Pluto is no longer a planet - WHY
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Rigel Stardin
SOC-13
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...
Spinward Scout
SOC-14 5K
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/Resonant_trans-Neptunian_object
http://en.wikipedia.org/wiki/Plutinos
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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.
Whatever. In my household Pluto is still a planet, but the others are not.
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.
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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!)
(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
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.Wikipedia quote of the IAU Resolution said:
Neptune's a planet because of footnote 1, not the definition.
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
So Neptune is not yet a planet...?
Peter Newman
SOC-12
Note also that the IAU definition says that planets are 'celestial bodies'. Celestial means 'sky'. The Earth is not in the Earths sky. (They haven't bothered to define their terms in a heliocentric manner.) Therefore, when you are on the Earth, the Earth is not a planet either - at least not without the footnote. Ignore the footnotes and we're down to six planets - Mercury, Venus, Mars, Jupiter, Saturn, and Uranus....
They may want a more precise definition. Even if they don't, I do.
They may want a more precise definition. Even if they don't, I do.
mike wightman
SOC-14 10K
Neptune has cleared its orbit - name one other planet or dwarf planet sized object in Neptune's orbit.
Of course you have to be careful with what you claim to be Neptune's orbit in the first place, since Neptune's orbit used to be closer to the Sun than Uranus. In current theories of our solar system formation and evolution Neptune and Uranus actually formed much closer to Jupiter and Saturn.
The gravity of Jupiter and Saturn conspired to throw Neptune and Uranus much farther out, with Neptune actually moving beyond Uranus. Neptune ended up in the region we know of today, which in turn caused a massive disturbance of the Kuiper belt which in turn lead to the period known as the late heavy bombardment.
It's all fascinating stuff and well worth reading up on.
Of course you have to be careful with what you claim to be Neptune's orbit in the first place, since Neptune's orbit used to be closer to the Sun than Uranus. In current theories of our solar system formation and evolution Neptune and Uranus actually formed much closer to Jupiter and Saturn.
The gravity of Jupiter and Saturn conspired to throw Neptune and Uranus much farther out, with Neptune actually moving beyond Uranus. Neptune ended up in the region we know of today, which in turn caused a massive disturbance of the Kuiper belt which in turn lead to the period known as the late heavy bombardment.
It's all fascinating stuff and well worth reading up on.
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Neptune has cleared its orbit - name one other planet or dwarf planet sized object in Neptune's orbit.
Of course you have to be careful with what you claim to be Neptune's orbit in the first place, since Neptune's orbit used to be closer to the Sun than Uranus. In current theories of our solar system formation and evolution Neptune and Uranus actually formed much closer to Jupiter and Saturn.
The gravity of Jupiter and Saturn conspired to throw Neptune and Uranus much farther out, with Neptune actually moving beyond Uranus. Neptune ended up in the region we know of today, which in turn caused a massive disturbance of the Kuiper belt which in turn lead to the period known as the late heavy bombardment.
It's all fascinating stuff and well worth reading up on.
I love Astronomical theory!
Pluto, Charon, and their three satellites. For starters.Neptune has cleared its orbit - name one other planet or dwarf planet sized object in Neptune's orbit.
From Wikipedia, the following shows a great number of the orbits with their major axis aligned, including neptune. (Full sized, click image to go to wikimedia image page)
The definition is clear enough - clear the neighborhood means pull them into either co-orbit (moons, Trojans), collision, or ejection from system.
Which means Neptune isn't a planet yet, except for the footnote.
Gravitationally dominate is quite different, and was a proposed definition. It has the problem of no planet's gravity is more significant that other planets much past the L4/L5 trojan points. Heck, at Neptune's trojans, it is possible for uranus to be closer.
Let's do some math... gravity effect is roughly inverse square of mass (density distribution can skew this somewhat, but not on the scale we're talking.
| Planet | Mass (kg) | Orbital distance (m) | Neptune L5 minimum distance | Graivtational Force | Relative Gravitational force |
| Jupiter | 1.90E+27 | 7.79E+11 | 6.55E+11 | 4.42E+03 | 8.73E+02 |
| Saturn | 5.68E+26 | 1.43E+12 | 1.44E+12 | 2.74E+02 | 5.42E+01 |
| Uranus | 8.68E+25 | 2.87E+12 | 1.62E+12 | 3.30E+01 | 6.51E+00 |
| Neptune | 1.02E+26 | 4.50E+12 | 4.50E+12 | 5.07E+00 | 1.00E+00 |
| Formula | —NA— | —NA— | DON–DOx | M/DNL52 | FGX/FGN |
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mike wightman
SOC-14 10K
In which case none of the planets in the solar system are planets - check out all the near Earth asteroids, earth crossing asteroids etc.
