Saturday, 18 November 2017

There Are Nine Planets in Our Solar System


CAPT Anni Potts


But are there? It may be what a great many of us were taught at school, but is this true? The simple answer is no … well, probably no. It's a bit of an ongoing argument and quite a controversial one, too, but the general consensus is no.
Of the original nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto, it is the latter that had its planetary status repeatedly argued, usually because of its size. Finally, in August 2006, the International Astronomical Union sat down to debate the issue and decided upon three criteria to which any object must comply in order to be considered a planet.
  1. A planet has to orbit the Sun.
  2. A planet needs enough gravity to pull itself into a sphere.
  3. A planet needs to have cleared its neighbourhood (or orbit) of other objects. That means it must be gravitationally dominant and that there are no other bodies of a comparable size in its orbit other than its satellites.
On that basis, of the traditional nine planets, eight of them met those criteria. However, Pluto failed on the third because its mass is only 0.07 times that of the mass of the other objects in its neighbourhood. (In other words, there's a lot of crap in its backyard.) As a result, it was it was relegated to being a dwarf planet instead.
At that same meeting, astronomers examined many of the other bodies in our solar system and reclassified them too. As a result, we now officially have eight planets and five dwarf planets in our solar system, but even that may change.

Dwarf Planets

Eris is the largest of our dwarf planets, although some sources say it is slightly smaller than Pluto. Discovered in 2005, because it was deemed larger than Pluto, it was considered to be the tenth planet for a while. Eris is particularly fascinating because of its orbit which is not 'round' but elliptical and on a very different plane to the rest of the planets and dwarf planets in our solar system.
Pluto is next in size and the most renown of the dwarf planets. Then comes Haumea was only recently discovered (in 2004). It is about a third the size of Pluto and only just has enough gravity to keep itself from falling apart. However, despite its size, it does have its own moons.


Ceres (discovered in 1801) is the smallest of the dwarf planets and was previously categorised as an asteroid. (In fact, it was the first asteroid ever discovered.) Like most of the dwarf planets, it's an icy world and it lies between the orbits of Mars and Jupiter. It may also have an ocean buried under its ice.
Makemake is another fairly recent discovery (2005) and is about two-thirds the size of Pluto and deosn't appear to have any moons. With a surface temperature of about −243.2°C, it is covered with methane, ethane, and possibly nitrogen ices.
Ceres (discovered in 1801) is the smallest of the dwarf planets and was previously categorised as an asteroid. (In fact, it was the first asteroid ever discovered.) Like most of the dwarf planets, it's an icy world and it lies between the orbits of Mars and Jupiter. It may also have an ocean buried under its ice.

However, that's not the end of it because there are hundreds of other known bodies in our solar system that could also be dwarf planets. So in answer to the question, how many planets, dwarf or otherwise, are there in our solar system, the answer is that we don't really know.

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