Planet Nine is probably just playing hard to get.
Evidence of a big, unseen world in the extreme outer solar system continues to mount. Take the recent discovery of the distant dwarf planet 2015 TG387, known as "The Goblin." This world's highly elliptical orbit appears to have been sculpted by the gravity of a sizable planet way out there in the dark depths — as have the orbits of more than a dozen other faraway objects, researchers say.
"Planet Nine really remains the only viable explanation for all of the stuff that we observe," said Konstantin Batygin, a theoretical astrophysicist at the California Institute of Technology (Caltech) in Pasadena. [The Evidence for 'Planet Nine' in Our Solar System (Gallery)]
Starting the hunt
Batygin is a major player in the hunt for Planet Nine, which really kicked off in 2014.
That year, astronomers Chad Trujillo and Scott Sheppard proposed the existence of a large "perturber" far beyond Neptune. Such a planet, the scientists said, could explain weird features in the orbits of the dwarf planets Sedna and 2012 VP113, as well as a few other distant objects.
In January of 2016, Batygin and fellow Caltech researcher Mike Brown marshalled more evidence for this hypothetical world, which the duo dubbed "Planet Nine." Batygin and Brown also took a stab at characterizing the planet, estimating that it's perhaps 10 times more massive than Earth and orbits about 600 astronomical units (AU) from the sun on average. (One AU is the Earth-sun distance, about 93 million miles, or 150 million kilometers.)
Further detections of distant objects with oddball orbits followed. Astronomers have now spotted 14 bodies that bear the imprint of a perturber's tug, Batygin said.
That imprint is primarily a pronounced clustering. Basically, the elongated portions of the objects' highly elliptical orbits point in the same direction, in a way predicted by the Planet Nine models.
The chances of such a configuration developing by chance alone are less than 0.1 percent, Batygin said. And other possible explanations fall short, he added.
For example, some researchers have proposed that the clustering resulted from the combined tugs of many small objects in the Kuiper Belt, the ring of icy bodies beyond Neptune's orbit. But a "self-modulating" Kuiper Belt would look very different from the actual belt that we see, Batygin said. And, he added, a recent study suggests that the entire Kuiper Belt (sans Planet Nine) harbors no more than 2 percent of the mass of Earth — not nearly enough to shape the bodies' orbits in the observed manner.
"The evidence for Planet Nine is really, really solid," Batygin told Space.com. [Our Solar System: A Photo Tour of the Planets]
He pegged the probability of the world's existence at "over 90 percent." Sheppard, who's based at the Carnegie Institution for Science in Washington, D.C., is similarly bullish.
"We think it's more likely than not," Sheppard told Space.com. "For me, personally, it's probably at the 80 [percent] to 90 percent level."
Where is Planet Nine hiding?
Batygin and Brown have systematically searched for Planet Nine over the past few years, as have Trujillo and Sheppard (who refer to the hypothetical world as "Planet X"). Both teams have been using Japan's 26-foot (8 meters) Subaru Telescope, which sits atop Mauna Kea on the Big Island of Hawaii.
Other research groups around the world have joined the hunt as well. Still, it's not all that surprising that the putative planet remains undiscovered, Sheppard said.
"Where we think the planet is — hundreds of AU away, if not 1,000 AU — something even as big as Neptune would be fainter than most telescopes could see," Sheppard said.
"And most of our surveys to date do not go that faint, do not go that deep. We've covered very little of the sky to the depth that's needed to be covered to find something this faint," he added. "You can hide a very big thing in the outer solar system very easily."
Indeed, the search has so far covered just 20 percent to 25 percent of "premium sky," the regions where Planet Nine is most likely to be, both Sheppard and Batygin said.
It's tough to make predictions about when Planet Nine will finally be spotted, because astronomers don't know the object's mass, brightness or precise orbit — or even that it exists at all. But Batygin said he suspects that Subaru is capable of seeing the planet. (Subaru combines good resolution with a wide field of view. That latter quality is key. Some instruments, such as NASA's Hubble Space Telescope, are probably sharp-eyed enough to spot Planet Nine but have such narrow fields of view that employing them in the hunt would be impractical.)
If Planet Nine is too faint for Subaru, then Batygin, Sheppard and other hunters likely won't despair. Help will soon arrive in the form of powerful new instruments, such as the Large Synoptic Survey Telescope (LSST), which is scheduled to come online in the Chilean Andes in the early 2020s.
"If we don't find it in the next five or so years," Batygin said, LSST "is definitely going to deliver the final word on Planet Nine."
Mike Wall's book about the search for alien life, "Out There," will be published on Nov. 13 by Grand Central Publishing. Follow him on Twitter @michaeldwall. Follow us @Spacedotcom or Facebook. Originally published on Space.com.