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Last updated 2008 September 24

 

New definitions for Solar System bodies

 

1)         Introduction

 

This is a slightly modified version of the article which was published in the December 2006 issue of the Journal of the British Astronomical Association. The presentation given at a BAA Ordinary Meeting can be found here.

 

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 (ie; asteroids and comets)

 

2)         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'.

 

3)         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.

 

4)         Planets

 

The Planets are the major bodies in the Solar System as known prior to March 1930 when Pluto was discovered ie; 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.

 

5)         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.

 

Name

Number/Name

Group

Diameter (km)

Comments

Ceres

1

Main Belt

942

 

Pluto

134340

EKBO/TNO -Plutoid

2306

 

Eris

136199

(2003 UB313)

EKBO/TNO - Plutoid

2400

 

Haumea

136108

(2003 EL61)

EKBO/TNO

1500

Outer and inner moons have been named Hi’ika and Nakama respectively

Makemake

136472

(2005 FY9)

EKBO/TNO - Plutoid

1540 (approx)

 

 

6)         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.

 

Plutinos are out and Plutoids are in !!! See IAU news release for full details

 

7)         What of the Asteroid and Remote Planets Section ?

 

Should we rename it 'Small Solar System Bodies (Asteroids), Dwarf Planets and Pluto-Like Objects Section' - SSSB(A)DPPLOS ? I think I will let the dust settle before making that decision !

 

Well at least we can drop the ‘LO’ (see above) and the ‘P’ now stands for ‘Plutoids’.

 

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