Sunday September 3, 2006

Resolutions on planetary status spark debate

THE most significant outcome of the recently concluded 2006 International Astronomical Union (IAU) General Assembly undoubtedly concerned Pluto. Two resolutions were made, namely Resolution 5A and 6A.

Resolution 5A stated that objects in the solar system could be divided into three categories, namely a “planet”, “dwarf planet” and “small solar-system bodies” – the last of which excludes satellites (i.e., moons).

According to the new definitions, three criteria have to be fulfilled for an object to be called a “planet”.

Firstly, it has to be in orbit around the Sun. Secondly, it has to be massive enough so that its self-gravity is able to pull itself into a “hydrostatic equilibrium” or a nearly round shape. Finally, it must clear all other objects out of the way of its orbit.

Pluto passed the first two but failed on the last criteria mainly because it shares its space with a ring of icy debris known as the Kuiper Belt and its orbit intersects that of Neptune.

Resolution 5A’s definition of a “dwarf planet” includes the first two criteria for “planet”, but adds two others – if it does not clear objects around its orbit, and is not a satellite.

The definition of “small solar-system bodies” would be for objects orbiting around the sun, and that would include objects such as asteroids, most trans-Neptunian objects (i.e., bodies with an orbit beyond that of planet Neptune's own) comets, and other small bodies.

And finally, Resolution 6A – just in case 5A was not clear enough – drove the final nail in Pluto's planetary status and confirmed it as “dwarf planet”.

Perhaps some solace was provided for Pluto lovers when IAU recognised Pluto as a prototype by which it would classify any trans-Neptunian object.

However, the resolutions have raised much controversy and sparked further debate.

One of the reasons is that there were only 424 astronomers who voted, hardly a fair representation of astronomers worldwide.

More significant, however, is the fact that the third criterion is considered ambiguous, as planets such as Earth and Jupiter have asteroids and comets that regularly cross its path. It is also argued that by that same measure, Neptune should likewise be stripped of its planetary status because Pluto intersects with its orbit.

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