The Fallen Gods of the Celestial Spheres

Owen Gingerich, Cambridge, MA (May 27, 2009)

How many planets are there in the Solar System? Cinques stellas errantes, noted the german naturalist Alexander von Humboldt, drawing from the ancient greek tradition of the ἀστέρες πλανῆται. Six, would say Copernicus; do not forget the one you are standing upon. Seven, added Herschel, as this bluish comet he was observing was definitely failing to develop a tail, and could only be Uranus, the father of Saturn, in turn father of Jupiter, ruler of the gods and father of everybody else. 

Wes Traub

That was however not to last. Take a large piece of paper and plot the distance of each planet with respect to the solar system center. A XVII century astronomer will notice that the space between the planets is somewhat regular, doubling as you go from Mercury to the farthest globe revolving around the Sun. With one exception: there is a large gap between Earth and Mars, a missing planet to hop through on your way to the red demon of war. Giuseppe Piazzi found it, Cerere Ferdinandea, Ceres for the ages. It was not alone: Pallas was found next, and then Juno, and finally Vesta, all sharing the same gap between Earth and Mars in such close orbital proximity that repulsed reason. "Two stars keep not their motion in one sphere", chimed in King Henry in Shakespeare. But they kept coming, the small planets and a new large one, found on the opposite end of this magic number sequence. Neptune god of the sea, the farthest of them all. Eleven planets and counting, but as small ones kept being discovered, inexorably crowding this oasis-belt separating us from the parched canal-builder martians, a reform was in order. Herschel himself proposed to call these solar system gnomes as asteroids, "star-like" wandering objects. Ceres and company were demoted, and the asteroid belt was born. It is estimated to contain almost 2 million bodies, of which Ceres is the largest member. The number of planets regressed back to eight, with great relief of all schoolchildren around the world, saved from having to memorize an ever increasing list of orbs with funny mythological names.

Menkind proposes but nature disposes... and in 1930 Clyde W. Tombaugh, "astronomer, teacher, punster and friend" discovered a new planet, farther away than even lonesome Neptune. He called it Pluto, after the suggestion of 11 years old Venetia Burney that decided that the roman god of the underworld would not mind to roam at such distance from the Sun. The nine planets were born, as well as a yellow canine, pet of a famous mouse and his goofy doglike friend (a Disney marvel of a dog which is pet of another dog).

Everybody knows that this was not the end of the story. Pluto is distant, but not alone. Chiron was discovered next, and then Pholus, and then others occupying the empty space just inside the orbit of Neptune. They were named Centaurs and nobody dared to call them planets in fear to upset once more the celestial spheres. But then come the cubewanos (after 1992 QB1, left nameless for having exhausted the greek Olympus) just outside Pluto, and then the plutinos (sharing orbits with Pluto), and the twotinos even beyond the cubewanos... The whole Kuiper belt, 20 times as wide and 200 times heavier than the asteroid belt, came into existence, debris of an inchoate solar system that never coalesced into proper planets. Some Kuiper belt body are large: Eris even larger than Pluto, with Makemake, Sedna, 2007 OR10, Quaoar and Orcus following suit. Eris is the goddess of chaos, strife and discord, and true to her name immediately plotted to unset the smaller Pluto from its undeserved planetary status.

David Charbonneau

And so it was. On August 24, 2006 the International Astronomical Union (IAU) voted to formulate a revised definition of planet, trying to limit this status to the larger bodies in the solar system. The winning proposal defined a planet as a spherical body orbiting the Sun, having sufficient mass to clear its orbits from other objects. Ceres in the middle of the populous asteroid belt, and Pluto as just one of many Kuiper belt objects, were not satisfying this definition, and were demoted once and for all to the class of dwarf planet. This somehow caused consternation for multitudes of school children around the world, that started to send hate-mail to the poor astronomers involved in the decision. The state legislature of Illinois even voted for Pluto to remain a planet, albeit to the same effect of the state of North Carolina when it voted global warming out of existence by legislative decree. If you read this, please don't be nasty, astronomers are nice people too and Pluto likes to be the king of the Kuiper belt, rather than the runt of the solar system. 

Pluto will be visited by the New Horizons spacecraft later this year. Launched on January 19, 2006, the probe has just been roused from hibernation as it approaches Pluto, for a two-weeks data-gathering flyby at the end of a 9 years trip. After Pluto, New Horizons will continue its voyage through the Kuiper belt, visiting another of its inhabitants, if one will be found within reach of the path that can be accessed with the remaining fuel onboard the spacecraft. New Horizons carries a few ounces of the ashes of Clyde Tombaugh, that died 10 years before the spacecraft launch, when Pluto was still a planet, and the goddess of discord had yet to uphend the orders in the celestial spheres.

The photo above shows Owen Gingerich, Chairman of the International Astronomical Union's Planet Definition Committee, at the time of the Pluto discussion. I took the photo during the same event I describe in the previous post. The two smaller photos show Wes Traub and David Charbonneau, hunters of extrasolar planets. On this respect, if you think that 8 planets in the solar system are not enough, console yourself with almost 2,000 planets already discovered orbiting over 1,000 stars other than the Sun: these are big objects and are not at risk to be demoted by a hasty IAU. Their names are kind of boring (resembling more a telephone number than a deity) but this will also change soon: the IAU decided to delegate the naming of extrasolar planets to the public. Despite the competition being open to amateur astronomers from the whole world, it appears that japanese groups were very efficient in registering for this task, and secured over one third of the total naming rights. Fortunately Japan has thousands of gods in its Olympus, so we should not run out of names, at least for a while! 

Not just planets: the arrow points to comet Lovejoy, promise (Ames, IA, Jan 23, 2015)