A decade ago, astronomers Scott Sheppard and Chad Trujillo hypothesized the existence of a sizable planet beyond Neptune. Dozens of papers have since come out with arguments both pro and con and sparked conversations among astronomers and the public alike. Discovery of the planet’s existence would raise questions about the solar system’s formation and the processes that produced its distant orbit. An additional planet would also explain the behaviors—the paths of the extreme trans-Neptunian objects (ETNOs) that orbit the Sun with trajectories that can't be explained by Neptune’s gravitational pull—that led to the initial hypothesis.
Following the 1846 discovery of Neptune, whose existence was predicted before it was directly observed, astronomers wondered if another faraway planet perturbs Uranus’s and Neptune’s orbits. In 1880, astronomer George Forbes was the first to formally propose the existence of trans-Neptunian planets. Nearly 150 years later, one may be found with the help of the Vera C. Rubin Observatory in Chile, which is slated to begin operations next year.
The orbits of six extreme trans-Neptunian objects (rendered in magenta) are aligned in one direction, which could suggest the existence of Planet Nine (orbit rendered in orange). (Courtesy of Caltech/R. Hurt/IPAC; diagram created using WorldWide Telescope.)
The orbits of six extreme trans-Neptunian objects (rendered in magenta) are aligned in one direction, which could suggest the existence of Planet Nine (orbit rendered in orange). (Courtesy of Caltech/R. Hurt/IPAC; diagram created using WorldWide Telescope.)
A group of 13 ETNOs—whose orbits have a semimajor axis of at least 150 astronomical units (AU)—ignited interest in Sheppard, of the Carnegie Institution for Science, and Trujillo, of Northern Arizona University. They observed that the ETNOs had elongated orbits around the Sun that were too distant to be affected by Neptune’s gravitational pull. The pair suggested that another planet was tugging on the orbits.
The puzzle inspired Caltech astronomers Konstantin Batygin and Michael Brown to take a closer look. They published research in 2016 detailing how 6 of the 13 identified ETNOs’ orbits point in the same direction in space and tilt about 30° downward from the plane of the solar system. They calculated the chance of both those behaviors occurring randomly to be about 0.007%. After testing and fine-tuning their models, Batygin and Brown supported the idea that a planet herds the ETNOs. They dubbed the hypothetical object Planet Nine. (See the Quick Study by Brown, Physics Today, March 2019, page 70.)
Their models put the planet somewhere between 5 and 10 Earth masses and position it near the outer Kuiper belt, 400–800 AU from the Sun. The small size and great distance mean that very little light would reflect off the planet, making it difficult to detect.
Presenting the evidence
The astronomy community is divided about Planet Nine. David Nesvorny, an astronomer at the Southwest Research Institute in Colorado, says he was skeptical at first. But, he says, “none of my ideas could explain what’s going on besides Planet Nine.” A ninth planet could be responsible for the retrograde motions observed in some Kuiper belt objects as well as the ETNOs’ clustered orbits, Nesvorny says. Using solar-system simulations, he has found that Planet Nine could also explain why some objects make their way inside Neptune’s orbit from beyond the Oort cloud, which is located far beyond the edges of the Kuiper belt.
Samantha Lawler, an astronomer at the University of Regina in Canada, remains skeptical about Planet Nine. “I am convinced there are strong observational biases in the ETNOs that we need to be very careful about,” she says. Because ETNOs reflect very little light at such great distances, they have been observed only when they are in certain areas of the sky and close to the Sun, she says. Lawler worked on the Outer Solar System Origins Survey, which detected more ETNOs with high perihelion distances. With the added ETNOs, according to the survey’s study published in 2017 in the Astronomical Journal, there’s no evidence of clustered orbits.
The Vera C. Rubin Observatory in Chile will take images of the Southern Hemisphere over the next 10 years, starting in 2025. It will access around 10 billion objects and is poised to detect Planet Nine—if it exists.
OLIVIER BONIN/SLAC NATIONAL ACCELERATOR LABORATORY
The Vera C. Rubin Observatory in Chile will take images of the Southern Hemisphere over the next 10 years, starting in 2025. It will access around 10 billion objects and is poised to detect Planet Nine—if it exists.
OLIVIER BONIN/SLAC NATIONAL ACCELERATOR LABORATORY
Others attribute the ETNO orbits to a dark-matter effect or a misunderstanding of how gravity works, but both theories are “very complicated explanations” for what is likely going on, says Lawler.
Astronomer David Tholen at the University of Hawaii says he is “in the gray area” when it comes to Planet Nine. He says that astronomers use different sets of ETNOs to measure clustering and alignments. Because the “cutoff for which objects are chosen is arbitrary,” more data are needed for him to support or reject the hypothesis. “I’m not sure if the evidence is leaning one way or another right now,” says Matthew Holman, a Smithsonian astrophysicist and lecturer at Harvard University. But, he adds, the possibility of Planet Nine is compelling enough to warrant research time.
This past April, Batygin and Brown published more evidence that Planet Nine exists. Their study focused on a new set of low-inclination trans-Neptunian objects with orbits that reach the inside of Neptune’s orbit. According to the study, those objects could only make their way into the inner solar system if another planet were there to slingshot them. A planet could also explain how the objects were pulled into the solar system and how their orbits were formed.
Planet Nine hunters have their sights set on the Rubin Observatory. It boasts an 8.4-meter telescope and the largest camera ever built for astronomy research. The camera will scan the Southern Hemisphere sky every night over the next decade and is expected to access around 10 billion objects. Planet Nine’s estimated brightness and distance fall into the observatory’s capabilities. “If we find something moving out around 500 to 600 AU, it will be Planet Nine,” says Brown. If Rubin doesn’t find Planet Nine within a few years, he adds, “we’ll try using radio telescopes.”
