Pluto; the (dwarf?) planet

[Demonperformer]

I was going to post this on another thread [https://stargazerslounge.com/topic/319600-pluto-august-4th-and-16th], but am starting a new one as it would probably drag that thread way off-topic if I posted this there and the OP of that thread deserves better.

Several comments have been put on that thread along the lines of "Pluto will always be a planet to me". And it got me wondering: do people who consider Pluto to be a planet (rather than a dwarf planet) also consider Eris (50km smaller diameter but 5% greater mass compared to Pluto) to be a planet (rather than a dwarf) as well?

And do these people still consider Ceres (which was regarded as a full planet until the mid-19th century) to be just an asteroid rather than a dwarf planet?

My gut-feeling is that the answers to both of these questions will be 'no', suggesting that the Pluto-thing is nothing more than sentiment (which is a valid - if not very scientific - position), but I am willing to be proved wrong on this .... just let me grab my tin helmet and dive for cover .... now .... discuss.

[Stu]

It is a very complicated subject really, because there are also quite a few moons in the solar system that could be classified as planets or dwarf planets.

For me, if it is a sphere and is not orbitting another larger sphere then it is a planet, but I’m happy with the term dwarf planet for the smaller ones and think that there are probably quite a few objects out there this will apply to.

[DRT]

The "scientific" basis of classification that caused Pluto to be demoted relies on the need for a Planet to have cleared its orbit around the Sun of significant debris. One of the arguments against that requirement is that the farther out you go the more difficult that test is to pass. I wonder if an Earth-sized body in the Kuiper Belt would pass the test?

[SIDO]

I think there are really to arguments coupled to a misunderstanding, nasa & the minor planetary folks clearly think its still a planet hence the term Dwarf Planet and the general population journalists included think or assume that the downgrade means its somehow not a planet anymore or at least not the one they once knew and loved for so long even though up until recently we dident even know what it looked like what it was really made of and even disagreed on how big it actually was, not to disappoint but Pluto is indeed still a planet all be its now a dwarf planet. Should we be talking about why we call a sunset a sunset because the sun isent setting folks, your location on earth is rotating away from it, or maybe that's too complex ?

[Stu]

17 minutes ago, SIDO said:

or maybe that's too complex

Can we avoid the sarcasm please.

I’ve never quite got the requirement for a planet to clear its orbit. Does that mean the whole orbit or just a bit around itself? Jupiter seems to fail in one regard here, with Trojans and Greeks sharing its orbit...

https://en.m.wikipedia.org/wiki/Jupiter_trojan

[SIDO]

The sunset term is one still in use since it was commonly believed the earth was at the center of the universe, the statement was not intended to be sarcastic in any fashion. I was simply making the point that we often take things out of context and even to the point of wanting to keep the outdated terminology...then you accused me of sarcasm Stu....I'm stunned and disappointed...

[dweller25]

@SIDO

Of course we should be talking about it - that is what forums are all about isn’t it

PS - I am in the “its a planet” camp - but accept it’s a very minor planet ?

[DRT]

48 minutes ago, SIDO said:

or maybe that's too complex ?

I suspect that is the bit of your post that Stu was referring to - it seems to be implying that some of us aren't clever enough to understand.

[DRT]

33 minutes ago, Stu said:

Can we avoid the sarcasm please.

I’ve never quite got the requirement for a planet to clear its orbit. Does that mean the whole orbit or just a bit around itself? Jupiter seems to fail in one regard here, with Trojans and Greeks sharing its orbit...

https://en.m.wikipedia.org/wiki/Jupiter_trojan

Earth, Mars, Saturn and Neptune also have Trojans. Perhaps they are all minor planets too? ? 

[Gina]

I agree that the clearing of objects in a planets orbit is not relevant.  Whether the point about the orbit being different, whereas all the "other" planets have the same orbital axis, is relevant I'm not sure.

[SIDO]

33 minutes ago, DRT said:

I suspect that is the bit of your post that Stu was referring to - it seems to be implying that some of us aren't clever enough to understand.

Going back to just reading posts seems that was working better for me, asking weather or not something to is complex for others to understand I will never be cured of. My humble apologies if I may have offended anyone ?

[domstar]

I hope the people who reclassified it know what they're talking about. I'd probably have to side with them on the Pluto issue. Maybe it was because Pluto-sized things were being discovered more and more and it would be strange to call the all planets of the solar system. 

On the sentimental side, I find it hard to think of Ceres as a planet or even a dwarf planet. 

[Demonperformer]

2 hours ago, Stu said:

there are also quite a few moons in the solar system that could be classified as planets or dwarf planets. 

