Scientists find evidence of ‘Planet Nine’
Two scientists from Caltech have discovered evidence of a giant planet with a bizarre, elongated orbit in the outer Solar System. The object, dubbed “Planet Nine” by the scientists, has a mass of about 10 times that of Earth and orbits the Sun at an average distance of approximately 20 times farther away than does Neptune—which orbits the Sun at an average distance of 2.8 billion miles (30.11 AU; 4.50 × 109 km). It would take this new planet between 10,000 and 20,000 years to make one orbit around the Sun.
The scientists, Mike Brown and Konstantin Batygin, discovered the planet’s existence through mathematical modeling and computer simulations but have not directly observed it yet.
“This would be a real ninth planet,” says Brown, the Richard and Barbara Rosenberg Professor of Planetary Astronomy. “There have only been two true planets discovered since ancient times, and this would be a third. It’s a pretty substantial chunk of our Solar System that’s still out there to be found, which is pretty exciting.”
Brown and Batygin’s findings were published in the current issue of the Astronomical Journal, and show how the existence of Planet Nine would explain the mysterious behavior of icy objects in the region beyond Neptune known as the Kuiper Belt.
“Although we were initially quite skeptical that this planet could exist, as we continued to investigate its orbit and what it would mean for the outer Solar System, we become increasingly convinced that it is out there,” says Batygin, an assistant professor of planetary science. “For the first time in over 150 years, there is solid evidence that the Solar System’s planetary census is incomplete.”
The research that resulted in the theoretical discovery began in 2014 when Chad Trujillo, a former postdoc of Brown’s, and his colleague Scott Sheppard published a paper noting that 13 of the most distant Kuiper Belt objects are similar with respect to an obscure orbital feature. Trujillo and Sheppard suggested that the presence of a small planet might be responsible for the similarity.
Brown thought the ninth planet solution was unlikely, but he discussed the problem with Batygin. The two scientists then began what was to be a year-and-a-half-long collaboration to study the distant objects. Batygin and Brown quickly realized that the six most distant Solar System objects analyzed in Trujillo and Sheppard’s study all follow elliptical orbits that point in the same direction in physical space.
“It’s almost like having six hands on a clock all moving at different rates, and when you happen to look up, they’re all in exactly the same place,” says Brown. The odds of having that happen are something like 1 in 100, he says. However, on top of that, the orbits of the six objects are also all tilted in the same way—pointing about 30 degrees downward in the same direction relative to the plane of the eight known planets. The probability of that happening is about 0.007 percent.
“Basically, it shouldn’t happen randomly,” Brown says. “So we thought something else must be shaping these orbits.”
After trying a variety of different scenarios, Brown and Batygin noticed that if they ran their simulations with a massive planet in an anti-aligned orbit—an orbit in which the planet’s closest approach to the Sun, or perihelion, is 180 degrees across from the perihelion of all the other objects and known planets—the distant Kuiper Belt objects in the simulation assumed the alignment that is actually observed.
Brown and Batygin are continuing to refine their simulations to learn more about the planet’s orbit and its influence on the distant Solar System. Brown and other astronomers have begun searching the skies for Planet Nine.
“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.”
Jim Sharkey is a lab assistant, writer and general science enthusiast who grew up in Enid, Oklahoma, the hometown of Skylab and Shuttle astronaut Owen K. Garriott. As a young Star Trek fan he participated in the letter-writing campaign which resulted in the space shuttle prototype being named Enterprise. While his academic studies have ranged from psychology and archaeology to biology, he has never lost his passion for space exploration. Jim began blogging about science, science fiction and futurism in 2004. Jim resides in the San Francisco Bay area and has attended NASA Socials for the Mars Science Laboratory Curiosity rover landing and the NASA LADEE lunar orbiter launch.