The cancelling of Pluto

 

It used to be a planet. Then in 2006 it was relegated and is now called 134340 Pluto. Our chief scientist looks at the story of the little (former) planet that caused a big fuss. 

In 2006 there was a right hoodoo happening up there in the Milky Way because boffins have been re-drawing the planets and the results of their work mean that what most of us have been taught- that there are nine planets in our solar system- is now wrong. Without recourse to a fleet of spaceships or even a nifty destructor ray scientists have destroyed Pluto, at least as far as its planetary status is concerned. In late August, a new definition of a planet was approved by a seemingly self-appointed clique of scientists with presumably nothing better to do.

It all happened at the grandly monikered General Assembly of the International Astronomical Union (AIU) held in Prague. 424 astronomers (who knew there were even that many?) voted on the definition after the sort of horse trading often seen at the United Nations over more serious matters. An initial proposal by the AIU’s planet definition committee chaired by Owen Gengerich, would have added three more planets to the roll call, perhaps assuming that Pluto’s position was unassailable. The suggestion was based on the fact that these bodies were the same size or larger than Pluto, one of which, the modestly named 2003 UN313 had been hailed as “the tenth planet” due to being slightly bigger whilst the other two new planets would have been the asteroid Ceres and Charon, in a promotion from being one of Pluto’s moons. This proposal caused a furore and after several days wrangling four alternative proposals were put forward.

Eventually the decision was taken that would relegate Pluto to the status of “dwarf planet”. This was based on several criteria including that a planet “must have cleared the neighbourhood around its orbit” which reflects the fact that larger objects either aggregate or fling away material in their path; Pluto fails this as its orbit overlaps that of Neptune. Unsurprisingly this caused an outcry and led to accusations that the vote was rigged; it was claimed that only 10% of the astronomers attending the event were able to vote; “you can’t even claim consensus” fumed the US space agency’s Dr Alan Stern. Even Gengerich was unable to vote as he had to return home and it was claimed that the vital vote had been scheduled deliberately at a time when people would already have left.  Whatever the circumstances Pluto had been sacked at the age of 76.

Pluto was discovered in 1930 by Clyde Tombaugh of Lowell University in Arizona. Calculations which later turned out to be in error had predicted a planet beyond Neptune based on the motions of that planet and Uranus and unaware of the error, Tombaugh undertook a survey which pinpointed Pluto The name comes from Roman mythology in which Pluto is the god of the underworld and was selected for the new planet because it is so far from the Sun as to be in perpetual darkness. Pluto's orbit is in fact highly eccentric. At times it is closer to the Sun than Neptune and rotates in the opposite direction from most of the other planets. Pluto's orbital period is exactly 1.5 times longer than Neptune's and its orbital inclination is also much higher than the other planets'. Thus though it appears that Pluto's orbit crosses Neptune's, it really doesn't and they will never collide.

Clyde Tombaugh: "I think I've found something..."

The former planet is 5,913,520,000km from the Sun (i.e. a long way), has a diameter of 2274km and a surface temperature that varies between about -235 and -210 C (38 to 63 K). Not recommended for holidays then! Its composition is unknown, but its density indicates that it is probably a mixture of 70% rock and 30% water ice. The bright areas of the surface seem to be covered with ices of nitrogen with smaller amounts of (solid) methane, ethane and carbon monoxide. The composition of the darker areas of Pluto's surface is unknown but may be due to primordial organic material or photochemical reactions driven by cosmic rays.

Pluto's atmosphere is also, as you’ve probably guessed by now, a bit of a mystery but probably consists primarily of nitrogen, some carbon monoxide and methane and is extremely tenuous, the surface pressure being only a few microbars. Pluto's atmosphere may exist as a gas only when Pluto is near its perihelion (the point at a planet’s orbit when it is nearest to the Sun). For most of Pluto's long year, the atmospheric gases are frozen into ice and near perihelion, some of the atmosphere escapes to space perhaps even interacting with Charon.

