The Hunt for “Planet X”: Is a Secret “Super-Earth” Lurking in Our Solar System?

Micah Hanks·

Planet X

Welcome to this week’s installment of The Intelligence Brief… Recently, a longstanding astronomical controversy has seen renewed debate, as researchers have presented what they believe to be new evidence for the existence of a secret “super-Earth” hidden somewhere in our distant solar system. In our analysis, we’ll be looking at 1) how anomalies in our solar system have long suggested the presence of a large, hidden object somewhere out there, 2) a look at the enduring quest for “Planet X”, 3) how new clues—and a potential candidate—have recently been unearthed from old data, and 4) what the implications of such a discovery, if proven, would be.

Quote of the Week

“We don’t have definitive proof yet that there’s a planetary-mass body out there, but something funny is going on that we don’t understand.”

– Nathan Kaib, Carnegie Institution Planet Formation Theorist

RECENT NEWS from The Debrief

Oddities in Our Planetary Neighborhood

For decades, astronomers have been aware that something just isn’t quite right about our solar system.

Over the years, detections of anomalies in the orbits of distant dwarf planets and icy objects in the Kuiper Belt and evidence of odd “clustering” some of these space objects exhibit, as well as high inclinations of distant populations of trans-Neptunian bodies, and even objects between giant planets that orbit the Sun, but in a retrograde direction, have all suggested that something is fundamentally incomplete about our understanding of our planetary neighborhood.

Such orbital oddities have caused many astronomers to consider whether a ninth planet—a hypothetical celestial body that remains invisible to astronomers, but the existence of which can be inferred from its influence on other objects—might be lurking somewhere in the farthest reaches of our solar system. But if so, how could it have remained hidden for so long, and what would its existence, if proven, actually mean?

The Quest For “Planet X”

The idea that a distant, hidden planet might exist in our solar system has been around for a while. In the early days of 20th-century astronomy, icons such as Percival Lowell theorized whether a hidden body, often referred to as “Planet X,” could be inferred from irregularities in Uranus’s orbit.

Eventually, these theories were validated with the discovery of Pluto in 1930, which secured its place as the solar system’s ninth planet until 2006, when the International Astronomical Union finally concluded it no longer met the criteria for being a planet (primarily because it does not clear its orbit of other objects) and downgraded it to a dwarf planet.

In the wake of Pluto’s reclassification, in 2016, Caltech astronomers Konstantin Batygin and Mike Brown proposed that an unknown Planet Nine might still exist in an orbit much further beyond Pluto in the solar system’s outermost region. The idea they proposed was that a hidden planet roughly the size of Neptune, and between 5 and 10 times the mass of Earth, might be orbiting the Sun on a highly elongated path that could be 30 times more distant than Neptune, meaning it would take up to 20,000 years to complete just one orbit.

While the idea remains possible based on extreme gravitational anomalies observed in trans-Neptunian objects (ETNOs), the question of Planet Nine’s existence remains theoretical. However, a potential new clue in the decades-long search for a hidden planet somewhere out there beyond Neptune has now emerged, and in a somewhat surprising place: within old satellite data that offers fresh momentum to the longstanding investigation into whether a giant planet has secretly been lurking in our solar system.

A Giant in the Shadows: New Clues from Old Data

In a recent study by Terry Phan, a doctoral student at Taiwan’s National Tsing Hua University, Phan and his colleagues probed archival data from the decommissioned IRAS and AKARI infrared satellites in search of slow-moving distant objects that might be ideal candidates for the mysterious, hypothetical ninth planet.

After filtering out known celestial bodies, Phan and his colleagues were indeed able to locate one good candidate: a faint dot that appeared consistently across both of their datasets and displayed promising characteristics, including moving in a way suggestive of an extremely distant planetary orbit.

“It’s motivated us a lot,” Phan said of the discovery, which was published as a preprint on April 24 and accepted for publication in Publications of the Astronomical Society of Australia (Phan and his colleague’s paper can be found here).

Scientific Skepticism and the Case for Caution

However, not everyone is as excited by Phan’s discovery, including even a few of the earliest proponents of the modern hunt for a ninth planet. Mike Brown, co-creator of the Planet Nine hypothesis, also examined the estimated orbit of the object Phan and his colleagues discovered, reporting what he believes are major discrepancies regarding the newly reported findings.

Specifically, the new candidate seems to exhibit an orbital tilt of 120 degrees, which Brown and others have suggested is too far beyond Planet Nine’s predicted tilt of 15–20 degrees. Adding to the doubt among astronomers, Brown said that Phan and his team’s candidate also tilts in the opposite direction from the solar system’s primary planetary plane, all of which would lessen the likelihood that such an object—planet or otherwise—could account for the anomalies present in our solar system that prompted the search for a hypothetical “Planet X” in the first place.

Intriguingly, these discrepancies don’t necessarily rule out the possibility that the newly discovered candidate might actually be an undiscovered planet. However, if this were the case, its peculiarities seem to suggest that it is not the mystery “Planet Nine,” but instead an entirely separate and unexpected celestial body that, if anything, might disprove original ideas about orbital instabilities in the outer solar system.

The Search Continues

While debate continues, the search for additional planets in our solar system remains underway. Ground-based observatories like Hawaii’s Keck and Subaru telescopes, along with citizen science initiatives like Backyard Worlds: Planet 9, continue to scour our solar system for additional candidates. Arguably. The Vera C. Rubin Observatory, which began initial on-sky operations late last year and will achieve system first light in July, may offer the best chance yet to confirm or refute Planet Nine’s existence. Equipped with the world’s largest digital camera, the observatory will conduct a decade-long survey of the Southern Hemisphere that could identify many more Kuiper Belt objects with suspiciously aligned orbits, if they exist.

Whether or not such observations reveal the long-suspected existence of a hidden planet in our solar system, or simply raise new questions about the existing anomalies in our solar system, remains to be seen. However, if eventually proven, the existence of Planet Nine may help to bring our solar system closer in line with more distant observations of exoplanets, which have revealed that the presence of “super Earths” is actually fairly common in other planetary systems.

For now, Planet Nine remains an intriguing possibility for astronomers, albeit one that holds a steady orbit in the realm of scientific theory—at least until its existence is proven, perhaps with the help of tomorrow’s advanced space observatories.

That concludes this week’s installment of The Intelligence Brief. You can read past editions of our newsletter at our website, or if you found this installment online, don’t forget to subscribe and get future email editions from us here. Also, if you have a tip or other information you’d like to send along directly to me, you can email me at micah [@] thedebrief [dot] org, or reach me on X: @MicahHanks.

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