‘Planet X’ – a promising candidate to replace Pluto?

Pluto was reclassified as a ‘dwarf planet’ in 2005, following the discovery of similar sized objects in the Kuiper belt.

Based on mathematical modelling and computer simulation, astronomers Konstantin Batygin and Mike Brown of Caltech have hinted at the existence of a new planet.

The presence of a new planet, currently called ‘Planet X’ or ‘Planet Nine’, beyond Neptune was first predicted in 1846 by Percival Lowell. The presence of this new planet has been suggested and since been disproved, up until now.

Discrepancies in Uranus’ orbit lead to the discovery of Neptune. Now, in the same way, the unusual behaviour of six dwarf planets in the Kuiper belt (an area of icy debris beyond Neptune) has provided strong evidence for a new ninth planet. The objects ‘clump’ together with elliptical orbits tilted out of the plane of the solar system when they are all closest to the sun. The odds of this happening are 0.007%. There is no visual proof for the planet, but this chance gives the theory a statistical significance of 3.8 sigma. The required threshold is normally 3, except in some fields like particle Physics, where 5 sigma is required.

The strange behaviour observed could be due to more Kuiper belt objects, but then the mass of the belt would have to be 100 times what it is currently thought to be. Or, it could be because of the gravitational pull of a passing star, or a nearby stellar nursery.

Computer simulations of a new planet, roughly the size of Neptune and with a mass between 5 and 15 times that of Earth’s, predict the same behaviour. The planet is tilted in the same way as the six dwarfs, and is in an elliptical orbit so far away that it takes the planet 15,000 years to go round the sun once. Its closest approach to the sun is seven times further away than Neptune, 200 Au (astronomical units, about 150 million km), and could get as far as 600 to 1200 Au away.

If Planet X turns out to exist, there are still more mysteries. How did it end up so far from the sun, where there isn’t enough dust and gas to form a planet? And even if it could form, how did it become so big? Batygin and Brown suggest the planet formed closer to the sun. Computer models suggest this is possible, so the planet could have formed there and been kicked outward by another gas giant. It would have to have been slowed down by drag from residual gas in the protoplanetary disk, enough to settle into a distant orbit within the solar system, which means it has to have been kicked out between 3 and 10 million years after the birth of the system, while there was enough gas about.

At first it seems unlikely that the planet could be stable in the elliptical orbit and not collide with other objects. However, due to mean-motion resonance the anti-aligned orbit of the planet prevents collisions with Kuiper belt objects. As orbiting objects approach each other they exchange energy, which gives the objects periodic nudges so that they stay in the same position relative to the planet.

So the planet has a feasible origin and its orbit is possible. The planet can also explain the strange orbits of two of the six objects in the Kuiper belt. One of these objects, called Sedna, never gets close to Neptune. It would be expected to get kicked out by gravity and then attracted back. This is not what happens, instead the new planet pulls Sedna and other similar objects away from Neptune, towards itself.

Now that the new planet has been modelled, other phenomena can begin to be explained. For example, the objects in the Kuiper belt with orbits perpendicular to the plane of the planets. A ninth planet would actually make our solar system more similar to other known planetary systems. These often have no single orbital range, so have some planets with very close orbits (like Earth, Mars, Venus) and some very far away (like the new planet), and with similar planet masses.

The planet’s precise orbit or location on the orbit is not known, but astronomers think it could be found in the next 5 years. Earlier this week a group of French astronomers announced that they have used modelling to halve the area where the new planet is predicted to be located. They studied data from Cassini, Nasa’s spacecraft orbiting Saturn and modelled what other bodies would potentially influence the planet. When the planet is furthest away from the sun it has no effect on the other planets, excluding two zones from its possible location. If Cassini’s mission, which finishes next year, is extended to 2020, the search field could be narrowed even more.

Planets in elliptical orbits move fastest when they’re closest to the sun and if Planet X was at its closest point to the sun it may already have been discovered. Instead, the planet spends most of its time near aphelion, at distances of 600 to 1200 Au. Objects at this distance can be seen by telescopes such as the Hubble Space Telescope or the Keck telescopes in Hawaii, but these have very small fields of view. Subaru, an 8 metre telescope owned by Japan and located in Hawaii, has a wide enough lens to see a field of view 75 times larger than can be seen using the Keck telescope, and can still detect an object as faint as Planet X.

Could this planet be the long-awaited replacement for Pluto (which happened to lose its planetary status because of the same Brown predicting the new one)? Well if it is out there then perhaps so. At the moment Brown’s calling it Planet Phattie – 90s slang for cool!

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