Last updated 2018 November 27

Introduction

This is a slightly modified and updated version of the article which was published in the December 2006 issue of the Journal of the British Astronomical Association.

The XXVIth General Assembly of the International Astronomical Union was held in Prague 14th to 25th August 2006. Two resolutions were passed, but not without considerable discussion, relating to planets, asteroids and comets. The outcome of these resolutions is that the Solar System is now made up of Planets, Dwarf Planets and Small Solar System Bodies (i.e.; asteroids and comets)

Resolution 5

A planet is a celestial body that;

(a) is in orbit around the Sun
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
(c) has cleared the neighbourhood around its orbit

A dwarf planet is a celestial body that;

(a) is in orbit around the Sun
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
(c) has not cleared the neighbourhood around its orbit
(d) is not a satellite

All other objects, except satellites, orbiting the Sun shall be referred to as ‘Small Solar System Bodies’.

Resolution 6

Pluto is a dwarf planet and is recognised as the prototype of a new category of Trans-Neptunian Object. An IAU process will be established to select a name for this category.

Planets

The Planets are the major bodies in the Solar System as known prior to March 1930 when Pluto was discovered i.e.; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Jupiter might appear to fall foul of Resolution 5, item (c) as it has not cleared its orbit of its Trojan asteroids ! However due to its dominant size compared with such bodies and the chances of that planet colliding or capturing such objects being small, Jupiter retains its classification.

Dwarf Planets

Objects included in this category are the larger asteroids in the Main and Edgeworth-Kuiper Belts and are listed in the table below possible candidates include; 2 Pallas, 4 Vesta, 10 Hygiea, 90482 Orcus, 90377 Sedna and 50000 Quaoar. All such asteroids have a diameter of greater than 200 km but no minimum size has yet been quoted by the IAU.

The total number in this category may be considerable. Data from the Infra Red Astronomical Satellite (IRAS) indicated that there were 30 Main Belt asteroids with diameters exceeding 200 km. Astronomer Dave Jewitt has estimated that there are 70,000 Trans-Neptunian Objects with diameters greater than 100 km.

Pluto-like objects

Such objects are currently referred to as Plutinos so the need for this resolution was not at all clear. Plutinos are in a 3:2 resonance with Neptune in that they complete 2 orbits for every 3 made by that planet. The first Plutino to be discovered, by Jane Luu and Dave Jewitt, was 1993 RO. Other examples are; 1996 TP66, 1993 SB, 1995 HM5 and 1996 TQ66.

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