For thousands of years, gazing upward was how astronomers studied the sky above. On a clear night, several planets in our solar system were visible without a telescope. However, as telescopes increased in sophistication, astronomers discovered more planets, confirming the eight in our solar system. As technology continues to progress, though, could we uncover a new one?

Astrophysicists don’t know if there are undiscovered planets in our solar system, but new technologies are enabling researchers to get closer to an answer. 

What Is Planet X?

Ancient people and early astronomers long knew about five planets: Jupiter, Mars, Mercury, Saturn, and Venus, though these planets seemed like stars at first. In 1781, a British astronomer identified Uranus as also being a planet and not a star.

In the coming decades, astronomers worked with mathematical equations that suggested there was a planet beyond Uranus. Only in the mid-1800s was a telescope powerful enough to put the planet in scientists’ sights.

“Neptune was discovered in 1846, and basically the speculation of a possible planet beyond Neptune began just a few years after that,” says Amir Siraj, an astrophysicist with Princeton University. 

Read More: The Mysterious World of Uranus, the Ice Giant

Renewed Interest in Planet X

The search for Planet X eventually led to the discovery of Pluto in 1930. “At the time, astronomers thought, this planet we’ve been talking about, it’s a giant planet beyond Neptune,” Siraj says.

As technology improved, scientists were better able to calculate Pluto’s mass, which prompted some astrophysicists to argue Pluto was not Planet X. Their argument was furthered when the  International Astronomical Union downgraded Pluto in 2006 to dwarf-planet status.  

“In the past 10 years, in particular, there has been a reignition in this search,” Siraj says.

One of the primary reasons some astrophysicists propose there is a planet beyond Neptune has to do with odd orbits.

“Orbits of the most distant small bodies — comets or asteroids — seem to be clustered on one half or one side of the solar system,” Siraj says. “That’s very weird and something that can’t be explained by our current understanding of the solar system.”

A 2014 study in Nature first noted these orbits. A 2021 study in The Astronomical Journal examined the clustering in the orbit and concluded that “Planet Nine” was likely closer and brighter than expected.

Read More: Planets Have a Gravitational Dance With Aligning Orbits

Searching for Planet Nine

Astrophysicists don’t agree whether the clustering in the orbit is a real effect. Some have argued it is biased because the view that scientists currently have is limited, Siraj says.

“This debate for the last decade has a lot of scientists confused, including myself. I decided to look at the problem from scratch,” he says.

In a 2024 paper, Siraj and his co-authors ran simulations of the solar system, including an extra planet beyond Neptune.

“We did it 300 times, about 2.5 times more than what was done previously,” Siraj says. “In each simulation, you try different parameters for the extra planet. A different mass, a different tilt, a different shape of the orbit. You run these for millions of years, and then you compare the distribution to what we see in our solar system.”

Siraj and his co-authors were able to compare 51 objects in the solar system. A previous work had only been able to compare 11. They found that the perimeters for this possible planet were different than what has been previously discussed in the scientific literature, and they supported the possibility of an unseen planet beyond Neptune.

Scientists hope a new telescope will have the potential to see deeper into the solar system. In 2025, the Vera C. Rubin Observatory on Cerro Pachón — a mountain in Chile, is expected to go online. The observatory boasts that in the time it takes a person to open up their phone and pose for a selfie, their new telescope will be able to snap an image of 100,000 galaxies, many of which have never been seen by scientists.

The telescope will have the largest digital camera ever built, the LSST. Siraj says he expects it will take “the deepest, all-sky survey that humanity has ever conducted.” 

Read More: If Planet Nine Is a Tiny Black Hole, This Is How to Find It

What the Future Holds for Planetary Science

So, what might the Rubin Observatory find past Neptune? Based on the current literature, Siraj sees a few possibilities. One is that the Rubin Observatory, with its increased capabilities, might be able to see a planet beyond Neptune. 

Another possibility is the telescope doesn’t identify a ninth planet but finds more distant objects than what was previously known. Siraj says there is also a possibility a planet isn’t found, and the lopsided objects are not confirmed. Or a planet isn’t found, but the clustering is established. The last scenario, Siraj says, would be “puzzling” to astrophysicists and may even suggest the objects are coming from beyond the Kuiper Belt. 

Either way, Siraj says the Rubin could answer a lot of questions while posing many new ones.

“Next year is going to be an enormous year for solar system science,” he says.

Read More: What Does the Future of Astronomy Hold? We’ll Find Out Soon

Article Sources

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Emilie Lucchesi has written for some of the country’s largest newspapers, including The New York Times, Chicago Tribune and Los Angeles Times. She holds a bachelor’s degree in journalism from the University of Missouri and an MA from DePaul University. She also holds a Ph.D. in communication from the University of Illinois-Chicago with an emphasis on media framing, message construction and stigma communication. Emilie has authored three nonfiction books. Her third, A Light in the Dark: Surviving More Than Ted Bundy, releases October 3, 2023, from Chicago Review Press and is co-authored with survivor Kathy Kleiner Rubin.