Now Its Official: Eight Planets (Pluto got bumped)

[Ouabache]

Some fascinating resolutions were made at this years'
IAU (international astronomical union) General Assembly 2006 Meeting

One that has caught our attention recently: definition of a "planet". Pluto is not amoung them. (instead it is placed in a new category: "dwarf planet" )

RESOLUTION 5A
The IAU therefore resolves that "planets" and other bodies in our Solar System be defined into three distinct categories in the following way:

(1) 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, 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) 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".

1-The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

2-An IAU process will be established to assign borderline objects into either dwarf planet and other categories.

3-These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.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.

 

[scott1]

We already have about 5 threads about this...

 

[dimensionless]

I'm no astronomer, but I don't like the new system. I think a planet should be anything that is rigid and has enough gravity to be round. This would mean that the sun has five planets:

Mercury
Venus
Earth
Mars
Ceres

It would also mean that the Earth had one planet and Jupiter, Saturn, and Uranus each have a mess of planets. All together the solar system has 20+ planets.

I'd say if it's really gassy and you can't stand on it, then it's not a planet. This means Jupiter and Saturn should get bumped from the list. I mean, how can we possibly put Mercury and Jupiter in the same category? Jupiter and Saturn are planetary systems just by themselves

I could also be persuaded of the view that only the gas giants are planets while Earth and many other familiar bodies are asteroids.

We could just dump the categorization system all together. It will never be perfect.

Edit:
The current system bumps planets if they have yet to clear their orbital path, or if they are not in a nice Earth-like orbit. In my opinion, this is the equivalent of saying that an electron is not an electron if it is not bound to a proton. If Saturn's moon Titan was orbiting the sun, it would definitely be called a planet. Titan has no intrinsic property that makes it a moon. It is thus my position that the current system makes no more sense than the previous one.

 

[DaveC426913]

In my opinion, this is the equivalent of saying that an electron is not an electron if it is not bound to a proton. If Saturn's moon Titan was orbiting the sun, it would definitely be called a planet. Titan has no intrinsic property that makes it a moon. It is thus my position that the current system makes no more sense than the previous one.

Unlike an electron, a planet's or moon's identity is partly about where it is.

What we understand as a moon is definitely not something that would be orbiting the sun; moons are dependent on planets. Planets, likewise, orbit the Sun.

So what I'm saying is that, true, Titan has no intrinsic property in a vacuum (that's metaphorically, not literally) that makes it a moon, it's about Titan's context. That's part of how we define them.

 

[dimensionless]

Yes, but I would say that Earth is a planet, and that Saturn is more like the Sun than the Earth. Therefore Titan is obiting a star-like body and should be given the designation of planet.

As far as moons go, what would we do if both objects orbit a point that lies somewhere in between the two objects?

 

[selfAdjoint]

Yes, but I would say that Earth is a planet, and that Saturn is more like the Sun than the Earth. Therefore Titan is obiting a star-like body and should be given the designation of planet.

As far as moons go, what would we do if both objects orbit a point that lies somewhere in between the two objects?

Newton help us all if you ever get on that committee

 

[Astronuc]

http://edition.cnn.com/2006/TECH/space/08/24/pluto.ap/index.html?section=cnn_space&ref=google

Pluto no longer a planet, say astronomers

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.

After a tumultuous week of clashing over the essence of the cosmos, the International Astronomical Union stripped Pluto of the planetary status it has held since its discovery in 1930. The new definition of what is -- and isn't -- a planet fills a centuries-old black hole for scientists who have labored since Copernicus without one.

Although astronomers applauded after the vote, Jocelyn Bell Burnell -- a specialist in neutron stars from Northern Ireland who oversaw the proceedings -- urged those who might be "quite disappointed" to look on the bright side.

"It could be argued that we are creating an umbrella called 'planet' under which the dwarf planets exist," she said, . . . .

The decision by the prestigious international group spells out the basic tests that celestial objects will have to meet before they can be considered for admission to the elite cosmic club.

For now, membership will be restricted to the eight "classical" planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Much-maligned Pluto doesn't make the grade under the new rules for a planet: "a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a ... nearly round shape, and has cleared the neighborhood around its orbit."

Instead, it will be reclassified in a new category of "dwarf planets," similar to what long have been termed "minor planets."

Maybe they should have told the world that they were considering changing the designation of Pluto as a planet and given everyone time to adjust to the idea.

I presume by cleared, one means that the planet has accreted or absorbed smaller masses to form a large unique mass.

Some more news on the matter and a nice history summary:

Pluto, the Un-Planet?
http://www.npr.org/templates/story/story.php?storyId=5705254
by David Kestenbaum

 

[Ouabache]

Here's a sister thread regarding the fate of Pluto, in one of our more laid-back forums.