“I try not to be religious about my own results. It’s important to keep a skeptical eye,” says Caltech's Konstantin Batygin. “But I actually feel more comfortable than I did two years ago, because the theory still holds up beautifully. The more we look, the more we see a solar system that makes no sense without Planet Nine.”
It’s been just over two years, writes Lee Billings in today's Scientific American, since Caltech astronomers Mike Brown and Konstantin Batygin made an explosive claim: Based on the orbital motion of objects in the Kuiper Belt—a region beyond Neptune that is home to Pluto and other icy bodies—there must be a very big something much farther out, hidden save for its subtle gravitational tugs on the rest of the solar system.
Brown and Batygin’s best models put this mysterious object at about 10 times Earth’s mass, perhaps 20 times more distant from the sun than Neptune and currently drifting through what might be a 20,000-year orbit in a patch of sky near the constellation Orion. Brown and Batygin called it “Planet Nine,” elevating it to the position once held by Pluto (which was demoted to “dwarf planet” status in 2006, when Brown discovered multiple Pluto-like worlds out past Neptune). Within months a small army of theorists and observers had thrown themselves into the search—which, so far, has come up empty. Planet Nine remains stubbornly in absentia.
Unknown planets far from the sun are not a new idea; they perennially pop up in astronomy. Such claims trace back to the 1800s, and fostered the discoveries of Neptune and Pluto. What makes Planet Nine different is how much more we now know about the outer solar system—a vast and stygian abyss in which hiding a planet is still possible, although getting harder all the time. Out there twirl frozen bits of flotsam left over from our solar system’s earliest moments. A big planet’s gravity can act like a thumb on a scale, subtly but substantially tweaking the movements of these so-called trans-Neptunian objects (TNOs). As astronomers use new telescopes and other instruments to rapidly map this last frontier of the solar system, they keep finding what seems to be a Planet Nine–shaped hole in it.
Brown and Batygin’s proposed planet handily explains orbital oddities observed in some TNOs. In their initial paper the pair showed how a recently discovered population of TNOs, bizarrely orbiting nearly perpendicular to the plane of the known planets, could be coaxed and kept there by the gravity of a far-out hidden world.
Other newfound TNOs move in a telltale filigree of orbital resonances, periodically perturbing one another in a web of complex patterns that hint at further interactions with some great, unseen mass. Planet Nine’s gravitational influence could even serve as a solution to the long-standing mystery of why the sun’s axis of spin is tilted six degrees askew to the orbits of the inner planets.
Planet Nine also aligns with an emerging awareness that the solar system’s early days were a chaotic mess, in which the early formation of Jupiter and Saturn scattered smaller and more embryonic worlds into the sun or the interstellar void. In this picture Planet Nine might have been an outbound world that plowed through enough debris to slow down and get trapped in the solar hinterlands. Or it could have been an alien outcast from another star, gravitationally captured when it wandered too close to our own. In an indirect way it could even be responsible for our existence—scattered inward rather than outward, it might have disrupted Earth’s orbit, preventing life’s genesis here.
Further afield, surveys of planets orbiting other stars have shown the most common worlds in our galaxy bear a passing resemblance to the putative Planet Nine—so-called “super-Earths” that are midway in size between Earth and Neptune, and appear around most stars we examine. If Planet Nine is real, it could be more than just another planet on the block; it could be the missing link between our familiar solar system and those we now see elsewhere in the Milky Way.
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