 180 years!" says Brown. The astronomer is referring to the official discovery of Neptune in 1846.

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The ice giant Neptune is the most distant known planet in our Solar System. Neptune orbits the Sun at a distance approximately 30 times that of Earth from the Sun. Photo: Corbis via Getty Images

Neptune's existence was predicted when astronomers noticed anomalies in the orbit of a neighboring planet—Uranus. German scientist Johann Gottfried Galle then used calculations from colleagues to locate Neptune in the sky. Later, however, it turned out that Galileo Galilei had observed Neptune in the sky as early as 1612, but he did not identify the object as a planet: Neptune's motion relative to the stars was very slow and imperceptible with the telescopes of that era.

Could history repeat itself with Planet Nine? Astrophysicist Malena Rice of Yale University suspects it very well might.

"I'm not at all convinced that Planet Nine isn't in the data we've already collected. We just need to study it very carefully," she says.

The Most Typical Size in Other Systems

This is precisely what a group of scientists from Taiwan, Japan, and Australia may have achieved last April. They analyzed sky surveys from two infrared telescopes, taken in 1983 and 2006, and found a pair of identical faint points that could indicate the motion of an unknown planet.

Some astronomers were skeptical of their findings. The scientists themselves are very cautious in their conclusions.

"It's too early to say that we have found Planet Nine," admits Terry Phan from Taiwan's National Tsing Hua University.

Instead, he speaks of detecting a "potential candidate" for the role of Planet Nine.

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The Vera Rubin Observatory in Chile, scientists hope, will put debates about Planet Nine in the past. Photo: Anadolu via Getty Image

For astronomers like Rice, confirmation of Planet Nine's existence would not be a shock. The hypothetical planet, according to scientists' calculations, is larger than Earth but smaller than Neptune. Rice says this is the most typical size of planets in other systems.

"We see planets of this kind around about half of stars, but they are absent in the Solar System," she explains.

If Not a Planet, Then What?

Opponents of the Planet Nine hypothesis use various arguments: from observational errors in Batygin and Brown's work to past examples of mysterious planets. In the early 20th century, scientists theorized that a Planet X influenced Uranus's orbit, but this theory was later disproven.

Skepticism is further fueled by the 2023 discovery of Amanita (2023 KQ₁₄) — a TNO whose orbit does not align with the six objects analyzed by Batygin and Brown.

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Sedna, discovered in 2003, is one of the TNOs whose unusual orbit led scientists to hypothesize about Planet Nine. Photo: NASA, ESA and M Brown (Caltech)

A team of astrophysicists from the Jülich Research Centre in Germany proposed an alternative hypothesis in 2025 based on computer simulations. The simulations suggest that billions of years ago, a close flyby of a massive star could have caused gravitational chaos that altered the orbits of TNOs.

"I wouldn't say there can't be a Planet Nine," says Susanne Pfalzner, who led the research. "But the probability is small."

According to Greenstreet from the Vera Rubin Observatory, in recent years, there has been less and less evidence for the existence of another planet. But even if the observatory's work doesn't yield evidence for Planet Nine, the researcher remains optimistic.

"There's a huge region in the outer Solar System that is still almost unexplored... Who knows what awaits us there," she says.

"With every question answered, new questions arise before us."

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