My Very Educated Mother Just Served Us Nothing: The Debate over Pluto’s Loss of Planetary Status


Pluto: No longer one of the big boys

The demotion of Pluto from planet to ‘dwarf planet’ status in 2006 caused an outcry in both the general and scientific communities. Children who had previously learnt the order of the planets with the mnemonic ‘My Very Educated Mother Just Served Us Nine Pizzas’  wrote letters to those who instigated the change demanding to know why their favourite planet was taken away. The reason for the change was because Pluto didn’t fit in with the rest of the planets and there were more and more like it being found beyond Pluto. Neil deGrasse Tyson, one of the public figures of the debate said:

 “What we found in the mid-90s was that researchers were discovering new objects in the outer solar system, objects that were small like Pluto, icy like Pluto, with elongated orbits like Pluto…we thought to ourselves, maybe it’s not that Pluto was the 9th planet. Maybe Pluto was the first object of a new class of objects that populates this outer zone in the solar system.”

Though the reasons make sense, the method of the demotion was questionable. The vote to demote Pluto was taken on the last day of the International Astronomical Union’s (IAU) General Assembly. The problem is that by this time only 424 of around 10 000 astronomers remained to take the vote. How can a vote taken by only five per cent of its constituents be deemed fair?

One of the reasons that this was, and still is such a contentious issue is because there was no technical definition of a planet before 2006. “It became clear that scientists do not have a widely accepted or clear definition of what a planet is” said Gibor Basri and Michael E .Brown, in a paper affliated with CERN (European Laboratory for Particle Physics). A planet was deemed a planet if those in the know agreed it to be. Seems pretty unscientific? Many will agree with you.

A planet is now defined as

 “a celestial body that is in orbit around the Sun, has sufficient mass… so that it assumes a …nearly round shape, and has cleared the neighbourhood around its orbit.”

according to the IAU definition. This was one of the great issues with Pluto, because its path crossed with that of Neptune.

Pluto is a strange little thing, with its eccentric orbit and a moon half its size: it’s obvious that it is very different from the other planets. It is understandable why scientists would wish to reclassify it as something other than a planet. Tyson’s point is a valid one. Pluto may be the first in a long string of icy dwarf planets. Then we would have: The ‘terrestrial planets’ (Mercury, Venus, Mars and Earth) that consist mainly of rock and metal and have a solid surface. The ‘gas giants’ (Jupiter, Saturn, Uranus and Neptune) which are far larger with a thick atmosphere consisting of hydrogen and helium; in the case of Jupiter and Saturn the gas makes up most of the planet with only the core being solid. And then, the icy, eccentrically-orbiting little Plutos of our galaxy.

So should Pluto be deemed a planet or not? In reality, we do not know enough about the nature of our galaxy to say whether it come under the same definition as the current planets. There are many good reasons against Pluto being a planet, but a vote cast by fiver percent of the voting group does not sound very democratic. It will likely continue to be a contentious issue for a while yet.

Whatever the outcome, the universe is an interesting place and I, for one, can’t wait to read up and find out more.




This is an edited assignment submitted for grading at The University of Western Australia

Definition of a planet from herefrom Neil deGrasse Tyson here and photo from here.

Article header is a CRIRES model-based computer-generated impression of the Plutonian surface, with atmospheric haze, and Charon. Sourced from Wikipedia.


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