Today, August 24th, will go down in astronomical history as the day little, but still fascinating, Pluto and its three moons got kicked out of a now Gated Community. The Gate & Fence is a definition chosen to exclude any world that does not fit the pre-1930 demographics of the Solar Club.
To do this, the International Astronomical Union [IAU] took the definition chosen by its committee tasked with creating a definition of a “Planet”
“a body orbiting the sun that was big enough so that gravity would overcome internal forces and squash it into a roughly spherical shape.”
and added one more qualifier:
“a planet must also be massive enough to clear other objects out of its orbital zone.”
This last phrase was designed to keep out Ceres and Pluto and Xena.
The trouble with this standard is that
a) this clearing operation is incomplete. If it were complete, we would not be threatened by near Earth objects, NEOs.
b) most of what clearing process that has occurred took the better part of a billion years.
c) Mighty Jupiter and Neptune didn’t exactly “clear” their orbits, but shepherded the stragglers into Trojan (Lagrange 4 and 5) orbital positions, 60 degrees ahead and behind them in their paths around the sun.
The winners are those who have always resented Pluto, and wanted a definition that would forever keep the Planetary Club membership fixed.
The losers are the public. Elevation of Ceres, a mini-planet in its own right, preservation of Pluto’s status, and an open door to other ice planets beyond would have much better fostered public interest in the solar system and the universe in general.
With this sad step, we took one giant leap backwards towards the days when the only worlds were Earth, Heaven, and Hell, and all the rest were just lights in the sky.
Creating gerrymandered definitions that preserve club membership to the historic planetary demographics is a big mistake.
It would have been so simple to just create classes of planets:
a) the four rocky planets: Mercury, Venus, Earth, Mars
b) dwarf rocky planets: Ceres, Pallas, Vesta
c) the four gas giant planets: Jupiter, Saturn, Uranus, Neptune
d) the small outer icy planets: Pluto, Xena, and others yet to be discovered.
The contrived nature of the new definition is clear from the fact that the rocky planets and gas giants are two vastly different classes of worlds.
To keep club membership at eight, the definition has to be vague enough to embrace both small rocky worlds and larger gas giants, yet specific enough to exclude the dwarf rocky worlds and outer ice worlds. Gerrymandered.
If we have a definition that has the latitude to include both of these groups, then why not one a bit more general to include the other groups?
“It is a puzzlement”, said the King of Siam.
I don’t think this is the end of the story. The public repercussions of Pluto’s demotion will erode public interest in astronomy. After all, astronomers clearly do not have their act together, and are acting like an Old Boys Club.
Meanwhile, we are busy discovering more and more exoplanets - planets around other suns, and you can be sure, that the demographics of these systems will sooner or later force us to back off this “gated community” definition.
Oh yes, one more thing. The committee had proposed elevating Charon. This may seem absurd but you should not think that Pluto and Charon would have been planets separately. Indeed, I think that given that they both rotate around a common center of gravity well above Pluto’s surface, should classify them as “a binary planet system.”
So not ...... Neptune, Pluto, Charon, and Xena
but .... Neptune, Pluto-Charon, and Xena.
Now that would have been wonderful!
But I am even more disappointed that Ceres’ claim is not being honored. It is a world with enough gravity to force it into a spheroidal shape, and to stratify its material into layers, densest at the core. Clearly, Ceres and Gaspra or Eros are different types of objects. Yes, Ceres was too small to force order among the fragmentary objects in its orbital zone. But that does not change what Ceres is in itself.
Ceres is bound to play an important role if humankind ever ventures beyond Mars and into the realm of the outer gas giants. See “Ceres: largest asteroid or mini-planet” in MMM #196 June 2006, pp 4-6. It is high time that Ceres got the recognition due it both as a mini world in itself and as to its strategic potential in the saga of homo solaris.
It is a pity that the astronomers couldn’t see the wider picture. But horse blinders are an occupational hazard of any “specialized” profession. Who would have suspected that persons whose occupation is to explore the universe at large, could have such closed small minds.
No apologies. Sometimes, punches shouldn't be pulled.