Why are Celestial Objects Spherical (and Others Are Not)?

BIS Space: SphereImage credit: NASA/Apollo 17 crew; taken by either Harrison Schmitt or Ron Evans [Public domain]

It’s mostly because of their size

The most massive objects in the universe have a spherical shape. Smaller objects can have almost any shape, but as their mass increases, so does their gravitational field. They eventually collapse until gravity and the pressure gradient counterbalance each other. This is hydrostatic balance. The sphere is the form where all matter can be as close to the centre as possible. This is the shape adopted by all bodies that have an important gravity.


BIS Space: Sphere

This collage is composed of Chandra images made over a span of several months (ordered left to right, except for the close-up). They provide a dramatic look at the activity generated by the pulsar (white dot near the centre of the images) in the Crab Nebula. The inner X-ray ring is thought to be a shock wave that marks the boundary between the surrounding nebula and the flow of matter and antimatter particles from the pulsar. Energetic shocked particles move outward to brighten the outer ring and produce an extended X-ray glow. The jets perpendicular to the ring are due to matter and antimatter particles spewing out from the poles of the pulsar.

Image by NASA / CXC / ASU / J. Hester et al.

Small dense objects can also be spherical

There is no limit for an object to be spherical. It depends on its size, on its mass and on its rotation speed. We generally observe this phenomenon on bodies more than 1000 kilometres in diameter, with many exceptions. Neutron stars can be spherical whereas they are only a few tens of kilometres in diameter. Their density and their phenomenal gravity do not generate irregularities.


The large spherical planets have a bead at the equator

BIS Space: Sphere













Image credit: A. Feild (Space Telescope Science Institute) [Public domain]

Planets and other large celestial bodies are not quite spherical, because of their rotation. They are a little more swollen at their equator. We observe this phenomenon on Earth, but it is even more apparent on the gas giants. Saturn, in particular, has an equatorial diameter 10% larger than its polar diameter. The most extreme case we know is Haumea. This dwarf planet is rotating so fast that it is twice as wide as it is high.

A flat planet?

However, a body breaks up beyond a limit. In theory, a very fast rotating planet made up of uranium could have an equatorial diameter five times larger than its polar diameter. If such an object exists, it is probably the closest thing to a flat planet in the universe.


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