The definitions are a bit pants but their intent is clear.
The definitions are a bit pants but their intent is clear.
Their intent may be clear, but the definition passed is as bad as not having one.
Gravitational Dominance can't be used, because any given body is affected by the other masses, and since there are trojans of Jupiter, Uranus and Neptune... and Jupiter is the dominant gravitational force when it is passing the outer world's trojan points.
Clearing the orbit is a bad definition, because several KBO's are non co-orbital but are orbit crossing with Neptune.
From what I've read, pretty much none of the IAU voting members objected to "rounded by own gravity" aka "Hydrostatic Equilibrium", nor to "not orbiting another body besides the sun" - there was much argument over the "clearing its own orbit".
The hydrostatic equilibrium appears to require about 200km diameter. The IAU has a draft recommendation of a minimum mass of 5E20kg. We'll use the common Astronomical Jargon for this henceforth: planemo.
Not orbiting another body gives us 20-something planemo bodies, but only 14 that the IAU has accepted confirmation of size and shape on: Ceres, Pluto, Haumea, Makemake, Eris. Orcus and Quaor have met the definition, but have not been formally accepted (but, barring redefinition, will be).
Ceres has no moons.
Pluto has 5 moons, including a planemo, Charon
Haumea has two moons, Hiʻiaka and Namaka. But haumea itself isn't actually round - it's elliptical.†
Makemake has no detected moons
Eris has Dysnomia
Orcus has Vanth
Quaor has Weywot
2007 UK126 has a reported moon - both are unnamed,
Sedna has no reported moons.
† Haumea may actually be two planemos collided that haven't yet fully rounded. Or it might not actually be a planemo.
The definition is only problematic for the third condition, which is, given the continued debates even still some 6 years later, a problem. We all know the intent was to not exclude any of the "traditional" planets except pluto. That definition, however, is not accepted by all of the IAU members, is likely to be brought to table again for clarification and/or revocation, especially since the IAU committee accepted an ellipsoid as a planemo.
At least that ellipsoid has moons....
Fritz_Brown
Super Moderator
Well, Earth certainly doesn't qualify as having swept its orbit. I mean, there is Gor to consider.......
Let's leave the fantasy stuff out of it. There's plenty to "clutter up" our orbit as is.
2010 TK7 (earth trojan) doesn't count as "not cleared" - trojans are one method of clearing an orbit.
Moons don't count as "not cleared"... Earth's only got one.
most so-called Orbit crossers like Pluto really are not in crossing orbits. Pluto's orbital plane is sufficiently different from neptunes that it's not going to intersect with neptune - it's vertically displaced as it crosses neptune's orbit. Cruithne, likewise, has an inclination of 18° or so, vs earth's around 7° , so as it crosses the 1 AU ring, it's at a considerably different elevation vs the ecliptic from earth.
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It was a political move - not a scientific one. Hence the underhanded way the 'vote' was taken ...
No planets have actually completely 'cleared' their orbits in any provable way, even discounting Trojan points there is still a lot we have already detected (and undoubtedly even more we have not) - and the other definitions leave Pluto a planet (and several more to add to the list).
The 'issue' about what to do with further planets as they are (inevitably) discovered has been around for a long time - while the 'IAU' didn't have a definition, accretion ability and orbiting a star (or more) has always been the accepted definition. Only subjective definitions have been offered (and not actually approved even by the AIU, IIRC) to avoid labeling Pluto (and other bodies) as planets.
Looking forward to New Horizon's rendezvous with the Planet Pluto in ~2 1/2 years...
(I enjoyed discussing Kuiper candidates with the PI some years back - and ideas regarding New Horizon's 2, which sadly never got funded.).
No planets have actually completely 'cleared' their orbits in any provable way, even discounting Trojan points there is still a lot we have already detected (and undoubtedly even more we have not) - and the other definitions leave Pluto a planet (and several more to add to the list).
The 'issue' about what to do with further planets as they are (inevitably) discovered has been around for a long time - while the 'IAU' didn't have a definition, accretion ability and orbiting a star (or more) has always been the accepted definition. Only subjective definitions have been offered (and not actually approved even by the AIU, IIRC) to avoid labeling Pluto (and other bodies) as planets.
Looking forward to New Horizon's rendezvous with the Planet Pluto in ~2 1/2 years...
(I enjoyed discussing Kuiper candidates with the PI some years back - and ideas regarding New Horizon's 2, which sadly never got funded.).
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