As a planet, Saturn has the highest flattening(ellipticity) which is 0.09796. So, it makes Saturn with the largest equatorial bulge as a planet and as such Saturn is the flattest planet.

However, I have encountered this statement from here:

Earth's Moon is even less elliptical, with a flattening of less than 1/825, while Jupiter is visibly oblate at about 1/15 and one of Saturn's triaxial moons, Telesto, is highly flattened, with f between 1/3 to 1/2 (meaning that the polar diameter is between 50% and 67% of the equatorial).

So, Telesto has a flattening value even higher than Saturn itself. I couldn't find any other body that has higher flattening value than Telesto. Does that mean Telesto has the highest recorded 'f' value?

Question: What celestial body inside the solar system has the highest recorded flattening value? Is it Telesto?

Note: Most of the asteroids, meteors, comets and small moons have irregular shapes and as such measuring the 'f' value is impossible and may have no useful value. I want to know which body has the highest recorded 'f' value.

  • $\begingroup$ Does stating a "flattening ratio" require that the flattening be due to rotation? Can crazy-shaped objects like ʻOumuamua or ravioli-like moons be considered as well? (see this answer and this answer) $\endgroup$ – uhoh Nov 17 '20 at 3:35
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    $\begingroup$ @uhoh It is hard to measure f value for 'crazy shaped objects' as they can have irregular dimensions as such it is suitable for objects having the ellipsoid/spheroid shape(similar to rugby ball) or any other shape as long as it maintains hydrostatic equilibrium. The flattening can be due to any reasons. It can be due to rotation, gravitational perturbation, orbital eccentricity, tidal locking etc. I want to know the highest known value which has been recorded by astronomers and not just a value from 'back-of-the-envelope' calculation. $\endgroup$ – Nilay Ghosh Nov 17 '20 at 5:20

I believe besides Telesto the dwarf planet Haumea in the Kuiper belt could also be in the mix for the highest flattening of objects in our solar system. Haumea is a triaxial ellipsoid where the 'f' value would around 1/2 between the largest and smallest major axis.


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