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What makes the worlds seem round?

26 Aug 2015, 13:23 UTC
What makes the worlds seem round? Voyager 2/NASA
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Are Mimas and Miranda the smallest known objects in hydrostatic equilibrium?
Dan – London
This question relates to the shape of astrophysical objects and as we discussed in the past, it’s an important characteristic of stars, and it’s in the definition of “planet”. An object is round if it has hydrostatic equilibrium, meaning the pressure generated by its components balance the weight of those same components. Now if you go on Wikipedia and look at the gravitationally rounded objects of the Solar System, you’ll find that Mimas and Miranda are the two smallest objects listed in the solar system.
Mimas is the Death Star looking moon of Saturn. It orbits Saturn between the A and the E ring, and has a radius of 198.2±0.4 Km. Miranda is the smallest and innermost of Uranus’s five round satellites, with a radius of 235.8±0.7 km.
Mimas (Credit: Cassini/ESA)
Miranda (Credit: Voyager 2/NASA)
Although both satellites are round it’s not exactly correct to claim they are in hydrostatic equilibrium. They are the smallest round objects due to self-gravity, but ’round’ has a wide definition in astronomy. The Earth is round, but it’s not a perfect sphere. It is an oblate spheroid (the polar radius ...

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