Talk:Dwarf planet

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For the discussion on developing a strategy for naming dwarf planet articles, please see Talk:Dwarf planet/Naming


Definition[edit]

'A dwarf planet is an object the size of a planet...' So if Mercury were the size of Ceres it would, having cleared its orbit, still be a planet? Just checking. Rothorpe (talk) 17:37, 7 December 2014 (UTC)

Yes. But that's unlikely, since a low-mass object like Ceres is not gravitationally strong enough to clear its neighbourhood completely. Serendipodous 18:11, 7 December 2014 (UTC)
Right, thanks. Rothorpe (talk) 18:36, 7 December 2014 (UTC)
It would have a Stern–Levison parameter of less than 1 (Λ~Μ2), and so would not clear its orbit over the age of the universe, let alone during its existence. However, if one would suddenly reduce Mercury's mass to that of Ceres, it would still appear to have cleared its neighborhood, and therefore still appear a planet. But that would be artificial. --JorisvS (talk) 18:47, 7 December 2014 (UTC)
Yes, that's what I was wondering about. Thanks. Rothorpe (talk) 20:14, 7 December 2014 (UTC)
The definition points up the fact that it is an unsatisfactory term, which I suppose is good. ('Planetoid' would perhaps have been a little better, but it still has the connotation of size. Which reminds me of 'asteroids', 'little stars', a much worse misnomer, long accepted.) Rothorpe (talk) 16:45, 8 December 2014 (UTC)
Don't "planetoid" and "asteroid" mean "like a planet/star", rather than "little planet/star"? Iapetus (talk) 10:19, 12 March 2015 (UTC)

"Nearly round"[edit]

We need to avoid this wording, even if some dumbed-down sources use it. Haumea is not nearly round, but is a DP, while Vesta *is* nearly round, but is not a DP. Utterly useless as a definition. Sometimes we just have to write beyond a grammar-school level. — kwami (talk) 18:40, 19 February 2015 (UTC)

I would not call Vesta nearly round. In fact I find that Vesta's criteria for inclusion have been overrated from the start. Serendipodous 19:38, 19 February 2015 (UTC)
Agreed, I don't see roundness by any definition in Vesta, not even the old Hubble pictures. From the Feb 12 pictures, Ceres looks pretty rough, not as smooth as the similarly-sized moons -- the main belt is one tough neighbourhood! Tbayboy (talk) 00:38, 20 February 2015 (UTC)
The IAU is the origin of the phrase, in the definition itself, so it's not really "dumbed-down" for sources to use it. "Round" is sometimes used in maths to mean "everywhere convex", which may explain why they used it (reminds me of "metal" in astronomy, which is not the dictionary definition). "Convex" might make a suitable shorthand, but I don't think we could use it, or any other such description, without sources. But I'm happy with the "...H-E..." phrase you used, ugly as it reads. Tbayboy (talk) 00:38, 20 February 2015 (UTC)
That would be okay, then, though we'd want to supply that definition of "round". Still, "nearly round" would mean not-everywhere-convex, which could include Vesta.
More critically, what about a body that is an ellipsoid but not in HE, like Saturn's mid-sized moons (if they weren't satellites)? Would they be DPs, or former DPs? — kwami (talk) 04:51, 11 March 2015 (UTC)

New NEWS today, for future editing[edit]

To quote: (CNN)NASA's tractor-trailer sized Dawn spacecraft will snuggle up to Ceres on Friday, getting close enough to be pulled into orbit and to complete the first mission to a dwarf planet. "I'm just delighted that Dawn is now on the doorstep of Ceres," Jim Green, director of NASA's planetary science division said in a news conference on Monday. Ceres was discovered in 1801 (Pluto wasn't found until 1930) and was the first object found in our solar system's main asteroid belt between Mars and Jupiter. ... ... ... [As you all know, right?]

Headline-1: NASA spacecraft nearing mysterious dwarf planet Ceres

QUOTE: "Ceres was demoted to an asteroid because 19th century astronomers couldn't be sure it was round. But it was bumped up to a dwarf planet when that category was created in 2006." -- AstroU (talk) 13:21, 3 March 2015 (UTC) -- PS: FYI for future editing.