For me, the interesting one would be Triton, given that Pluto crosses (albeit at a large inclination difference) the orbit of Neptune. Pluto is about 10% smaller diameter than Triton, so, if their orbits had lain closer together (inclination-wise) it could presumably have been captured by Neptune or, conversely, if Triton's original orbit had lain further from Neptune (inclination-wise) it would presumably be a dwarf planet.

[Demonperformer]

The "clearing its orbit" requirement has always struck me as problematical. I wonder if "trojan" objects are in a slightly different category as the planet itself is required to form the legrange points that hold them in their orbits. If one does not ignore such things, one could also argue that any planet with a moon has not "cleared its orbit".

[michael.h.f.wilkinson]

The problem with any classification is whether the classes referred to are really distinct. If the classes have fuzzy boundaries, there will always be problematic cases. In this case I feel that the IAU does have a point. Despite the Trojans and Greeks, Jupiter has definitely cleared its orbit in the sense that there is no major object in its orbit that isn't profoundly affected by the gravity of Jupiter: the motions of both its satellites and the Trojans and Greeks are dictated by its gravity and that of the sun. The same holds true for the other major planets. The fact that Pluto crosses Neptune's orbit means it is not in a clear orbital path of its own. Of course, that implies Neptune's path hasn't been fully cleared either, but it is gravitationally dominant by a huge degree in its neck of the woods. Ceres shares its orbit with many other objects of similar sizes. If in the Kuiper Belt there are several earth-size objects sharing roughly the same orbit, that might give reason for a rethink, and perhaps change of definitions.

[Macavity]

To paraphrase an academic I knew: [Pluto is a planet] "For Historical Reasons". ?
I have no quibble with leaving it to remain so. I think it makes little difference.
My regret is that I didn't re-start Astronomomy earlier!!! Triton is well within
range of my 8" video setup, but Pluto is too low + a huge stellar background? ?

[MarsG76]

As you stated, it's sentiment, tradition and old habits of what we have been taught at school.. people do not like change, but unfortunately facts are that Pluto is indeed a dwarf planet, a member of the Kupier belt, just like Ceres also being a dwarf plane, being a member of the asteroid belt..... neither of which clear their immediate orbit of other debris.

[Macavity]

You think Astronomers have problems of classification? ?
https://en.wikipedia.org/wiki/List_of_particles

Conventions are GREAT... Exceptions prompt thinking? The
latter at cost of a rather minimal number of extra words? ?

Pluto was formerly known as a Planet, but now... ?

[DRT]

1 hour ago, MarsG76 said:

 but unfortunately facts are that Pluto is indeed a dwarf planet, a member of the Kupier belt, just like Ceres also being a dwarf plane, being a member of the asteroid belt..... neither of which clear their immediate orbit of other debris.

What is a fact is that those two bodies are not planets based on a current arbitrary definition of what a planet is ?

1 hour ago, michael.h.f.wilkinson said:

 The fact that Pluto crosses Neptune's orbit means it is not in a clear orbital path of its own. Of course, that implies Neptune's path hasn't been fully cleared either, but it is gravitationally dominant by a huge degree in its neck of the woods. 

Being gravitationally dominant in its neck of the woods is not the test that has been applied to Pluto.  Shouldn’t Neptune have captured Pluto as a Moon or obliterated it in order to meet the criteria for being a planet?

[chiltonstar]

In my heart, it's a planet. In my head, it isn't.

I still want to see it naked eye though!

Chris

[MarsG76]

3 minutes ago, DRT said:

current arbitrary definition of what a planet is

Exactly.. tomorrow a new definition will state that Jupiter isn't a planet but indeed a failed star..... 

 

[Demonperformer]

8 minutes ago, chiltonstar said:

In my heart, it's a planet. In my head, it isn't.

My point exactly.

[Mr Spock]

Mercury is smaller than Ganymede or Titan. Should that be a minor planet too?

 

[Paul M]

5 minutes ago, Mr Spock said:

Mercury is smaller than Ganymede or Titan. Should that be a minor planet too?

 

That's the difference between being a minor planet and being a Minor Planet

Can planets develop inferiority complexes?

Venus is often inferior too! It'll be wanting to hide round the back of the Sun avoiding its impending inferiority...

Venus and Mercury are the only 2 that can't present any opposition to the semantics...

Where will it end??!! 

[LukeSkywatcher]

I'm in "Pluto is a planet" camp. Also in the "Eris and Ceres are just asteroids" camp. It's a sentimental thing with me about Pluto. I do understand the characteristics for something to be classed as a planet.