Pluto in the doghouse

This article was published on the Guardian’s Comment is Free website on 30 August 2006.

Please note that the Gulf Daily News republished this article without permission, and in clear violation of its syndication agreement with the Guardian. The Gulf Daily News refused to pay a retrospective syndication fee, and continued to host the article on its website until September 2008. The Guardian legal department declined to pursue the matter.

At at lively meeting of the International Astronomical Union (IAU) held recently in Prague, representatives of the world’s space scientists finally settled on the definition of a planet, and in the process demoted Pluto to the status of “dwarf planet”. That means school textbooks will have to be rewritten, I may get out of it some writing and editing commissions, and astrologers will look more stupid than one would have ever thought possible.

Pluto was the last ‘planet’ to be discovered. Astronomers in the late 19th century had predicted that a massive body was to be found beyond the orbit of Neptune, owing to peculiarities in the latter’s orbit. In February 1930, while comparing photographs taken during the previous month using a telescope at the Lowell Observatory in Arizona, astronomer Clyde Tombaugh found an object in the area of sky predicted by his slide rule-wielding colleagues, and the discovery was confirmed the following month.

But one problem with this everyday story of sky-gazing folk is that Pluto is far too small to cause the predicted effect on Neptune’s orbit; another is that astronomers had miscalculated Neptune’s mass, leading to an error in Pluto’s predicted orbit. Given the combination of errors, it’s all the more surprising that Clyde Tombaugh discovered the body, as its presence in the region of sky predicted was pure coincidence. This goes to show how science proceeds often by trial and error, with the emphasis on error.

What is Pluto? The little we know is that Pluto is a round, icy body with an extremely thin nitrogen and methane atmosphere, and a diameter of just over 2300 km. This is 18% of the Earth’s diameter, and makes Pluto smaller even than our humble Moon. Pluto has a highly elliptical orbit that extends some 50 times the distance between the Earth and the Sun, and so for part of its orbit is closer to the Sun than Neptune. Pluto also has a moon, Charon, over 1200 km in diameter, and a couple of other minor satellites.

Another thing that sets Pluto apart is that it orbits the Sun in a plane offset 17 degrees from that of the other planets. It was thus never a full member of the club, accepted by astronomers as a true planet. What allowed Pluto to be accepted, albeit reluctantly, as a planet was the scientific community’s inability until yesterday to settle on the strict definition of a planet. The astronomers may now have got their act together, but it could take a long time for the IAU decision to filter through and be accepted by the masses.

Not that this matters much in the wider scheme of things, but the reclassification of Pluto could focus scientific studies of the solar system, and raise the relative importance of its smaller bodies.

For example, in the asteroid belt between Mars and Jupiter orbits Ceres, which could have become the fifth planet had the IAU decision gone another way. And beyond the orbit of Neptune exists the Kuiper Belt of icy planetoids, of which Pluto is now but one of many. Some are larger even than Pluto, and when 2003 UB313 (nicknamed “Xena”) was discovered a few years ago, Pluto’s position as a planet became untenable, and something had to be done.

So what did the squabbling astronomers actually do yesterday that resulted in Pluto’s demotion? They finally accepted that a planet must not only orbit the Sun and be large enough settle into a spheroid shape, but it must also have cleared its orbit of other objects. Given the peculiar nature of Pluto’s orbit, it fails the third test of planetdom.

So, there you have it; Pluto is finally assigned its proper place in the firmament. Now, if the ex-planet’s “energy” is, according to astrologers, “…subtle, but its results will hit you like a ton of bricks.”, what will they have to say about 2003 UB313? I can hardly wait.