For centuries, the Solar System has revealed its secrets gradually, yielding new planets, moons, and asteroids to the patient observer and the diligent theorist. Yet even in the 21st century, with robotic eyes combing the heavens and telescopes probing distant stars, our own cosmic backyard may still harbour a giant secret. Planet Nine – a hypothesised world lurking in the frozen outer reaches of the Solar System – has captivated astronomers, stirred imaginations, and challenged our understanding of planetary formation and dynamics. With recent developments, including a tantalising re-examination of IRAS data and the imminent activation of the Vera C. Rubin Observatory, the hunt is heating up.
A Brief History of the Hunt
The story of unseen planetary influences is not a new one. In the 19th century, discrepancies in the orbit of Uranus led to the discovery of Neptune in 1846. This success inspired a belief that perturbations in known planetary orbits could signal the presence of other hidden worlds. After Pluto’s discovery in 1930, astronomers continued to speculate about additional planets, particularly to explain unusual orbital behaviours at the fringes of the Solar System.
However, it was not until the 2010s that Planet Nine – or “Planet X”, as it was often called in prior decades, re-entered serious scientific discussion. In 2016, Caltech astronomers Konstantin Batygin and Mike Brown published a paper suggesting that a group of distant Kuiper Belt Objects (KBOs) shared orbital features that could best be explained by the gravitational influence of an as-yet-undiscovered planet, roughly 5 to 10 times the mass of Earth, orbiting the Sun at a distance of 400 to 800 AU (astronomical units).
Their modelling showed that such a planet could shepherd distant objects into aligned orbits through resonant interactions, causing the clustering observed. The idea gained traction because the odds of such alignments occurring by chance were calculated to be very low, less than 0.1%. Since then, evidence has slowly accumulated, not of the planet itself, but of the celestial fingerprints it may have left on the outer Solar System.
The IRAS Data Surprise
The Infrared Astronomical Satellite (IRAS), launched in 1983, performed the first all-sky survey in the infrared spectrum. While its resolution and sensitivity were crude by modern standards, IRAS amassed a wealth of data that astronomers are still mining decades later.
In early 2025, a re-analysis of IRAS archival data made headlines in the astronomy community. A team at the University of Arizona reported that a previously unidentified infrared source, observed fleetingly in 1983, exhibited motion consistent with a trans-Neptunian object in the 400 – 600 AU (29.92 x 109 km – 89.76 x 109 km) range. While the data are not conclusive, the object was only captured in two frames, and its motion could potentially be attributed to other causes, it has reignited hopes that Planet Nine might finally be within reach.
Further infrared missions, such as WISE and its NEOWISE extension, have also combed the sky, but Planet Nine’s slow movement and faint thermal signature make detection extremely difficult. It is too far and too cold to be visible in most optical wavelengths, and it would appear to move only a few arcseconds per year across the sky, easily mistaken for background noise or missed entirely.
Enter the Vera Rubin Observatory
The Vera C. Rubin Observatory, located in Chile, represents a generational leap in astronomical capability. Equipped with a powerful 8.4-metre mirror and a staggering 3.2-gigapixel camera, it is poised to begin its 10 year-long Legacy Survey of Space and Time (LSST). Designed to survey the entire visible sky every few nights, the observatory will generate an unprecedented time-lapse movie of the universe – and huge volumes of data to be searched through for new discoveries.
For Planet Nine hunters, Rubin’s wide field of view and rapid cadence make it the best chance yet to directly detect the elusive world. Its optical sensitivity will allow it to pick up the faint reflected light of distant objects down to about 24th magnitude, potentially sufficient to see Planet Nine, depending on its size, albedo, and exact location.
Unlike earlier efforts that focused on targeted searches of suspected sky regions, the Rubin Observatory’s approach is broader and more systematic. Its ability to detect slow moving objects will be key, as it can distinguish true planetary candidates from background stars and galaxies by analysing their motion across successive images.
The observatory is expected to begin full operations in late 2025. Should Planet Nine be within its detection parameters, the LSST could capture its movement over time, enabling triangulation of its orbit and mass, potentially confirming or refuting decades of speculation.
The Impact of Discovery
Should Planet Nine be found, it would be a transformative moment in planetary science. Not only would it expand the Solar System’s known architecture, but it would also force a re-evaluation of planetary formation theories. A planet of this size so far from the Sun would raise several pressing questions.
Did it form in situ, amid the icy detritus of the early Solar System? If so, it would imply that the protoplanetary disc was much more massive and extended than previously thought. Alternatively, could Planet Nine be a rogue planet, formed closer to the Sun and later ejected outward by gravitational interactions with Jupiter or Saturn? Or was it perhaps a captured object, snagged by the Sun’s gravity from another star system during the chaotic days of the Sun’s birth cluster?
Each scenario carries profound implications for our understanding of planetary systems. If Planet Nine formed near its current orbit, then our models of planetary accretion need to accommodate gas or ice giants forming in extremely distant and sparse regions. If captured, it would be a concrete example of interstellar planetary exchange, hinting that such phenomena could be common across the galaxy.
Are We Alone Out There?
Perhaps the most thrilling question raised by the Planet Nine hypothesis is whether it is truly alone. Could the outer Solar System harbour not one, but several large planets?
Some astronomers speculate that the gravitational oddities observed in extreme KBOs could require more than a single perturber. There may be smaller planets, Mars sized or super Earths, hiding in the dark, uncharted territories beyond Neptune. The detection of one planet could open the floodgates, leading to a cascade of discoveries as new surveys and targeted searches are launched, or the Vera C Rubin Observatory could find these planets in its data sets.
Furthermore, finding Planet Nine might help us resolve other long-standing mysteries. For example, the unusual tilt of the Sun’s rotational axis relative to the plane of the planets has puzzled scientists for decades. A distant, inclined massive planet could exert enough torque over billions of years to produce this effect. Similarly, some of the unusual orbital properties of comets and distant centaurs might also find explanations in a multi-planet outer Solar System.
Even the hypothetical Oort Cloud, that spherical shell of icy objects thought to encircle the Solar System, could be influenced or shaped by distant planets. Their presence might help explain why some long-period comets arrive from unusual directions or with unexpected velocities.
Looking Ahead
The discovery of Neptune in the 19th century was hailed as a triumph of mathematical prediction. A similar triumph may be on the horizon, as modern computational modelling, archival data, and next-generation telescopes converge.
But there is also a poetic resonance in this search. Planet Nine represents a modern mythos, a new frontier if you will, not in a distant galaxy, but in our own celestial backyard. In an age when humans have walked on the Moon, landed robots on comets, and sent probes beyond the heliopause, it is humbling to think that we likely still do not know the full inventory of our own Solar System.
Whether or not Planet Nine is ultimately found, the search itself is advancing science. It is refining our understanding of orbital mechanics, stimulating improvements in survey techniques, and sharpening our theories of how planets form and evolve. It is also uniting professional and amateur astronomers in a shared quest, one in which even citizen scientists poring over data may yet play a pivotal role.
As the Vera Rubin Observatory prepares to open its robotic eye to the heavens, excitement is mounting. If a new planet is out there, cold, distant, and silent, it may not remain hidden for much longer.
And should we find it, the Solar System will never look the same again.