To try and gain more solid facts than all this supposition the first ever spacecraft to Pluto was launched in January 2006 with the intention of arriving in 2015 and presumably the recent change in status will not mean it is called back! Some mapping of Pluto has been achieved thanks to a satellite called Charon discovered in 1978 just before its orbital plane moved edge-on toward the inner solar system. It was therefore possible to observe many transits of Pluto over Charon and vice versa. By carefully calculating which portions of which body would be covered at what times, and watching brightness curves, astronomers were able to construct a rough map of light and dark areas on both bodies. In 2005, a team using the Hubble Space Telescope discovered two additional tiny moons called Nix and Hydra that are estimated to be between 60 and 200 kilometers in diameter.

The allure of Pluto as an exotic and mysterious place about which speculation can run riot has made it an understandable magnet for science fiction writers with many books and stories involving Pluto. Amongst the myriad of examples are Stephen Baxter’s 1997 story Gossamer in which stranded astronauts discover a life form on Pluto during perihelion. Larry Niven’s Wait it Out has its protagonist trapped on Pluto where he discovers a super fluid form of life. Robert Heinlein seemed to love the place; his 1958 novel Have Space Suit, Will Travel showed it as an alien base used for exploration of Earth while Starship Troopers included a research station on Pluto. He also included the place in a 1953 story called Sky Lift. John De Chancie’s 1980s series of Starrigger books had Pluto as the location of a dimensional gate to an interstellar Skyway whilst Kim Stanley Robinson’s 1985 novel Icehenge centres around a mysterious structure found on the planet, oops dwarf planet. More recently Vaccum Diagrams written by Stephen Baxter had a portal in the orbit of Pluto and suggested a form of life there that is a bit like a snowflake.

Some TV sci-fi has plundered Pluto too; Doctor Who got in on the act in 1977 with The Sunmakers which had the place covered in colonies lit by artificial suns that made it look remarkably like England while in Futurama the place was inhabited by penguins! Back in the 1960’s Space Patrol had an episode in which there was a colony on Pluto where conditions were freezing which begs the question of why they settled there at all! Music-wise of course there is the tale of The Planets, Gustav Holst’s well known classical smorgasbord composed a decade before Pluto was discovered. In the late 1990s, the Halle Orchestra took it upon themselves to commission Colin Matthews to add a Pluto movement, entitled `Pluto- The Renewer` which had its first performance in 2000 and which presumably won’t be getting many more airings.

Despite the controversial vote, Pluto does have its allies and after the AIU’s decision, some astronomers were fuming; Gengerich put the blame for the decision on `dynamicists` -astronomers who are experts in the motion and gravitational effects of celestial objects. His definition was one favoured by planetary geologists and he reckoned it was all down to the dynamicists being insulted. “Its sloppy science,” said Alan Stern, “it’s inconsistent”. He pulled holes in the definitions claiming for example that Earth, Jupiter, Mars and Neptune have also not fully cleared their orbital zones, Jupiter in fact is accompanied by 100,000 Trojan asteroids on its rounds.  He added with a common sense that even non-scientists can grasp; “If Neptune had cleared its path, Pluto wouldn’t be there”. Stern started a petition to try and get Pluto reinstated

The arguments for the decision seem rather weak willed by contrast; Professor Iwan Williams the IAU’s president of planetary science commented with childish smugness; “Pluto has lots and lots of friends, we’re not so keen to have Pluto and all his friends in the club because it gets crowded.” His main argument seemed to be against having too many planets; “By the end of the decade we would have had 100 planets!” he rather overstated. Whatever the pros and cons, it seems that the change is here to stay and no amount of blustery statements and grave pullover clad pronouncements are likely to change minds.

By way of a postscript, that icy rock whose discovery started all the kerfuffle, our old friend 2003 UN313 was finally named by the IAU in September as Eris appropriately enough after a Greek goddess of chaos and strife. Amidst all the arguments it’s refreshing to see that there is still a sense of humour in the astronomical community.

This article was originally published in This Way Up fanzine in 2006. To date Pluto's status as a planet has not been restored.