Talk:90377 Sedna

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Mass[edit]

I restored the note about Sedna's mass on the theory that it's a routine calculation, and as such not OR per WP:CALC. Please feel free to revert my undo if you disagree. —RP88 (talk) 01:22, 27 March 2014 (UTC)

In case it's not clear, the use of Pluto's density in the calc is presumably the source of a possible claim of OR. —RP88 (talk) 01:26, 27 March 2014 (UTC)

Possible that it's not a dwarf planet??[edit]

To qualify as a dwarf planet, Sedna must be shown to be in hydrostatic equilibrium. It is bright enough, and therefore large enough, that this is expected to be the case,[56] and several astronomers have called it one.

Are there some astronomers who believe it doesn't qualify as one?? Georgia guy (talk) 14:47, 2 April 2014 (UTC)

I don't know if there are any astronomers who have unequivocally said it wasn't (I don't think they would do so without pretty strong evidence, of which there is none), but not everyone is completely ready to say that it is. Some won't be happy without a telescope image, while others are happy with a diameter estimate. Serendipodous 15:06, 2 April 2014 (UTC)
At a diameter 995±80 km, no one seriously thinks that it could be irregular, AFAIK. Note that even rocky Vesta at a diameter of 525 km is rather round, aside from Rheasilvia basin (which has precluded it from regularly being called a dwarf planet) and at a diameter of 213 km icy Phoebe used to round but was severly battered. Then again, Saturn's moons Mimas, Enceladus, Tethys, Dione, and Iapetus all are clearly round, but careful measurements of their dimensions has revealed that they deviate from strict hydrostatic equilibrium (of Saturn's moons only Rhea and Titan have dimensions in correspondence with hydrostatic equilibrium). So if you go for hydrostatic equilibrium in its strict sense, none of the IAU-accepted dwarf planets (Ceres, Pluto, Eris, Makemake, and Haumea) are really known to fit. --JorisvS (talk) 15:09, 2 April 2014 (UTC)
If 2002 UX25 has the <0.9 density that Brown says it does, it would not be a dwarf planet (since DP can't be porous). UX25 was thought to be too large to be porous: how large can a porous object get? There's also the Saturn moons, as you mention. Would Iapetus be considered a DP if it was a TNO? I think the reason that Sedna is not generally labelled a DP is a recognition of the lack of understanding of these bodies (or, rather, lack of proof of current understanding), rather than uncertainty about Sedna's size or other properties. Most days, though, I think it's simply that whether or not something is a DP is not scientifically interesting, so most astronomers just blindly follow the IAU list, or just ignore the issue altogether. Tbayboy (talk) 18:37, 2 April 2014 (UTC)
A big strike against Sedna is that since it does not have a known moon, we do not know its mass or (estimated density). -- Kheider (talk) 18:56, 2 April 2014 (UTC)
I suspect that were Iapetus in solar orbit, it would be considered a DP (but would likely also be generating discussion about that it is not technically in HE). 2002 UX25 is an interesting case! Large enough that is should be a DP, but a very low density possibly requiring porosity and lightcurve measurements that cast doubt on its DPness...--JorisvS (talk) 21:04, 2 April 2014 (UTC)
I think this should remind us that DP is a rather stupid class. It was created to save Pluto's planetary status, but since that never happened, it serves no purpose except to determine which body at the IAU gets to name a new object. Iapetus would not be a DP if it were in Solar orbit, and there's a good chance that Haumea and Makemake are not DPs either. If they aren't, then Sedna probably isn't, but they would each require an orbiter, not just a fly-by, to determine if they're actually in HE. In other words, a completely useless category.
Serendipodous, if astronomers are willing to accept telescope images, then they're effectively saying the IAU definition can be ignored.
I think we should follow Brown's rec and use "planetoid" (literally, "planet-like") for bodies which we can reasonably expect to be round (that is, to be "worlds"), without demanding that they fit an technical definition that we can't actually determine. There's no problem calling Sedna a "planetoid". — kwami (talk) 21:24, 2 April 2014 (UTC)
Well, the class is fine, but we now know that requiring technical hydrostatic equilibrium for it is meaningless, thanks to the Saturnian moons. Those are all clearly rounded and really should be considered DPs if they were in solar orbit. Looking for rounded objects, one notices also Phoebe and Vesta, both of which appear to have been battered out of a nicely round shape. Because technical HE does not do it, a simple impact on a DP should not suddenly make it lose its DPness. --JorisvS (talk) 14:42, 14 April 2014 (UTC)
Because of Saturn's moons, we know that equilibrium shape does not mean HE. But without close-up observations, it's also impossible to know if irregular bodies like Phoebe were once in HE. Certainly if a HE body were split in half, the two halves would not count as separate DPs, so where do we draw the line? What if an ex-HE body is disrupted and comes back together as a rubble pile? Or 21 Lutetia, if it proves to be the battered core of a DP? — kwami (talk) 22:09, 14 April 2014 (UTC)
See 31 Euphrosyne: With a density over 6, it must be a fragment or core of something big. Tbayboy (talk) 01:07, 15 April 2014 (UTC)