Search for Planet Nine advances

Fresh location, brightness projections raise hopes of actually spotting distant world

More clues about where to search for a possible ninth planet lurking in the fringes of our solar system are emerging from the Kuiper belt, the icy debris field beyond Neptune. And new calculations suggest that the putative planet might be brighter — and a bit easier to find — than once thought.

Evidence for the existence of Planet Nine is scant, based on apparent alignments among the orbits of the six most distant denizens of the Kuiper belt.
Their oval orbits all point in roughly the same direction and lie in about the same plane, suggesting that a hidden planet, about five to 20 times as massive as Earth, has herded them onto similar trajectories.

Planetary scientists Mike Brown and Konstantin Batygin, both at Caltech, announced this evidence in January. Now they’ve used it to refine Planet Nine’s properties and narrow in on where it might be hiding. Their results appear in the June 20 Astrophysical Journal Letters.

Planet Nine’s average distance from the sun is most likely between 500 and 600 times as far as Earth’s, Brown and Batygin report. Its orbit is highly stretched and tipped by about 30 degrees relative to the rest of the solar system, taking it well above and below the orbits of the eight known worlds. And right now, it’s probably near its farthest point from the sun — possibly as far as 250 billion kilometers away — in a large patch of sky around the constellation Orion.

But the evidence depends on orbital oddities among just six frozen worlds. “The argument that a planet is there is not ironclad,” cautions Renu Malhotra, a planetary scientist at the University of Arizona in Tucson. “I think it’s worth studying. There’s enough there to not ignore this evidence,” she adds. “We just shouldn’t get depressed if the planet’s not there.”

Malhotra and colleagues have been looking for independent evidence for a ninth planet. And they think they’ve found another clue: The orbital periods of those six bodies are roughly synced to one another, her team reports in the same journal. For example, the most distant one, the dwarf planet Sedna, goes around the sun five times in about the same amount of time that its neighbor, 2010 GB174, completes eight orbits. Such synced orbits usually hint at some gravitational link among all the bodies involved.