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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:

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)?

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    $\begingroup$ A related and I thought it was a fun read, at least the top answer: quora.com/… $\endgroup$
    – userLTK
    Commented Nov 27, 2016 at 6:31
  • $\begingroup$ @userLTK that is absolutely fascinating! I've asked a follow-up question. Thank you for that! $\endgroup$
    – uhoh
    Commented Nov 27, 2016 at 7:36
  • $\begingroup$ I couldn't find tags for planetoid or planetessimal, would something like these be appropriate now? $\endgroup$
    – uhoh
    Commented Feb 4, 2018 at 17:35
  • $\begingroup$ @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. $\endgroup$
    – uhoh
    Commented Feb 4, 2018 at 17:37
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    $\begingroup$ @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. $\endgroup$
    – chirlu
    Commented Feb 4, 2018 at 18:02

2 Answers 2

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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.

enter image description here enter image description here

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  • $\begingroup$ 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 and here. $\endgroup$
    – uhoh
    Commented Nov 26, 2016 at 23:02
  • $\begingroup$ Wow! It seems the triaxial ellipsoid certainly can be the minimum energy solution (or at least close to it). Stackexchange to the rescue! $\endgroup$
    – uhoh
    Commented Nov 28, 2016 at 5:45
  • $\begingroup$ Dunham et al. (2019) argue that Haumea's triaxial shape is indeed consistent with hydrostatic equilibrium. $\endgroup$
    – Peter Erwin
    Commented Jun 17, 2021 at 15:40
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To add to LocalFluff's answer, the largest known clearly irregular body is the asteroid 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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