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How many planets are there in our Solar System? Nine, you think. Yet, the body that regulates the naming and classification of astronomical objects, the IAU, is currently debating whether there should be more or less than nine. All this stems from the status of Pluto and how we differentiate a planet from the other space rock floating around the solar system.
The field of astronomy that I work in is called Star Formation and that’s closely related to the field of Planet Formation. The broad paradigm we’re currently working under is that the sun formed when a rotating cloud of gas collapsed along its axis of rotation to form a disc. The small proto-sun was at the centre of that disc and gas spiraling inwards from the disc was used feed its growth. After the Sun has finished forming the scattered rocky remnants of the disc started to bumb and stick together forming proto-planets. Some of those rocks become big enough to capture atmospheres from the remaining gas in the disc. This is a very violent era with proto-planets crashing into each other or being thrown out of the disc.
However, eventually things begin to settle down to a more recognisable state. Most of the mass is consolidated into eight major planets all orbiting in the what use to be the plane of the disc. There is still a fair amount of “builders rubble” left, but this is mainly left at the edge of the solar system in a massive extended shell of material called the Opik-Oort Cloud (which is where comets come from) and in a belt of larger objects called the Edgeworth-Kuiper Belt. The largest object in the Edgeworth-Kuiper Belt and the only one we were able to detect until recently is the object we call Pluto.
Pluto was discoverd in 1930 by American astronomer Clyde Tombaugh and was added to the text books as the ninth planet. As we’ve come to understand Pluto’s place in the solar system there have been calls for it to be down graded from planet status and reclassified as just a Edgeworth-Kuiper Belt object. That has been constantly rejected by more historically minded astronomers. But it does create the problem that if Pluto is a planet then there are seveal other objects in the solar system which really should be considered as candidates for planet status.
Personally I think this is all a waste of time and man power. The latin root of the word Planet means “to wander” and that is pretty much the classical definition of a planet. They are objects in the night sky that appear to move, to wander, relative to the fixed pattern of background stars. Astronomy, like taxonomy, has always been a science fixated with the classification of things and we’ve constantly been searching for a more technical definition of a planet. However, we fail at every step and even if the IAU adopts the new scheme there will be legions of astronomers who will disagree or ignore it. And that’s side from the fact that American astronomers would never allow the only planet to be discovered by an American to be taken away from them
The solution isn’t to reclassify Pluto as that just confuses the public and vexes small children. Better to leave it where it is and use it as an excuse to teach people about the Kuiper Belt and the smaller members of our solar family.
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