Is there a ninth planet in our Solar System? Planet Nine is thought to be a trans-Neptunian object – a minor planet that orbits the Sun at a distance further out than Neptune. Both the Kuiper belt and the Oort cloud are included in this region, and Planet Nine is thought to orbit somewhere between the two.
The Oort cloud is a spherical region of icy debris thought to surround the Solar System. Although it was thought to extend from 5,000 to 100,000 Earth-Sun distances, it may start much closer to the Sun. Image credit: NASA and A. Feild (Space Telescope Science Institute)
There are strange goings-on in the Kuiper belt, and these have been known about for a while. In 2014, American researchers Chadwick Trujillo and Scott Sheppard tried to create a simulation of objects in the Oort cloud based on their observations of the distant solar system, and found that they were unable to explain the clustering of the orbits of Kuiper Belt objects (KBOs) in that region. Two Spanish scientists found what they refer to as “unexpected patterns” in the parameters of the orbits of so called “extreme trans-Neptunian objects”, and they consider their results to confirm the findings of Trujillo and Sheppard. These results were further confirmed by Caltech planetary scientists Konstantin Batygin and Michael “Pluto Killer” Brown, who found that objects in the Kuiper belt were clustering together, despite this being a less favourable configuration in terms of the configuration of their orbits and the space they take up.
The Caltech scientists worked out the probability of this configuration happening by chance was just 0.007% – which led them to conclude that something must have caused the Kuiper belt objects to group together as they have. They suggest that such celestial nudging could be caused by a planet whose mass is at least ten times that of the Earth, and whose longest orbital radius is 700 AU – the fabled Planet Nine. In fact, when the Outer Solar System Origin Survey first found the KBO with an eccentric orbit (called uo3L91), Mike Brown tweeted that it was
exactly where Planet Nine said it should be.
All of the (known) planets in our Solar System orbit the Sun in the same plane – that is, you could draw all of their orbits on a flat a piece of card (though it would have to be a very large piece of card).
The Sun doesn’t spin in this plane – the plane it spins in is about six degrees off. Now, this might seem quite inconsequential to us, but it’s significant enough that scientists are using it to suggest that there’s another planet in our Solar System that we’ve never even seen. One that orbits it in another plane, just to be different. Two separate studies have examined whether Planet Nine could have caused the six degree deviation. One international group of researchers worked together to show it would be possible for the tilt to be caused by Planet Nine. They suggest Planet Nine tugged on the orbits of the planets, tilting them by a smidge. Others agreed that a planet whose mass was between five and twenty times heavier than Earth, and whose closest point to the Sun was approximately 250 AU, might explain the six degree tilt.
A European team of physicists suggest that Planet Nine was captured from another star. Their idea is formed from the hypothesis that the Sun wasn’t created on its own; rather, it formed in a group of thousands of stars, known as its birth cluster. They propose that Planet Nine was pinched by the Sun from another star in it’s birth cluster. They state three conditions which must be satisfied by any potential planet-theft scenario: it has to happen far enough away that it won’t set the Kuiper belt
off-kilter; the “victim” star must have a planet with a wide orbit (easy to steal); and, once stolen, the planet must end up in an orbit that explains all the phenomena that made us suspect it existed in the first place (including the Kuiper belt
clustering and the Sun’s tilt). For Planet Nine, all these conditions could be satisfied simultaneously
There are a few things we can assume about Planet Nine (if it exists): in terms of its orbit, it is probably always at least 250 times further away from the Sun than the Earth is, but unlikely to be further away than 700 Earth-Sun distances.
Planet Nine is a hypothetical large planet in the far outer Solar System, the gravitational effects of which would explain the improbable orbital configuration of a group of trans-Neptunian objects (TNOs) that orbit mostly beyond the Kuiper belt. Recent speculation about the alignment of the eTNOs being due to a distant massive planet began with a 2014 letter to the journal Nature, in which astronomers Chad Trujillo and Scott S. Sheppard inferred the possible existence of a massive trans-Neptunian planet from similarities in the orbits of the distant trans-Neptunian objects Sedna and 2012 VP113. On 20 January 2016, researchers Konstantin Batygin and Michael E. Brown at Caltech explained how a massive outer planet would be the likeliest explanation for the similarities in orbits of six distant objects, and proposed specific orbital parameters. The predicted planet could be a super-Earth, with an estimated mass of 10 Earths (approximately 5,000 times the mass of Pluto), a diameter two to four times that of Earth, and a highly elliptical orbit with an orbital period of approximately 15,000 years.In addition to the clustering of the perihelia and the orbital poles of distant objects, Planet Nine offers explanations for the high perihelion of Sedna and 2012 VP113, for objects with orbits roughly perpendicular to those of the planets, for high inclination TNOs with semimajor axes less than 100 AU, and for the tilt of the Sun’s rotation axis. The objects it dynamically controls would form a cloud centered on its semi-major axis with a wide range of inclinations. Batygin and Brown suggested Planet Nine was a primordial giant planet core that was ejected from its initial orbit by an encounter with Jupiter during the nebular epoch of the Solar System and was later perturbed into a stable orbit by a distant encounter with a passing star or by the solar nebula. Others have proposed that it was captured in a similar manner from another star, or that it formed on a very distant circular orbit and was perturbed into its current eccentric orbit during a distant encounter with another star.