CNN is not a RS. And in this case they're wrong. — kwami (talk) 04:52, 11 March 2015 (UTC)

Discovering new dwarf satellite[edit]

A group of astronomers of university of Cambridge have discovered 9 new dwarf satellites which orbit the Milky way galaxy. The largest number ever discovered. Read more:

http://phys.org/news/2015-03-dwarf-galaxies-orbit-milky.html?utm_source=nwletter&utm_medium=email&utm_content=splt-item&utm_campaign=daily-nwletter

MansourJE (talk) 07:53, 11 March 2015 (UTC)

Those are dwarf galaxies, not dwarf planets, so not relevant to this page. Tbayboy (talk) 12:17, 11 March 2015 (UTC)

Sedna[edit]

There has been a change by an IP removing Sedna, which has been reverted. However, I agree with the removal of Sedna as of now - as has been stated, it isn't on the IAU's list and while it is "accepted by many" (as in the reasoning behind the revert), so is e.g. Quaoar. I cannot see the reasoning behind including Sedna and at the same time excluding the other "mostly certain" candidates. I would be more than happy to include them all, but as it stands, the IAU is the authority we are referring to (as they started the "dwarf planet" category) and they do not include Sedna - yet. --Ulkomaalainen (talk) 02:46, 20 March 2015 (UTC)

