In 2008, Makemake and Haumea have been named under the expectation that they would prove to be dwarf planets. Well, did they prove to be ones? Haumea obviously did since we know its shape which is a pretty weird ellipsoid. What about Makemake? Did we learn meanwhile whether Makemake is in hydrostatic equilibrium and/or whether it has an ellipsoidal shape? Or is there still the possibility that it's no dwarf planet according to the IAU's rules?
$\begingroup$
$\endgroup$
14
-
1$\begingroup$ what exactly is 'asteroid-shaped'? There's plenty of them which are too small for hydrostatic equilibrium $\endgroup$– planetmakerAug 4, 2020 at 8:46
-
$\begingroup$ @planetmaker Plenty of what? A dwarf planet must be in hydrostatic equilibirum, otherwise it's no dwarf planet according to the IAU. $\endgroup$– IoannesAug 4, 2020 at 9:04
-
1$\begingroup$ Makemake and Haumea are both well above the potato radius of 200 to 300 km, by a factor of more than two. Unless something happened recently (e.g., a collision), where a few tens of millions of years qualifies as "recently", its almost a certainty that those objects have more or less pulled themselves into a state close to hydrostatic equilibrium. $\endgroup$– David HammenAug 4, 2020 at 10:46
-
$\begingroup$ @DavidHammen That also depends on whether Makemake is an ice dwarf (like Pluto and many moons) or a terrestrial body (like the Earth's moon or Vesta). $\endgroup$– IoannesAug 4, 2020 at 11:01
-
1$\begingroup$ @Ioannes - That's reflected in the range of the potato radius. As ice is more easily deformed than is rock, the potato radius for an icy body is smaller (~200 km) than is the potato radius for a rocky body (~300 km). In any case, at over 700 km mean radius, the size of those two bodies almost certainly dictates that they will have pulled themselves into something close to hydrostatic equilibrium. $\endgroup$– David HammenAug 4, 2020 at 11:05
|
Show 9 more comments
1 Answer
$\begingroup$
$\endgroup$
4
Intention to answer
"Is there still a possibility that Makemake is not ellipsoidal but asteroid-shaped?"
Probably not under reasonable assumptions.
From Ortiz et. al. (2012) "Albedo and atmospheric constraints of dwarf planet Makemake from a stellar occultation" https://www.nature.com/articles/nature11597
Our preferred solution that fits the occultation chords corresponds to a body with projected axes of 1,430 ± 9 km (1σ) and 1,502 ± 45 km, implying a V-band geometric albedo pV = 0.77 ± 0.03. This albedo is larger than that of Pluto, but smaller than that of Eris. The disappearances and reappearances of the star were abrupt, showing that Makemake has no global Pluto-like atmosphere at an upper limit of 4–12 nanobar (1σ) for the surface pressure, although a localized atmosphere is possible. A density of 1.7 ± 0.3 g cm−3 is inferred from the data.
Concerning the irregular shape of Haumea there are some uncertainties, it might not be a homogenous body but one with a ring and it has satellites.
From Ortiz etl a. (2017) "The size, shape, density and ring of the dwarf planet Haumea from a stellar occultation" https://www.nature.com/articles/nature24051
Secondary events observed around the main body of Haumea are consistent with the presence of a ring with an opacity of 0.5, width of 70 kilometres and radius of about 2,287 kilometres. The ring is coplanar with both Haumea’s equator and the orbit of its satellite Hi’iaka. [...] The occultation by the main body provides an instantaneous elliptical projected shape with axes of about 1,704 kilometres and 1,138 kilometres.
The paper's also on Arxiv it seems.
-
1$\begingroup$ These observations are fitted results that assume the object is ellipsoidal. While this is a reasonable assumption, it does not prove that the object is ellipsoidal. It assumes the object is ellipsoidal. $\endgroup$ Aug 4, 2020 at 11:14
-
1$\begingroup$ To me, Makemake fits the concept of a dwarf planet quite nicely. It's Haumea that's more puzzling with its apparently triaxial shape and very fast rotation. $\endgroup$ Aug 4, 2020 at 11:34
-
1$\begingroup$ At a mean radius of ~9 km, Ultima Thule (486958 Arrokoth) is much smaller than the potato radius, so it's not surprising that it's lumpy. $\endgroup$ Aug 4, 2020 at 11:40
-
1$\begingroup$ @DavidHammen: Read the papers. Of course you can question anything but "fitted results" suggests cheating is just a baseless claim. The papers offer well founded, preferrable solutions to the observations. Limitations of lightcurve interpretation are openly discussed or linked to. Of course, like allways, future results are open. $\endgroup$– user34599Aug 4, 2020 at 13:05