Evidence of a ninth planet in our Solar System

Researchers at Caltech University (Pasadena CA, USA) believe they have found a new planet in our outer Solar System. Although they have not yet observed it directly, computer simulations hint at its existence.

The planet has been nicknamed ‘Planet Nine’. Its mass is roughly 10 times that of Earth, it is believed to be gaseous and its orbit around the Sun is in average 20 times larger than the 2.8 billion miles one of Neptune.

Calculated orbit of Planet Nine (credit: Caltech/R. Hurt/IPAC)

Calculated orbit of Planet Nine (credit: Caltech/R. Hurt/IPAC)

Researchers Konstantin Batygin and Mike Brown derived the existence of Planet Nine from mathematical models and computer simulations. Being about five thousand times more massive than Pluto it would certainly fit amongst the ‘Planet’ class. “This would be a real ninth planet,” says Professor Brown, co-author of the study published yesterday in The Astronomical Journal. “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.”

Planet Nine’s discovery was inferred from the motion of six small objects in the Kuiper Belt that could be nicely explained by assuming the presence of a massive planet in an anti-aligned orbit – i.e. an orbit in which the planet’s closest position (perihelion) to the Sun is 180 degrees opposite to the perihelion of all the other objects. This scenario is quite unlikely, but nonetheless possible. “I was very sceptical,” says Batygin. “I had never seen anything like this in celestial mechanics.”

As strange as it might seems the presence and orbit of Planet Nine are consistent with other observations made by Brown and collaborators – specifically the orbits of two Kuiper Belt’s objects called Sedna and 2012 VP113 together with those of other more distant ones.

So, what is the origin of Planet Nine? Scientists thought that in the early Solar System there were four planetary cores which attracted the nearby gases and formed the four gas planets Jupiter, Saturn, Uranus and Neptune. “But there is no reason that there could not have been five cores, rather than four,” says Brown. Planet Nine could represent that fifth core, possibly ejected into its distant orbit upon getting too close to Jupiter or Saturn.

At the moment, only the planet’s rough orbit is known, but it is feasible that the W. M. Keck Observatory and the Subaru Telescope, both on Mauna Kea in Hawaii, could actually spot it at some point. The discovery of Planet Nine is interesting because it makes our Solar System more similar to other systems in which planets often follow exceptionally distant orbits and for which the most common planets’ mass is one- to ten-fold that of Earth. “Until now, we’ve thought that the solar system was lacking in this most common type of planet. Maybe we’re more normal after all,” Batygin concluded.

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Carlo Bradac

Carlo Bradac

Dr Carlo Bradac is a Research Fellow at the University of Technology, Sydney (UTS). He studied physics and engineering at the Polytechnic of Milan (Italy) where he achieved his Bachelor of Science (2004) and Master of Science (2006) in Engineering for Physics and Mathematics. During his employment experience, he worked as Application Engineer and Process Automation & Control Engineer. In 2012 he completed his PhD in Physics at Macquarie University, Sydney (Australia). He worked as a Postdoctoral Research Fellow at Sydney University and Macquarie University, before moving to UTS upon receiving the Chancellor Postdoctoral Research and DECRA Fellowships.

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