For decades, our solar system seemed neatly wrapped up with eight planets circling the Sun like a cosmic waltz. But deep in the icy reaches beyond Neptune, a celestial puzzle has astronomers scratching their heads. Could there be a ninth planet—one massive enough to shepherd objects on the edge of our solar system but stealthy enough to elude discovery? Recent research offers tantalizing hints that this elusive “Planet 9” might be more than just science fiction.
A Mystery Born in the Outer Solar System
Back in 2015, two astronomers from Caltech, Mike Brown and Konstantin Batygin, dropped a cosmic bombshell. They discovered that six distant objects beyond Neptune’s orbit—part of a class known as trans-Neptunian objects (TNOs)—had something strange in common. Their orbits were eerily clustered, as if an invisible force were guiding them. The culprit? Possibly a massive, unseen planet lurking in the solar system’s outskirts.
In the new study, we explored the opposite end: the most unstable, long-period TNOs (perihelion <30 AU) near the solar system’s plane. There are 17 such objects with a>100AU, and notably, their perihelion distribution is almost flat between ~15 to 30 AU. pic.twitter.com/Nw8GgrGNSB
— Konstantin Batygin (@kbatygin) April 18, 2024
But skeptics were quick to point out that statistical flukes and observational biases might explain the anomaly. Still, the idea of a hidden planet was too tantalizing to dismiss.
Fast forward to today, and a new team of researchers believes they’ve found the strongest statistical evidence yet for Planet 9. After examining a broader population of long-period TNOs, they noticed that these objects consistently reach their closest approach to the Sun—known as their perihelion—within a specific range of 15 to 30 astronomical units (AU). For reference, 1 AU is the distance from Earth to the Sun. This clustering defies randomness, suggesting that something big is exerting a gravitational influence.
Simulations Point to a Hidden Giant
The researchers didn’t stop at observations. They ran detailed simulations to see what could explain the strange orbits of these objects. Their findings? A model that includes a massive, undiscovered planet fits the data far better than one without it. This hypothetical planet would likely be several times the mass of Earth and orbit hundreds of AU away—so far out that it would take thousands of years to complete a single revolution around the Sun.
However, pinpointing where to look for Planet 9 remains a challenge. Unlike planets closer to the Sun, whose positions can be tracked with relative ease, this hypothetical giant is a needle in a haystack of cosmic proportions.
Could a Rogue Star Be the Culprit?
Not everyone is sold on the Planet 9 hypothesis. Another team of researchers suggests an entirely different explanation for the TNOs’ unusual behavior. Their theory involves a “close encounter” between our Sun and another star billions of years ago.
Using 3,000 simulations, the team modeled scenarios in which stars of varying masses passed near the Sun. They concluded that a flyby of a star slightly smaller than our Sun—about 0.8 times its mass—at a distance of roughly 16.5 billion kilometers (10.3 billion miles) could have sculpted the outer solar system as we see it today. To put that distance in perspective, it’s about four times farther than Neptune’s orbit.
A New Observatory Could Solve the Mystery
Whether Planet 9 exists or the answer lies in our Sun’s ancient brush with a rogue star, the mystery might soon be unraveled. The upcoming Vera Rubin Observatory, set to become operational in the near future, promises to revolutionize our view of the outer solar system. Its unparalleled ability to scan the skies will offer astronomers the best chance yet to confirm or debunk the Planet 9 hypothesis.
Stay tuned—this cosmic detective story is just getting started. Whether we find a hidden planet, uncover evidence of a stellar encounter, or discover something entirely unexpected, the secrets of the solar system’s outer edges are waiting to be revealed.
