Ordinary Meeting, 2006 October 25

Reclassification of Pluto

Mr Dymock explained that two of the resolutions passed at the General Assembly of the IAU in Prague on August 14-25 had concerned the classification of solar system bodies – numbers 5 and 6. Resolution 5 prescribed that such bodies should be divided into three categories. The first of these, 'planets', contained those objects which satisfied three requirements – that they (a) were in orbit around the Sun, (b) were of sufficient mass for self-gravity to mould them into 'round' shapes, and (c) had cleared all material from the neighbourhood of their orbits. Mr Dymock remarked that (c) seemed somewhat vague – what degree of clearance was required? No doubt, with increasing study of extrasolar planets in years to come, this would be a debate for the future. In the meantime, there were eight confirmed 'planets': the traditional list, minus Pluto.

The second category of Resolution 5 was 'dwarf planets', members of which had to satisfy (a) and (b) above, but did not have to have cleared the neighbourhoods of their orbits. Thirdly, 'small solar system bodies' were all other solar-orbiting bodies, excluding artificial satellites.

Resolution 6 referred specifically to Pluto, determining that it should be classified as a dwarf planet, adding that it was the prototype of the set of trans-Neptunian bodies. The speaker pointed out that this second resolution seemed largely superfluous; Pluto's classification was already clear from the previous resolution.

Returning to Resolution 5, the speaker added that the category of dwarf planets would include a wide range of bodies in the outer solar system, including plutinos – objects just outside Neptune's orbit, and, like Pluto, locked into a 2/3 orbital resonance with it – trans-Neptunian objects, Classical Kuiper Belt Objects (KBOs), Scattered Disc Objects (SDOs), and some of the larger members of the Asteroid Belt such as Ceres. Such sub-categorisations would not officially be recognised by the IAU; these objects would officially be classed only as 'dwarf planets' or 'small solar system bodies'. As far as the speaker was aware, however, no definitive list of candidate dwarf planets had yet been published, though several unofficial lists could be found on the web.

To close, the speaker remarked that the name of his Section was no longer entirely accurate; it should surely be changed to 'The Small Solar System Bodies (Asteroids), Dwarf Planets and Pluto-Like Objects Section (SSSB(A)DPPLOS)'. He suspected the name wouldn't catch on, though.

Following applause, a member asked why Pluto's moon Charon was on the list of bodies which might be classified as dwarf planets, given that it was in orbit about Pluto, not the Sun. Mr Dymock replied that it was unclear what decision would be made here. Charon's size was sufficiently similar to that of Pluto that the system might be classified as a binary pair of dwarf planets. As yet, there was no definition which could be used to distinguish binary planet pairs from planet-moon pairs – scope for more IAU legislation, perhaps.

After thanking Mr Dymock for his clear account of the matter, the President invited the evening's final speaker, Mr Nick James, to become the first Sky Notes speaker of the post-Mobberley era.