A team of astronomers at the California Institute of Technology has found strong evidence of a massive gaseous planet – informally named Planet Nine – tracing an extremely elongated orbit in the outer Solar System.
Planet Nine is thought to be gaseous, similar to Uranus and Neptune; hypothetical lightning lights up the night side. Image credit: R. Hurt, IPAC / Caltech.
Planet Nine is likely a gas giant about 10 times more massive than Earth, according to the astronomers – Dr. Konstantin Batygin and Prof. Mike Brown, both of Caltech’s Division of Geological and Planetary Sciences.
The planet orbits 20 times farther from the Sun on average than does Neptune. It would take this gas giant approximately 15,000 years to make just one full orbit around the Sun.
The scientists discovered the planet’s existence through mathematical modeling and computer simulations but have not yet observed the object directly.
“This would be a real ninth planet. 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,” Prof. Brown said.
“The planet 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, Planet Nine gravitationally dominates its neighborhood of the Solar System.
In their study, Prof. Brown and Dr. Batygin realized that the six most distant known objects in the Solar System – 2007 TG422, 2013 RF98, 2004 VN112, 2012 VP113, 2012 GB174, and the minor planet 90377 Sedna – all follow elliptical orbits that point in the same direction in physical space. That is surprising because the outermost points of their orbits move around the Solar System, and they travel at different rates.
The six most distant known objects in the Solar System – 2007 TG422, 2013 RF98, 2004 VN112, 2012 VP113, 2012 GB174, and the minor planet 90377 Sedna – mysteriously line up in a single direction. Also, when viewed in three dimensions, they tilt nearly identically away from the plane of the Solar System. Image credit: R. Hurt, IPAC / Caltech.
“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. The odds of having that happen are something like 1 in 100,” Prof. Brown said.
The orbits of these distant objects are also all tilted in the same way – pointing 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%.
The scientists describe their work in the Jan. 20 issue of the Astronomical Journal and show that a massive planet in a distant eccentric orbit anti-aligned with the other six objects is required to maintain this configuration.
“We find that the observed orbital alignment can be maintained by a distant eccentric planet with mass >10 Earth masses whose orbit lies in approximately the same plane as those of the distant Kuiper Belt objects, but whose perihelion is 180 degrees away from the perihelia of the minor bodies,” they explained.
“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,” Dr. Batygin said.
“For the first time in over 150 years, there is solid evidence that the Solar System’s planetary census is incomplete.”
_____
Konstantin Batygin & Michael E. Brown. 2016. Evidence for a Distant Giant Planet in the Solar System. Astronomical Journal 151, 22; doi: 10.3847/0004-6256/151/2/22
