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Resolution on the Definition of a Planet
At the second session of the 26th General Assembly which was held at 14:00
Thursday August 24 in the Congress Hall in Prague, members of the IAU voted on the
resolutions presented here. There were separate sequential votes on Resolution 5A and
Resolution 5B. Similarly, there were separate votes on Resolutions 6A and 6B.
Resolution 5A is the principal definition for the IAU usage of planet
and related terms.
Resolution 5B adds the word classical to the collective name of the
eight planets Mercury through Neptune.
Resolution 6A creates for IAU usage a new class of objects, for which Pluto
is the proto-type.
Resolution 6B introduces the name plutonian objects for this class.
The Merriam-Webster Dictionary defines plutonian as:
Main Entry: plu*to*ni*an - Pronunciation: plü-'tO-nE-&n - Function: adjective -
Usage: often capitalized -: of, relation to, or characteristic of Pluto or the lower world.
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IAU Resolution:
Definition of a Planet in the Solar System
Contemporary observations are changing our understanding of planetary systems, and it is
important that our nomenclature for objects reflect our current understanding. This applies,
in particular to the designation 'planets'. The word 'planet' originally described
'wanderers' that were known only as moving lights in the sky. Recent discoveries lead us
to create a new definition, which we can make usin currently available scientific information.
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Resolution 5A
The IAU therefore resolves that planets and other bodies in our Solar System be defined into
three destinct categories in the following way:
| (1) | A planet¹ 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) shape², and
(c) has cleared the neighbourhood around its orbit. |
| (2) | 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) shape²,
(c) has not cleared the neighbourhood around its orbit, and
(d) is not a satellite. |
| (3) | All other objects³ orbiting the Sun shall be referred to
collectovely as Small Solar System Bodies. |
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The eight planets are: Mercury, Venus, Earth, Jupiter, Saturn, Uranus, and Neptune.
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An IAU process will be established to assign borderline objects into either dwarf
planet or other categories.
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These currently include most of the Solar System asteroids, most Trans-Neptunian
Objects (TNOs), comets, and other small bodies.
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Resolution 5B
Insert the word classical before the word planet in Resolution 5A,
Section (1), and footnote 1. Thus reading:
(1) A classical planet¹ is a celestial body ...
and
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The eight planets are: Mercury, Venus, Earth, Jupiter, Saturn, Uranus, and Neptune.
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IAU Resolution: Pluto
Resolution 6A
The IAU further resolves:
Pluto is a dwarf planet by the above definition and is recognized as the prototype of a new
category of trans-Neptunian objects.
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Resolution 6B
The following sentence is added to Resolution 6A:
This category is to be called plutonian objects.
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For your convenience, you can download a
PDF file
which contains above resolution and first comments of some scientists
present at the General Assembly in Prague. Some are quite funny, or sad
depending on your views. Also scientists are only ordinary people.
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30 March 2020 (00:32)
Christian Stütz
Kurt Tichy Gasse
Wien
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