<p>Among the asteroids 4 Vesta is the largest non-spherical object in the Solar system. But far out there in the Kuiper Belt is 136108 Haumea, and maybe yet undiscovered objects, that are very elongated. Haumea is 1,960 × 1,518 × 996 km. That's larger than the 960 km diameter Ceres and hardly spherical with 2:1 proportions. Its fast rotation and composition of malleable volatiles makes this possible. Maybe Haumea is "hydrostatically relaxed" without being spherical?</p>

<p>Haumea images below from <a href="https://en.wikipedia.org/wiki/Haumea" rel="nofollow noreferrer">Wikipedia</a>.</p>

<p><a href="https://i.sstatic.net/58wJy.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/58wJy.jpg" alt="enter image description here"></a>
<a href="https://i.sstatic.net/9Y6K5.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/9Y6K5.png" alt="enter image description here"></a></p>

CC BY-SA 3.0
2016-11-26T23:02:37.623Z
Thanks for the quick answer! We should probably iron out the remaining question still. I think for a relaxed body it would be spherical if not rotating much or an oblate spheroid if rotating - either way the surface would have reached equilibrium. I don't think a tri-axial ellipsoid is the minimum energy shape. See [here](https://en.wikipedia.org/wiki/IAU_definition_of_planet#First_draft_proposal) and [here](https://en.wikipedia.org/wiki/Dwarf_planet).
uhoh
CC BY-SA 3.0
2016-11-28T05:45:17.973Z
Wow! It seems the triaxial ellipsoid [certainly **can be**](http://astronomy.stackexchange.com/a/19207/7982) the minimum energy solution (or at least close to it). Stackexchange to the rescue!
uhoh
CC BY-SA 4.0
2021-06-17T15:40:22.427Z
[Dunham et al. (2019)](https://ui.adsabs.harvard.edu/abs/2019ApJ...877...41D/abstract) argue that Haumea's triaxial shape is indeed consistent with hydrostatic equilibrium.
Peter Erwin
CC BY-SA 3.0
2016-11-26T15:21:11.530Z
2016-11-26T15:27:42.257Z
2016-11-26T15:27:42.257Z
LocalFluff
<p>To add to LocalFluff's answer, the largest known clearly irregular body is the <a href="https://en.wikipedia.org/wiki/3_Juno" rel="nofollow noreferrer">asteroid Juno</a> (mean diameter: 246.6 km / 153.2 mi) in the main belt. However, celestial bodies can be spherical by chance, while far from hydrostatic equilibrium, like in case of Martian moon Phobos. A better way to distinguish asteroids from planets is by interior differentiation. Differentiated bodies are per se somewhat spheroidal. And if we take 'overcoming rigid body forces' literally, even some IAU-recognized planets aren't in hydrostatic equilibrium anyway.</p>

CC BY-SA 4.0
2021-01-18T12:20:21.470Z
2021-01-18T12:20:21.470Z
Plutos Loyer
<p>I was showing a recent video about Ceres to a friend and we looked up asteroids. Ceres has attained a fairly spherical shape, but Vesta still has a way to go. From <a href="https://en.wikipedia.org/wiki/4_Vesta#Physical_characteristics" rel="nofollow noreferrer">Wikipedia</a>:</p>

<blockquote>
  <p>Vesta's shape is close to a gravitationally relaxed oblate spheroid, but the large concavity and protrusion at the southern pole (see 'Surface features' below) combined with a mass less than $5×10^{20}$ kg precluded Vesta from automatically being considered a dwarf planet under International Astronomical Union (IAU) Resolution XXVI 5.</p>
</blockquote>

<p>I don't know the proper term to use here. Perhaps it is "hydrostatically relaxed" or something similar – I'm looking for the term that would suggest a body has reached a fairly round shape without major excursions, under the influence of its own gravity.</p>

<p>I think Ceres meets this criterion, and Vesta doesn't. Is Vesta the largest such body (by dimension or volume, not sure which would be the most reasonable measure)?</p>

CC BY-SA 3.0
2016-11-27T06:31:34.950Z
A related and I thought it was a fun read, at least the top answer:   https://www.quora.com/How-and-why-are-planets-spherical-What-makes-them-round#!n=96
userLTK
CC BY-SA 3.0
2016-11-27T07:36:23.603Z
@userLTK that is absolutely fascinating! I've asked a [follow-up question](http://astronomy.stackexchange.com/q/19202/7982). Thank you for that!
uhoh
CC BY-SA 3.0
2018-02-04T17:35:58.940Z
I couldn't find tags for planetoid or planetessimal, would something like these be appropriate now?
uhoh
CC BY-SA 3.0
2018-02-04T17:37:42.233Z
@Chirlu I've moved the sentence you proposed deleting into a comment, then accepted your edit. It should have been a comment from the beginning, however next time consider moving rather than simply discarding helpful suggestions.
uhoh
CC BY-SA 3.0
2018-02-04T18:02:00.627Z
@uhoh: OK, but I thought after more than one year, it's not going to happen anyway. ;-) In any case, as it is now, it is properly associated with your account.
chirlu
<asteroids><planetary-formation>
Ceres is spherical, Vesta isn't – is it the largest non-spherical solid body in the solar system?
uhoh
# Ceres is spherical, Vesta isn't – is it the largest non-spherical solid body in the solar system?

