<p>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?</p>

CC BY-SA 4.0
2020-08-04T08:46:48.520Z
what exactly is 'asteroid-shaped'? There's plenty of them which are too small for hydrostatic equilibrium
planetmaker
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2020-08-04T09:04:01.863Z
@planetmaker Plenty of what? A dwarf planet _must_ be in hydrostatic equilibirum, otherwise it's no dwarf planet according to the IAU.
Ioannes
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2020-08-04T10:46:20.763Z
Makemake and Haumea are both well above the [potato radius of 200 to 300 km](https://arxiv.org/pdf/1004.1091.pdf), 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.
David Hammen
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2020-08-04T11:01:29.997Z
@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).
Ioannes
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2020-08-04T11:05:54.000Z
@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.
David Hammen
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2020-08-04T11:13:48.347Z
@DavidHammen I wonder then why the IAU doesn't recognize any KBOs larger than 1400 km in diameter as dwarf planets.
Ioannes
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2020-08-04T12:38:15.513Z
@a_donda Haumea and Makemake are expected to be dwarf planets by the IAU, but they aren't recognized in the way Pluto, Eris and Ceres are. Gonggong, Quaoar and Sedna are borderline.
Ioannes
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2020-08-04T14:57:28.407Z
Re *I wonder then why the IAU doesn't recognize any KBOs larger than 1400 km in diameter as dwarf planets* Because size doesn't matter, at least as far as the definition of a dwarf planet is concerned. What matters is that the object in question has observably pulled itself into a roundish shape. The observations to date are not good enough. The definition is flawed, IMO. The category of roundness was a bone tossed to geophysicists who didn't like that any sane definition of "planet" would exclude Pluto.
David Hammen
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2020-08-04T15:00:19.283Z
Roundness is a terrible metric. It is not well defined, it has no clear boundary, and for the foreseeable future, it limits planets and dwarf planets to objects orbiting the solar system. The concept of "planet" can easily be extended beyond the solar system by getting rid of two concepts: (1) that planets are objects that orbit the Sun, and (2) that planets are roundish. The first mean that exoplanets are not planets, and the second means that until we have imaged an exoplanet in high resolution, it will remain a non-planet.
David Hammen
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2020-08-04T15:49:30.677Z
@DavidHammen "Sane" definitions _can_ include Pluto if you accept all other dwarf planets as planets as well, or set a border like the 700 km radius. Bodies that are _clearly_ ellipsoidal can't count as asteroids, e.g. Saturn has seven spherical moons while the other Saturnian satellites count as asteroidal, Uranus has five, Neptune one and Jupiter four. If I understand you correctly, you'd like to classify certain moons as planets? Otherwise, one could simply replace "orbit the Sun" by "orbit a star (including remnant stars and brown dwarfs)".
Ioannes
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2020-08-04T15:50:46.770Z
@DavidHammen And also include rogue planets of course.
Ioannes
CC BY-SA 4.0
2020-08-04T16:14:52.020Z
Re *Bodies that are clearly ellipsoidal can't count as asteroids* -- That rules out the largest asteroid (Ceres), and possibly others, as asteroids, something the IAU will not condone. The IAU already has already had one over-the-top battle over names. And while "asteroid" has no official definition, calling the moons of Saturn, Uranus, and Neptune "asteroids" is a far stretch, BTW, Pluto is not a planet.
David Hammen
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2020-08-04T17:11:57.383Z
@DavidHammen Ceres is no asteroid, it's a dwarf planet. Hygiea should be one too, and since Haumea is a dwarf planet, Vesta and Pallas perhaps should be ones too. But known to be clearly spherical are Ceres, Hygiea, Pluto, Eris and Makemake only (out of dwarf planet candidates). And why do you say Pluto is not a planet? I don't claim anything different except that it depends on definition.
Ioannes
<dwarf-planets><definition>
Is there still a possibility that Makemake is not ellipsoidal but asteroid-shaped?
Ioannes
# Is there still a possibility that Makemake is not ellipsoidal but asteroid-shaped?

(Asked by: username_0 on 2020-08-04)

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?

**Tags:** dwarf-planets, definition
> **Comments:**
> **username_1**: what exactly is 'asteroid-shaped'? There's plenty of them which are too small for hydrostatic equilibrium  
> **username_0**: @planetmaker Plenty of what? A dwarf planet _must_ be in hydrostatic equilibirum, otherwise it's no dwarf planet according to the IAU.  
> **username_2**: Makemake and Haumea are both well above the [potato radius of 200 to 300 km](https://arxiv.org/pdf/1004.1091.pdf), 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.  
> **username_0**: @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).  
> **username_2**: @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.  
> **username_0**: @DavidHammen I wonder then why the IAU doesn't recognize any KBOs larger than 1400 km in diameter as dwarf planets.  
> **username_0**: @a_donda Haumea and Makemake are expected to be dwarf planets by the IAU, but they aren't recognized in the way Pluto, Eris and Ceres are. Gonggong, Quaoar and Sedna are borderline.  
> **username_2**: Re *I wonder then why the IAU doesn't recognize any KBOs larger than 1400 km in diameter as dwarf planets* Because size doesn't matter, at least as far as the definition of a dwarf planet is concerned. What matters is that the object in question has observably pulled itself into a roundish shape. The observations to date are not good enough. The definition is flawed, IMO. The category of roundness was a bone tossed to geophysicists who didn't like that any sane definition of "planet" would exclude Pluto.  
> **username_2**: Roundness is a terrible metric. It is not well defined, it has no clear boundary, and for the foreseeable future, it limits planets and dwarf planets to objects orbiting the solar system. The concept of "planet" can easily be extended beyond the solar system by getting rid of two concepts: (1) that planets are objects that orbit the Sun, and (2) that planets are roundish. The first mean that exoplanets are not planets, and the second means that until we have imaged an exoplanet in high resolution, it will remain a non-planet.  
> **username_0**: @DavidHammen "Sane" definitions _can_ include Pluto if you accept all other dwarf planets as planets as well, or set a border like the 700 km radius. Bodies that are _clearly_ ellipsoidal can't count as asteroids, e.g. Saturn has seven spherical moons while the other Saturnian satellites count as asteroidal, Uranus has five, Neptune one and Jupiter four. If I understand you correctly, you'd like to classify certain moons as planets? Otherwise, one could simply replace "orbit the Sun" by "orbit a star (including remnant stars and brown dwarfs)".  
> **username_0**: @DavidHammen And also include rogue planets of course.  
> **username_2**: Re *Bodies that are clearly ellipsoidal can't count as asteroids* -- That rules out the largest asteroid (Ceres), and possibly others, as asteroids, something the IAU will not condone. The IAU already has already had one over-the-top battle over names. And while "asteroid" has no official definition, calling the moons of Saturn, Uranus, and Neptune "asteroids" is a far stretch, BTW, Pluto is not a planet.  
> **username_0**: @DavidHammen Ceres is no asteroid, it's a dwarf planet. Hygiea should be one too, and since Haumea is a dwarf planet, Vesta and Pallas perhaps should be ones too. But known to be clearly spherical are Ceres, Hygiea, Pluto, Eris and Makemake only (out of dwarf planet candidates). And why do you say Pluto is not a planet? I don't claim anything different except that it depends on definition.  

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## 0 answers

