Why Are Things In Space Round?

We’ve all noticed that most things in space are round, right? It’s common knowledge. But have you ever wondered why things in space are round or spherical?

The answer is gravity. Gravitational force is a central force. Its magnitude depends only on the distance separating the two masses. Thus, gravitational force possesses spherical symmetry. Gravity bring matter together and form a structure given they have enough mass. Only in a sphere can every point on the surface be equidistant from the centre making the gravitational force experienced on the surface uniform. Hence, any other shape would be impossible for massive objects.

The celestial objects like stars, planets and moons are made up of gases, rocks, ice etc. Given enough mass, the pull is going to arrange all the stuff into a roughly spherical shape. Less massive objects, like comets and asteroids, have less gravity. So the pull is not strong enough to mold them into a perfect sphere.

ARE ALL OBJECTS PERFECT SPHERES?

Not every astronomical object is a perfect sphere. Some are rounder than the other.

Celestial objects rotate around an axis. When something spins, like planets and stars, things on the outer edge have to move faster than things on the inside. The constant and rapid rotation flattens out the middle, and makes them wider around the equator compared to the poles. Faster the angular speed of rotation; more flattened the object is around the equator. When big objects spin very fast, they bulge in the middle and the extra width is called equatorial bulge.

Jupiter spins very rapidly (completes one rotation in 9.9 hours) and hence it is about 71,500 km across the equator and 66,900 km from pole to pole which means it is around 6.4% thicker around the middle. Saturn bulges the most in the solar system. It is about 10.7% thicker around the middle as compared to the distance from pole to pole.

Our sun spins too. However, it spins very slowly around its axis. Hence, it is almost a perfect sphere. The fastest spinning star in our cosmic neighbourhood, VFTS 102, a massive star in the Tarantula nebula in Large Magellanic Cloud, spins 100 times faster than our sun. The star is approaching the point when it will be torn apart by centrifugal forces if it spins any faster.

WHY DOES GRAVITY MAKE THINGS ROUND?

Gravity doesn’t always make things round. With the addition of mass, the gravitational pull will get bigger and eventually it would get to a point where things get so massive and gravitational pull on the surface would be so strong that it would start to squash the very material it is made up of into a sphere as this shape allows the gravitational potential to be same everywhere on the surface. How much matter do we need to get the gravity strong enough to make something a sphere? The minimum size is called the “potato radius” because things that are too small to be made a sphere are shaped like misshaped potatoes. Cool name though.

In the skies, there are lot of things like planets, their moons etc. If the radii of these objects are greater than 200km, they are spheres. However, if the radii of the objects are less than 200 km, they look like oddly shaped potatoes. These objects include asteroids and the two moons of Mars, Phobos and Deimos with radii 11.27 km and 6.2 km respectively and comets.

CONCLUSION

The things that determine the shape of astronomical objects are gravity and hence their masses and their size. No wonder “flat-earthers” deny the existence gravity! On top of that, the spins of objects also cause aberration in the spherical shape of the object. To conclude this article, most massive things in this universe are spherical.

Thank you for reading and most importantly keep on being curious!!

et me know what you think about giant spheres in space in the comment section below.