(Asked by: username_0 on 2016-11-26)

I was showing a recent video about Ceres to a friend and we looked up asteroids. Ceres has attained a fairly spherical shape, but Vesta still has a way to go. From [Wikipedia](https://en.wikipedia.org/wiki/4_Vesta#Physical_characteristics):

> Vesta's shape is close to a gravitationally relaxed oblate spheroid, but the large concavity and protrusion at the southern pole (see 'Surface features' below) combined with a mass less than $5×10^{20}$ kg precluded Vesta from automatically being considered a dwarf planet under International Astronomical Union (IAU) Resolution XXVI 5.

I don't know the proper term to use here. Perhaps it is "hydrostatically relaxed" or something similar – I'm looking for the term that would suggest a body has reached a fairly round shape without major excursions, under the influence of its own gravity.

I think Ceres meets this criterion, and Vesta doesn't. Is Vesta the largest such body (by dimension or volume, not sure which would be the most reasonable measure)?

**Tags:** asteroids, planetary-formation

> **Comments:**
> **username_1**: A related and I thought it was a fun read, at least the top answer:   https://www.quora.com/How-and-why-are-planets-spherical-What-makes-them-round#!n=96  
> **username_0**: @username_1 that is absolutely fascinating! I've asked a [follow-up question](http://astronomy.stackexchange.com/q/19202/7982). Thank you for that!  
> **username_0**: I couldn't find tags for planetoid or planetessimal, would something like these be appropriate now?  
> **username_0**: @Chirlu I've moved the sentence you proposed deleting into a comment, then accepted your edit. It should have been a comment from the beginning, however next time consider moving rather than simply discarding helpful suggestions.  
> **username_2**: @username_0: OK, but I thought after more than one year, it's not going to happen anyway. ;-) In any case, as it is now, it is properly associated with your account.  

---

## 2 answers

### Answer by username_3 on 2016-11-26. Score: 2 | Accepted answer

Among the asteroids 4 Vesta is the largest non-spherical object in the Solar system. But far out there in the Kuiper Belt is 136108 Haumea, and maybe yet undiscovered objects, that are very elongated. Haumea is 1,960 × 1,518 × 996 km. That's larger than the 960 km diameter Ceres and hardly spherical with 2:1 proportions. Its fast rotation and composition of malleable volatiles makes this possible. Maybe Haumea is "hydrostatically relaxed" without being spherical?

Haumea images below from [Wikipedia](https://en.wikipedia.org/wiki/Haumea).

[![enter image description here](https://i.sstatic.net/58wJy.jpg)](https://i.sstatic.net/58wJy.jpg)
[![enter image description here](https://i.sstatic.net/9Y6K5.png)](https://i.sstatic.net/9Y6K5.png)

> **Comments:**
> **username_0**: Thanks for the quick answer! We should probably iron out the remaining question still. I think for a relaxed body it would be spherical if not rotating much or an oblate spheroid if rotating - either way the surface would have reached equilibrium. I don't think a tri-axial ellipsoid is the minimum energy shape. See [here](https://en.wikipedia.org/wiki/IAU_definition_of_planet#First_draft_proposal) and [here](https://en.wikipedia.org/wiki/Dwarf_planet).  
> **username_0**: Wow! It seems the triaxial ellipsoid [certainly **can be**](http://astronomy.stackexchange.com/a/19207/7982) the minimum energy solution (or at least close to it). Stackexchange to the rescue!  
> **username_4**: [Dunham et al. (2019)](https://ui.adsabs.harvard.edu/abs/2019ApJ...877...41D/abstract) argue that Haumea's triaxial shape is indeed consistent with hydrostatic equilibrium.  

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### Answer by username_5 on 2021-01-18. Score: 2

To add to username_3's answer, the largest known clearly irregular body is the [asteroid Juno](https://en.wikipedia.org/wiki/3_Juno) (mean diameter: 246.6 km / 153.2 mi) in the main belt. However, celestial bodies can be spherical by chance, while far from hydrostatic equilibrium, like in case of Martian moon Phobos. A better way to distinguish asteroids from planets is by interior differentiation. Differentiated bodies are per se somewhat spheroidal. And if we take 'overcoming rigid body forces' literally, even some IAU-recognized planets aren't in hydrostatic equilibrium anyway.


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