CNN.com - Pluto gets the boot - Aug 24, 2006

CNN.com - Pluto gets the boot - Aug 24, 2006:

"PRAGUE, Czech Republic (AP) -- Leading astronomers declared Thursday that Pluto is no longer a planet under historic new guidelines that downsize the solar system from nine planets to eight. ..."

When Pluto was discovered in 1930 its planetary status was a bit borderline, but it was added anyway to the eight previously known planets of our Solar System.

Now those of us who memorized the planets as school children need to learn to stop at Neptune.

Gustav Holst composed his orchestral suite, The Planets, in 1918, so musical history will not need to be revised.

But what IS a planet? According to this CNN article, there has been no clear definition of planets since they were first discovered.

The International Astronomical Union, meeting in Prague, released their definition of a planet on August 24, 2006:

(1) A "planet"¹ 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, 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 except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".

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¹The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
²An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
³These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

Pluto (whose orbit intersects with Neptune's) met the first two criteria for planets, but inasmuch as it is not the only spherical object in the Kuiper Belt (or the region beyond Neptune), nor is it even the largest, it failed to meet the third criterion. The object UB313 ("Xena") is larger.

Owen Gingerich (Harvard University) was chair of the Planet Definition Committee. Gingerich is well-known in Christian circles and a quick Google search turned up a piece entitled Is the Cosmos All There Is? in which he discusses ways of looking at reality.

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