Sedna is there as a representative of that region of the solar system. Whether or not it is a dwarf planet, it's the largest known object in a distinct dynamic class. The other major dynamic classes with known candidates have representatives amongst the IAU DPs. Tbayboy (talk) 03:21, 20 March 2015 (UTC)
Also, the IAU isn't much of an authority. They've declared certain criteria for which commission gets to name which body. They aren't actually in the business of deciding whether s.t. is a DP. Since no other bodies meet their dual-commission criteria, they aren't bothering with DPs any more. The IAU five are a bureaucratic set, not an astronomical one. For the life of me I can't understand why we're using them an the authority, when they plainly aren't trying to be. — kwami (talk) 03:55, 20 March 2015 (UTC)
Basically because including Sedna and excluding others would need another authority. Otherwise we would end up using WP:OR. And the dismissal of the question of "whether or not it is a dwarf planet" seems strange on an article about dwarf planets. There are other articles here about candidates, trans neptunian objects, asteroids and so on, and even here they can be mentioned. But adding Sedna to a table with the designated dps is in my opinion confusing the reader unless we find an authority which includes Sedna (and excludes Quaoar, Orcus, Varuna etc.). --Ulkomaalainen (talk) 11:36, 20 March 2015 (UTC)
Not so much. It is not meant as a exhaustive list of μs or Λs. Only the most massive objects in a region need to be considered. If they fail, all others automatically fail, too. Quaoar, Orcus, Varuna etc. all orbit squarely in the Kuiper belt, so if Makemake fails (it does), then these do, too. Their μs are lower than that of Makemake. No point in including them. --JorisvS (talk) 12:34, 20 March 2015 (UTC)
Since we are talking DPs, not planets, it's not so much about "failing planetary criteria", but about what makes Sedna included in this list which would be understandable. In an article about DPs, this list as of now includes (a) the eight planets, (b) the five Dwarf Planets as chosen/accepted by the IAU, (c) Sedna. And this kind of grouping fails to be explained or sourced to a credible source. It is not about the criteria being technically wrong - I am not enough of an expert to discuss this - but about this sole inclusion seeming like OR to me. The source quoted for the table does not include Sedna (nor Makemake or Haumea for that matter) so it cannot justify this inclusion. If this were a non exhaustive list of "some celestial bodies", it should say so and make clear criteria for inclusion and exclusion. But as it stands it seems like a maybe very useful list, but made by Wikipedia itself which would constitute OR and presented in a way that furthers the suggestion that this set of six (Ceris, Eris, Makemake, Haumea, Pluto, Sedna) has a common ground which is not explained (my first reaction when scrolling across this table was "Oh, the IAU included Sedna now?" - which of course was easily countered by the lead paragraph. Long story short, I still think this list does not explain its criteria for inclusion, especially its deviation by one entry if compared with the easily spotted set up of "planets and IAU recognized dwarf planets" and at the same time does not correctly reference the quoted source. --Ulkomaalainen (talk) 21:36, 20 March 2015 (UTC)
I agree that the rationale for inclusion should be explained (better). These values remain accurate even if Sedna would turn out not to be a DP, they just wouldn't have any relevance. Maybe we could prune the IAU-5 down, because I think what I said above may apply to the Pluto–Makemake–Haumea set, but that should be analyzed better first. --JorisvS (talk) 23:13, 20 March 2015 (UTC)
Note that this is in the section "Orbital dominance", which is a criterium for planets. The table is there to show how DPs (both official and candidate) are not planets, as explained in the section text, and to illustrate the huge gap in dominance. JorisvS is correct that Makemake and Haumea aren't really necessary, since Pluto and Eris together cover the TN belt and are clearly larger than anything else. Also note that Pluto's µ is incorrect in the source, since Soter forgot to include Neptune in Pluto's orbital zone! Tbayboy (talk) 00:24, 21 March 2015 (UTC)
Resonances are a tricky thing with these values. Even though Jupiter and Neptune are by far the most massive in their orbital zone (and the dominant one; else there couldn't be also those minor planets in resonance with them), the resonant objects are not going to be scattered away by them. Rather, it is a special case of clearing their orbital zone. Including these objects' mass in the calculation can be misleading as can be understood from the hypothetical case of two co-orbital planets of roughly equal mass in an exchange orbit (say like that of Saturn's Janus and Epimetheus): A naive calculation of their planetary discriminant leads to a value on the order of ~1, which would lead to the false conclusion that these are not planets. Similarly, two (massive) planets (ice giants or even gas giants) could be in a non-co-orbital resonant configuration on crossing orbits, yet it would be ridiculous that this would disqualify them from being planets. The latter case is not unlike that of Neptune and Pluto, so I don't think it is actually correct to include Neptune's mass in the calculation of Pluto's planetary discriminant. --JorisvS (talk) 00:52, 21 March 2015 (UTC)
You're right. And Soter explicitly excludes resonant objects in his definition. I always think of an object in resonance as a kind of satellite, and that in turn as a back-handed accretion. Would Pluto and Orcus be considered resonant with each other? Their orbits should be synchronised, but obviously not because of their own gravity. Tbayboy (talk) 14:24, 21 March 2015 (UTC)
That's tricky. Beyond some 10–20 million years (its Lyapunov time), Pluto's orbit is unpredictable (nevertheless stable), and something similar should be the case for Orcus. So although they currently stay very far away from each other, wouldn't this mean they may come to orbit much closer to each other in the future/past, meaning one would scatter the other if they had sufficient mass? --JorisvS (talk) 16:46, 21 March 2015 (UTC)
They could also be at different points along their orbit (e.g., one at perihelion and one at aphelion) so that even if they're in the same orbit they might never get close. I wonder how much "slippage" there is; i.e., breaking the 3:2 resonance then getting pulled back into it, but at a different point in the orbit. Or does that manifest as a different argument of perihelion? Anyway, I don't think resonance is transitive, so Pluto and Orcus would count towards each others orbital zone material. Tbayboy (talk) 14:24, 22 March 2015 (UTC)
Could be, but not necessarily so. Don't their orbits librate/circulate? And what about the perturbations from the other planets, and even other large minor planets, slowly reorienting their orbits? --JorisvS (talk) 14:37, 22 March 2015 (UTC)
I think a note should be added to the table's caption that the table includes 8 planets, 5 dwarf planets and Sedna as a representative of Ooort cloud population. Ruslik_Zero 20:01, 21 March 2015 (UTC)
Agreed, but if a correct note (i.e. that the most massive object in an orbital region is shown, because these values are necessarily lower for all other objects) is added, is there still a point in keeping Makemake and Haumea in there? --JorisvS (talk) 20:29, 21 March 2015 (UTC)
I took a try at it. Lightened the purple to be printer friendly. Commented out Haumea and Makemake, so we don't lose the numbers. It's too much text to put above the table with the caption. Tbayboy (talk) 15:12, 22 March 2015 (UTC)
I like the version a lot. Thanks for the change and the constructive and polite discussion. --Ulkomaalainen (talk) 03:22, 23 March 2015 (UTC)