Brown and a handful of colleagues have now pretty well scanned the zone around Pluto, out to around 5 billion miles from the sun, for large objects. A frozen ball known as V774104, currently the most distant solar system object known, is twice that far. Go just a little deeper, though, and all kinds of things could be circling about, invisible to even the best telescopes.
For now, the evidence for Planet 9 comes solely from its gravity, not from its light. Starting about a decade ago, astronomers noticed odd patterns in the distant solar system. Brown and Batygin were especially struck by the orbits of six of the most extreme objects in the Kuiper Belt (the population of outer objects that includes both Pluto and Eris), all of which cluster on one side of the sky.
They deduced that a large planet, roughly 10 times the mass of Earth, was lurking on the other side of the sun, its gravitational pull sweeping any smaller stuff out of the way. The pattern of clustering indicated a likely orbit for Planet 9. The fact that nobody had noticed it yet offered another clue: The planet must be on the darkest, farthest part of its looping path, possibly 100 billion miles away.
That still leaves a lot of sky to examine. To visually track down his putative Planet 9, Brown is requesting 20 nights of observing time on the Subaru telescope in Hawaii, the only large instrument with a wide enough field of view to practically pull off such a search. Even if he gets his observing time (there’s a lot of competition), the project will take at least a year and a half.