Researchers at the California Institute of Technology (Caltech) have found evidence of a giant planet tracing a highly elongated orbit in the far distant reaches of our Solar System. The object, which has been nicknamed "Planet Nine", has a mass about 10 times that of Earth and orbits about 20 times farther from the Sun on average than does Neptune (which orbits the Sun at an average distance of 2.8 billion miles). With a semi-major axis of 300,000 light seconds, or about 0.01 light years, it would take this new planet an estimated 20,000 years to make just one full orbit around the Sun.
The researchers, Konstantin Batygin and Mike Brown, discovered the planet's existence through mathematical modelling and computer simulations but have not yet observed the object directly.
"This would be a real ninth planet," says Brown. "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 notes that Planet Nine, at 5,000 times the mass of Pluto, is sufficiently large that there should be no debate about whether it is a true planet. Unlike the class of smaller objects now known as dwarf planets, it gravitationally dominates its neighbourhood. In fact, it dominates a region larger than any of the other known planets, a fact that Brown says makes it "the most planet-y of the planets in the whole Solar System."
Batygin and Brown describe their work in the current issue of the Astronomical Journal and show how Planet Nine helps explain a number of mysterious features of the Kuiper Belt. The orbital correlations of six distant trans-Neptunian objects, pictured below, were key to their model. After they plotted the orbits of these and various other objects, they matched their simulations perfectly: "When we found that, my jaw sort of hit the floor," says Brown.
Batygin, an assistant professor of planetary science, comments: "Although we were initially quite sceptical that this planet could exist, as we continued to investigate its orbit – and what it would mean for the outer Solar System – we became increasingly convinced it is out there. For the first time in over 150 years, there is solid evidence that the Solar System's planetary census is incomplete."
In a couple of ways, this ninth planet – which seems like an oddball to us – would actually make our Solar System more similar to systems that astronomers are finding around other stars. Firstly, most of the planets around other Sun-like stars have no single orbital range – that is, some orbit at extremely close range to their host stars, while others follow exceptionally distant orbits. Secondly, the most common planets around other stars vary between one and 10 Earth-masses.
"One of the most startling discoveries about other planetary systems has been that the most common type of planet out there has a mass between that of Earth and that of Neptune," explains Batygin. "Until now, we have thought that the Solar System was lacking in this most common type of planet. Maybe we're more normal after all."
The team continue to refine their simulations and learn more about the planet's orbit and its gravitational influence on the Solar System. They believe their study will trigger a worldwide hunt by astronomers – both amateur and professional – to obtain the first direct visual images of Planet Nine. A new generation of observatories such as the James Webb Telescope may do just that. It might also be spotted in old images captured by previous surveys. Given its high mass, it is possible that various moons are there too.