Far beyond the eight planets of the solar system, beyond even Pluto and the diminutive dwarf planets, may lurk a major new world called “Planet Nine.” Few if any discoveries can be as sensational as finding another planet orbiting our sun, making the feat a Holy Grail for astronomers, who have managed to pull it off only a few times over the centuries. No one yet knows exactly where this ephemeral world might be—or even if it really exists at all. But in the race to find it researchers are now narrowing down its location through its influence on the rest of the solar system, roughly halving the amount of space they thought they had to search only a few months ago. The scientists detailed their latest advances in the quest last week at the meeting of the American Astronomical Society Division for Planetary Sciences and the European Planetary Science Congress in Pasadena, Calif.
In January astrophysicists Konstantin Batygin and Michael Brown at the California Institute of Technology revealed evidence for a big, undiscovered ninth planet. Their computer models suggested the gravitational pull of such a world might explain the strange, tilted orbits of several bodies in the Kuiper Belt of icy objects that haunt the solar system's outer reaches. Scientists are now scrambling to be the first to spot Planet Nine using some of the biggest telescopes on Earth, such as the Subaru Telescope in Hawaii.
Batygin and Brown's work narrowed down the planet’s possible mass and orbit to areas where previous observations might have missed it. Their calculations suggest that it has a mass between five to 20 times that of Earth—a figure that is key to knowing the approximate size of the object they are looking for. They also suggest that its orbit is likely tilted about 30 degrees compared to the plane of the solar system—the relatively thin, flat zone in which the eight major planets orbit. They also propose that the planet is now likely near its farthest point from the sun, in the sky's northern hemisphere, and that it likely has an elongated orbit averaging between 380 and 980 astronomical units (AU) from the sun. (One AU is the average distance between Earth and the sun.)
However, these estimates still leave a swath of sky "about 1,500 square degrees large," says astronomer Scott Sheppard at the Carnegie Institution for Science, who with astronomer Chadwick Trujillo first suggested the existence of Planet Nine in 2014. (In comparison, the full moon as seen from Earth covers about 0.2 degrees of sky.) This swath described by Sheppard corresponds to about 20 nights of observations on Subaru, "and if we get seven nights or so this year, that's three years—if it's not rainy any of those nights," Sheppard says.
So the strategy in the race is now largely a matter of reducing the search area by eliminating theoretical possibilities. In an as-yet unpublished set of about 100 new high-resolution computer simulations, Batygin says he and Brown have narrowed down Planet Nine's location to a roughly 600- to 800-square degree patch of sky. They first modeled the solar system over the course of about 4 billion years, focusing on how the gravitational pulls of the system’s largest planets—Jupiter, Saturn, Uranus, Neptune and Planet Nine—might have sculpted the orbits of thousands of randomly scattered Kuiper Belt objects (KBOs). "We're searching for all of the things that Planet Nine does to the solar system," Brown says.
In an attempt to refine Planet Nine’s likely orbit, the researchers next compared their results with how the Kuiper Belt looks now. "Our work yielded a synthetic solar system that looks a lot like the real one," Batygin says. "My confidence that Planet Nine is really out there has reached near-completion, given how our results are so close to what we actually see in the solar system."
Further strategies involve examining Planet Nine’s own possible gravitational influence on a variety of other bodies. Astronomers Yuri Medvedev and Dmitri Vavilov at the Institute of Applied Astronomy of the Russian Academy of Sciences looked at 768 comets entering the solar system for the first time, noting five that might have came close to Planet Nine—whose gravity would have altered their paths—sometime in the past. Their analysis suggests that "maybe Planet Nine made these comets go into the solar system," Vavilov says. "We think comets could help narrow down Planet Nine's location. It would help to find other comets." Sheppard says he would “be wary about using comets to help find Planet Nine, since there may be a lot of forces besides Planet Nine that could influence the comets' orbits. ... Still, while I'm skeptical, it could be helpful."
Analyses of Pluto by astrophysicists Matthew Holman and Matthew Payne at the Harvard-Smithsonian Center for Astrophysics failed to find conclusive signs for or against Planet Nine. This was partly due to the messy nature of the old archival images of Pluto, which make it difficult to see whether Pluto veered in its orbit in ways that would hint at Planet Nine's presence or absence, Holman says. However, he notes that the high-quality data on Saturn from the ground-based radio-dish network that monitors the position of NASA's Cassini spacecraft is very promising, and is in line with what Batygin and Brown have reported so far.
Analyzing variations in Mars' orbit may also help find Planet Nine, Holman says. Although Planet Nine would have less of an effect on Mars than Saturn—a consequence of Mars being closer and therefore more tightly gravitationally bound to the sun—there are more orbiters around Mars than Saturn, and they have been observing the Red Planet for a longer time, so "they're much more precise with their observations," Holman says. In addition, "Cassini ranging data is good to tens of meters, while ranging data between Earth and Mars is as good as one meter."
More potential evidence of Planet Nine's influence may be found in how long it takes outer solar system bodies to orbit the sun. For instance, the four KBOs with the longest-known orbits revolve in patterns most readily explained by the presence of Planet Nine, says astronomer Renu Malhotra, chair of theoretical astrophysics at the University of Arizona at Tucson. Work by Malhotra and her colleagues also suggests two likely tilts for Planet Nine's orbit, one closer to the plane of the solar system at 18 degrees and the other steeper at about 48 degrees—information that could help shrink the vast part of the sky to be searched.
But some research appears to limit possible locations to such an extent that it may rule out Planet Nine’s existence altogether. For example, while previous studies suggest Planet Nine exists because of the way certain KBO orbits cluster together, another explanation might be that the limited number of available KBO observations only make it look like their orbits are grouped, say astrophysicists Cory Shankman at the University of Victoria and Samantha Lawler at the National Research Council of Canada.*
Brown contends that he and Batygin have accounted for the possibility of such observational biases—and that other lines of evidence also suggest Planet Nine is real. For instance, Brown and his colleagues say they have found that the ghostly world’s influence might solve the longstanding mystery of why the plane of the solar system is tilted with respect to the sun.
Malhotra at the University of Arizona says she remains agnostic about Planet Nine, but notes that she and her colleagues have found that the orbits of extremely distant KBOs seem tilted in a way that is difficult to otherwise explain. "The amount of warp we see is just crazy," she says. "To me, it's the most intriguing evidence for Planet Nine I've run across so far."
*Editor's Note (10/26/16): This sentence was changed after posting to correct Samantha Lawler's institutional affiliation.