# What is the most asymmetric known planet?

Planets are more or less spherical, can you tell which one is the most asymmetric? Do they figure out such a property from an eccentric behaviour, or what?

I have a simple technical question: an asymmetric body gets a torque in a gravitational field, is the rotational energy/acceleration to be subtracted from the regular energy/acceleration? In other words: does an asymmetric planet get less radial acceleration than a perfect spherical one?

• Saturn, oblateness: astronomynotes.com/tables/tablesb.htm I'm not sure what you're asking with the rest of your question. Jul 31, 2016 at 13:33
• If we assume identical mass, identical average density and identical angular momentum, then the symmetric planet should rotate faster than the asymmetric one. The dense core being off-center and lighter side of the planet extending further from the gravitational center would cause more rotational inertia. As for gravitational torque, assuming the planet rotates, the gravitational torque would work both ways, accelerating and decelerating as the planet rotates. Tidal forces would tend to draw the heavy side of the asymmetric planet towards the object it orbits. (all that's pretty basic). Jul 31, 2016 at 17:04

What is the most asymmetric known planet?

That depends on what you mean by asymmetric, on what counts as planet, and on what parts of a planet are to be considered / excluded. The dominant feature of many planets is that they aren't quite spherical due to their rotation. If that counts then Saturn is the winner. Saturn's poles are 5900 km closer to Saturn's center of mass than are points on Saturn's equator.

That oblateness is however exactly the shape an object such as Saturn is expected to take on. It means that Saturn is in an equilibrium shape. Moreover, it has a very definite axis of symmetry, the rotation axis. So that's probably not what is meant in the question as being asymmetric. All of the giant planets are quite symmetric by this standard. We need to look to the terrestrial planets.

One concept that might be of aid is the center of mass - center of figure offset of each of the four terrestrial planets. Do the oceans count? Do the icecaps count? Excluding the oceans but not the ice leads to COM-COF offset of about 2100 meters. Including both leads to a COM-COF offset of about 800 meters, about the same as Mercury and Venus. Mars however has the Earth beat by far. Various estimates place the Mars COM-COF offset between 2.5 km and 3.3 km.

Another approach is to look at how lumpy the planet's gravitational field is. Mars once again is the winner in this regard.

I have a simple technical question: an asymmetric body gets a torque in a gravitational field, is the rotational energy/acceleration to be subtracted from the regular energy/acceleration? In other words: does an asymmetric planet get less radial acceleration than a perfect spherical one?

No. In a system of particles bound by central forces such as self-gravitation, Coulomb forces, and chemical bonds, the acceleration of the system's center of mass is the net sum of the external forces acting on the individual particles, divided by the total mass.

• While not a planet, I'd wager the Moon has Mars beat. The least asymmetric are likely the gas giants, because (wait for it), they're gas and far more flexible. Much of the asymmetry is in the outer crustal layers (I would think), that's where you get the most measurable mass variation. Aug 2, 2016 at 15:56
• @userLTK The comments are now deleted, but I requested that the asker make any questions about satellites or dwarf planets into a separate post. Aug 2, 2016 at 16:30
• @userLTK -- In many regards, Mars beats the Moon. Multiple bad things apparently happened to Mars during its formation. Mars' global dichotomy (northern lowlands vs southern highlands) is more stark than is the near side / far side dichotomy of the Moon. Mars' 3+ km center of figure-center of mass offset beats the 1.9 km offset for the Moon. The impact craters on the Moon pale in comparison to those on Mars, and the mountains of the Moon are tiny compared to Mars' volcanoes, particularly Olympus Mons. Io, on the other hand, may be Mars in weirdness. Aug 3, 2016 at 12:26
• @DavidHammen Speaking of an additional acceleration of a rotator that is not perfectly spherical or has some surface features, I would mention the subtle Yarkovsky effect. Negligible for planets in the solar system, it is measurable on asteroids. It may, perhaps, play some role for close-in Earths of small size, though it is impossible to warrant this without numerics. Jan 24, 2023 at 0:45