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.
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. 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.
Caltech researchers have found mathematical evidence suggesting there may be a "Planet X" deep in the solar system. This hypothetical Neptune-sized planet orbits our sun in a highly elongated orbit far beyond Pluto. The object, which the researchers have nicknamed "Planet Nine," could have a mass about 10 times that of Earth and orbit about 20 times farther from the sun on average than Neptune. It may take between 10,000 and 20,000 Earth years to make one full orbit around the sun.
The announcement does not mean there is a new planet in our solar system. The existence of this distant world is only theoretical at this point and no direct observation of the object nicknamed have been made. The mathematical prediction of a planet could explain the unique orbits of some smaller objects in the Kuiper Belt, a distant region of icy debris that extends far beyond the orbit of Neptune. Astronomers are now searching for the predicted planet.
In January 2015, Caltech astronomers Konstantin Batygin and Mike Brown announced new research that provides evidence of a giant planet tracing an unusual, elongated orbit in the outer solar system. The prediction is based on detailed mathematical modeling and computer simulations, not direct observation.
This large object could explain the unique orbits of at least five smaller objects discovered in the distant Kuiper Belt.
"The possibility of a new planet is certainly an exciting one for me as a planetary scientist and for all of us," said Jim Green, director of NASA's Planetary Science Division. "This is not, however, the detection or discovery of a new planet. It's too early to say with certainty there's a so-called Planet X. What we're seeing is an early prediction based on modeling from limited observations. It's the start of a process that could lead to an exciting result."
The Caltech scientists believe Planet X may have has a mass about 10 times that of Earth and be similar in size to Uranus or Neptune. The predicted orbit is about 20 times farther from our sun on average than Neptune (which orbits the sun at an average distance of 2.8 billion miles). It would take this new planet between 10,000 and 20,000 years to make just one full orbit around the sun (where Neptune completes an orbit roughly every 165 years).
When was it Discovered?
Planet X has not yet been discovered, and there is debate in the scientific community about whether it exists. The prediction in the Jan. 20, 2015 issue of the Astronomical Journal is based on mathematical modeling.
What is its Name?
Batygin and Brown nicknamed their predicted object "Planet Nine," but the actual naming rights of an object go to the person who actually discovers it. The name used during previous hunts for the long suspected giant, undiscovered object beyond Neptune is "Planet X."
If the predicted world is found, the name must be approved by the International Astronomical Union. Planets are traditionally named for mythological Roman gods.
Why Do They Think It's There?
Astronomers studying the Kuiper Belt have noticed some of the dwarf planets and other small, icy objects tend to follow orbits that cluster together. By analyzing these orbits, the Caltech team predicted the possibility that a large, previously undiscovered planet may be hiding far beyond Pluto.
They estimate the gravity of this potential planet might explain the unusual orbits of those Kuiper objects.
Astronomers, including Batygin and Brown, will begin using the world's most powerful telescopes to search for the object in its predicted orbit. Any object that far away from the sun will be very faint and hard to detect, but astronomers calculate that it should be possible to see it using existing telescopes.
"I would love to find it," says Brown. "But I'd also be perfectly happy if someone else found it. That is why we're publishing this paper. We hope that other people are going to get inspired and start searching."
"Anytime we have an interesting idea like this, we always apply Carl Sagan's rules for critical thinking, which include independent confirmation of the facts, looking for alternate explanations, and encouraging scientific debate," said Green. "If Planet X is out there, we'll find it together. Or we'll determine an alternate explanation for the data that we've received so far.