Planet 9?

Ken Tapping, 2nd February, 2016

Until recently our Solar System had 9 known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. The outermost planet, Pluto, was always a bit of a misfit. All the others keep to their own lanes as they orbit the Sun, and move in the same plane, like marbles rolling around a plate. Pluto does not. Its orbit crosses the orbit of Neptune, so that sometimes it is closer to the Sun, and its orbit is sharply angled to the plane in which all the other planets move.

Pluto remained an exception until other bodies just like it started to be discovered. They lie outside the orbit of Neptune, are Pluto-sized and smaller, and move in paths that are usually highly elliptical and inclined at all sorts of angles. There are many of these bodies; they form what has become known as the Kuiper Belt, and its members as Kuiper Belt Objects.  Pluto fits fairly well into this category. So in 2006, after a rather acrimonious vote at the International Astronomical Union, Pluto was reclassified as a “Dwarf Planet”, and as just a particularly large member of the Kuiper Belt. This meant that instead of the Solar System comprising 9 planets, it now had 8. However, recent research suggests that there could once again be (at least) 9 planets in the Solar System.

One of the most powerful methods for finding new bodies is through careful study of the orbits of known bodies. As Isaac Newton proposed, every piece of material in the universe pulls at every other piece of material with a force we call “gravity”. The more massive the lump of material is, the stronger its gravity, and the further you are from a body, the weaker its gravity pulls at you. Even though we still do not really know what gravity actually is, we know precisely what it does and can calculate its effects.

The Sun is by far the most massive object in the Solar System, so its gravitational pull on the planets plays the dominant role in determining their orbits. The gravitational tugging the planets exert on each other is much smaller. However, it can still be measured. We can look for small perturbations in what would otherwise be a simple orbit around the Sun, and then calculate the mass and position of the body that could be causing them. This is how discrepancies in the position of Uranus led to the discovery of the planet Neptune.

Thanks to the large telescopes now available, we can measure the motions of those distant Kuiper Belt Objects. This is exactly what astronomers at the California Institute of Technology have been doing, and they discovered that something is perturbing the orbits of some of the Kuiper Belt Objects. These measurements are really difficult.

The objects are very faint, and at their large distances from the Sun, they move very slowly in their orbits. However, the measurements suggest the existence of a planet about ten times the mass of the Earth, and lying about 90 billion kilometres from the Sun, compared with Pluto’s roughly 6 billion kilometres. If it does exist, the surface of the new ninth planet would be gloomy and very cold. The Sun would just be a very bright star in its sky. Of course, to confirm the existence of the new ninth planet, we have to see it, which will require searching for it. However, the prospect of discovering a new planet will ensure no shortage of people trying to find it.

The objects are very faint, and at their large distances from the Sun, they move very slowly in their orbits. However, the measurements suggest the existence of a planet about ten times the mass of the Earth, and lying about 90 billion kilometres from the Sun, compared with Pluto’s roughly 6 billion kilometres. If it does exist, the surface of the new ninth planet would be gloomy and very cold. The Sun would just be a very bright star in its sky. Of course, to confirm the existence of the new ninth planet, we have to see it, which will require searching for it. However, the prospect of discovering a new planet will ensure no shortage of people trying to find it.

Such groupings of the planets are quite rare. However, they are just sights to enjoy. There will be no significant consequences of all of them pulling at us from more or less the same direction.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355
E-mailken.tapping@nrc-cnrc.gc.ca

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