Jan 5, 2007
Well, no, it is, and it isn’t. It all depends on your perspective and what you feel about there being more than 20 planets in the Solar System rather than the more usually seen 9.
David Weintraub takes us on a cosmic tour through the history books from Aristotle’s logical fallacies of aether and perfect spheres moving in perfect circles to the discovery of dozens upon dozens of shapely and shapeless objects littering the once perfect heavens.
On 24th August 2006, as reported on Sciencebase (and everywhere else, admittedly), the International Astronomical Union decreed that Pluto should be demoted to the status of dwarf planet. After all, it’s discovery was a pure accident, it shouldn’t really have been spotted where it was in the 1930s at all, and it’s just so small, and really just the biggest of what we now refer to as the Kuiper Belt Objects.
However, there is no scientific reason to label Pluto as “not a planet”.
In one sense, Weintraub’s argument hinges on the fact that we cannot define what is and what is not a planet on the basis of a mnemonic taught to science students – My Very Earthly Mother Just Served Us Nasty Pizza.
Space is far more messy than that. Between Mars and Jupiter, where earlier astronomers hoped to find a planet that fit the now debunked Titius-Bode rule (which never quite became law), we find some startlingly large asteroids instead, among them Ceres. Then there is Eris (formerly known as UB313 and colloquially as Xena), and a myriad swarm of Kuiper belt objects, trans-Neptunian object, Oort cloud objects…
The list goes on. But, in the final reckoning is it for us to draw lines and say such and such an icy rock whirling around the sun billions of miles from earth is any more planet than the next chunk of ice and rock.
Pluto looks like a planet, moves like a planet, and quacks like a planet. Obviously that last one isn’t quite right. But, it’s not a planet like the inner planets, it’s not a gas giant, and it’s not like an asteroid, which would have been much more appropriately named planetoids rather than being labelled literally as “star-like”.
Weintraub anticipates that there will be no problem for the young, upcoming astronomers to simply add qualifiers to all the different kinds of planet we find. Nothing will be less alien than terms such as giant, terrestrial, icy, pulsar, belt-embedded prefixing the word planet and allowing is to create a sophisticated taxonomy that allows us to understand the nature of the universe around us.
It will make for an unwieldy mnemonic with our Earthly Mother having to add all kinds of toppings to that Nasty Pizza to make it stick. But then planets are intrinsically unwieldy.