Stop the presses, there are now only 8 planets!

Thursday, August 24th, 2006 at 12:25 PM by Dave

Well, the IAU has changed their minds. We now have 8 Planets!

The International Astronomical Union (IAU) has changed their minds on the topic of planets. They initially proposed 12 planets but now have decided that there are 8 “Classical” Planets consisting of Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

The IAU resolution 5A states, “Comtemporary 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 of “planet”. The word “planet” originally described “wanderers” that were known only as moving lights in the sky. Recent discoveries lead us to create a new definintion, which we can make using currently available scientific information.

“The IAU therefore resolves that “planets” and other bodies in our Solar System be defined into three distinct categoreis in the following way:

(1) A “planet” is a celestial body that
(a) is in orbit around the Sun,
(b) has suffiecient 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.

(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) shape,
(c) has not cleared the neighbourhood around its orbit, and
(d) is not a satellite.

(3) All other objects except satellites orbiting the Sun shall be referred to collectively as “Small Solar-System Bodies”.”

Resolution 6A states, “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 (TNO).”

(I always thought there were 8 planets! - Read my previous posts.)

Kids, have fun learning the planets (and the other newly defined objects)!

Posted in Astronomy News

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