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-   -   Pluto is no longer a planet - WHY (http://www.travellerrpg.com/CotI/Discuss/showthread.php?t=28805)

JAFARR January 13th, 2013 09:45 PM

Pluto is no longer a planet - WHY
 
I am not sure why, but kids these days days are now taught that our solar system has 8, not 9 planets. Pluto is called a "plutoid". Anyone know what this is all about. "A rose by any other name..."

Rigel Stardin January 13th, 2013 10:06 PM

In the late 90's they classifed Pluto as a planetiod. The reason being it was to small to be a planet and I believe it has something to do with it's orbit as well. You problably can google it and find out the real reason...

JAFARR January 13th, 2013 10:21 PM

How or does that effect Traveller?

Spinward Scout January 13th, 2013 11:10 PM

Quote:

Originally Posted by JAFARR (Post 418296)
Pluto is called a "plutoid". Anyone know what this is all about?

Pluto is smaller than some moons. Pluto's orbit is elliptical and crosses Neptune's orbit and there's a possibility that Pluto may impact Neptune in the future. Neal DeGrasse Tyson wanted to put himself more in the public eye. Pick two.

http://en.wikipedia.org/wiki/Resonan...ptunian_object

http://en.wikipedia.org/wiki/Plutinos

robject January 13th, 2013 11:40 PM

I also heard that there are potentially a double-handful of things in orbit around the sun which are Pluto's size. Dwarf planets like Eris and Sedna. Too many and just a bit too irregular to really qualify as planets, and the line has to be drawn somewhere, so.

Whatever. In my household Pluto is still a planet, but the others are not.

aramis January 14th, 2013 12:12 AM

Quote:

Originally Posted by Rigel Stardin (Post 418298)
In the late 90's they classifed Pluto as a planetiod. The reason being it was to small to be a planet and I believe it has something to do with it's orbit as well. You problably can google it and find out the real reason...

No, the change was made in 2008.

Dr. Niel DeGrasse Tyson has been railing for de-planetizing Pluto since about 1998 or so, but the IAU voted to adopt Dr. Tyson's definitions. There were reports by his opponents that they'd been told the issue was tabled, but a last minute vote was held.

The problem is that the IAU had three competing definitions for a planet floating about, but no official one.

Definition 1 was "Any body orbiting the Sun and rounded by it's own gravity, but not orbiting another body which orbits the sun" - which, if adopted, would have given us 22 known planets at that time (26 now).

Definition 2 was "Any body that has cleared its own orbit." Which ruled out Pluto.

Definition 3 was "Any body not smaller than Pluto."

From what I've read, no one objected to part 1. Lots objected to part 2, because it means that Pluto was excluded, thanks to Sedna, Haumea, and Make-make.

Lots objected to #3 because it was highly arbitrary, and would still give us 1 more at that time, and 2 more were in doubt as to their sizes and thought to be as big or bigger.

At present, there are a baker's dozen+ "Dwarf Planets" - Ceres, Pluto, Sedna, Makemake, Haumea, Eris, Orcus, Quaor, 2007 OR10, Varuna, Ixion, 2002 TC302, Possibly Vesta, and possibly Charon. Dozens more KBOs are candidates for the status.

Note that Charon only possibly counts because Pluto-Charon co-orbit with a barycenter just past Pluto's surface; the IAU hasn't made a formal ruling on "double planet", so it can be said that Charon is not... but even Dr. Tyson has described Charon as a dwarf planet, and he's the man who's definition was accepted by the IAU. Formally, Charon is at present a moon, but is good argument for defining what a double planet is, and it's commonly taught that double planet means a barycenter outside the surface of both.

Interestingly enough, by the strictest application, Neptune is ALSO a dwarf planet, since it has NOT cleared its orbit, as lots of KBO's cross its orbit, but since it's an Ice Giant, no one takes seriously that contention. The most notable is pluto.

inexorabletash January 14th, 2013 02:20 AM

The intent of the definition is, of course, more subtle than "cleared its orbit"; "gravitationally dominates its orbit" is clearer. All 8 planets have several orders of magnitude more mass than anything else that shares an orbital zone - KBOs, asteroids, Trojans, etc. The dwarf planets do not.

(Earth/Moon could be considered the anomaly, as the other 7 planet's moons are covered by the "orders of magnitude" in mass difference!)

aramis January 14th, 2013 02:32 AM

Quote:

Originally Posted by inexorabletash (Post 418312)
The intent of the definition is, of course, more subtle than "cleared its orbit"; "gravitationally dominates its orbit" is clearer. All 8 planets have several orders of magnitude more mass than anything else that shares an orbital zone - KBOs, asteroids, Trojans, etc. The dwarf planets do not.

(Earth/Moon could be considered the anomaly, as the other 7 planet's moons are covered by the "orders of magnitude" in mass difference!)

Their actual definition is
Quote:

Originally Posted by Wikipedia quote of the IAU Resolution
The IAU ... resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct 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) 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,2 (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects,3 except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies."

Footnotes:
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 either dwarf planet or other status.
3 These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

Note that Neptune specifically has NOT cleared out its neighborhood. Sure, it's orders of magnitude larger... but it doesn't meet the definition passed.

Neptune's a planet because of footnote 1, not the definition.

MCEvans January 14th, 2013 10:06 AM

Quote:

Originally Posted by JAFARR (Post 418299)
How or does that effect Traveller?

IMHO it does nothing to Traveller, unless you want it to in YTU.
To me Pluto is a planet with a Navel Base and Research Lab (TD13 pg25) & a undeclared Ancients site and Jump Space research Lab :)

Shonner January 14th, 2013 10:42 AM

What I heard was that telescopes were finding more chunks of rock outside our system and they resembled Pluto a lot in their behavior and appearance. So Pluto got re-classified as more of the same out there.


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