Science Questions

Why are astronomical objects usually spherical?

Sun, 14th Mar 2010

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Question

Christine Watchorn asked:

As I stared at the perfect circle of a setting sun, I wondered why all stars, planets etc known to us (or me) are perfect spheres, rather than elliptical or totally irregular shapes. Or are there some out there which aren't spherical that I don't know about??? Eager to have an answer and just love the show which makes science accessible to all, including 40 something, middle-aged Mums like me!

Answer

Chris -   Itís a very good question and thereís obviously some important science there for the simple reason that it doesnít matter what you look at, most of these objects are round, absolutely. 

The simple answer is itís down to gravity.

If we take our own solar system as a really good example, you have the Sun in the centre and thatís round, and the reason thatís round is because a big ball of gas collapsed in on itself and squeezed hard enough to start fusion.

Around that ball of gas wouldíve been initially an envelope - like a shroud - of material comprising dust, debris and gas, which slowly condensed into a disk called a proto-planetary disk.

 Over time, all of this material, thanks to gravity - which is a function of mass - wouldíve pulled this material slowly together. It would have accreted - or got together - and slowly wouldíve built up planetessimals, miniature planets, and then they grew to make big planets as they hoovered up - under increasingly powerful gravitational fields - the rest of that residual material.

Because gravity is pulling things together, everything thatís being attracted wants to get us close to everything else as it can.  The most effective way for that to happen is if objects are spherical. 

Itís the same as a raindrop, because water wants to get us close to other water molecules as it can without being in contact with too much air. Thatís why raindrops form round blobs, not a flat sheet of water - this way, as many water molecules can get as close together and stick together as they can.

Thatís also whatís happening with these nascent planets, or other objects in space.  The material squeezes together, and the way in which you can get as much material in as close a configuration as possible to other material is if itís a round shape.

Now, if you go in at high resolution and look closely, obviously, thereís not a perfect round surface because the Earth has mountains and things and so does Mars - itís got Olympus Mons, a giant volcano.

But, to all intents and purposes, at low resolution, these things are round because gravity has made them that way.

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Christine Watchorn asked the Naked Scientists: As I stared at the perfect circle of a setting sun, I wondered why all stars, planets etc known to us (or me) are perfect spheres, rather than elliptical or totally irregular shapes. Or are there some out there which aren't spherical that I don't know about??? Eager to have an answer and just love the show which makes science accessible to all, including 40 something, middle-aged Mums like me! What do you think? Christine Watchorn, Tue, 23rd Feb 2010

While the answer listed is mostly correct; it isn't actually entirely true; What musn't be forgotten is that gravity isn't the only thing affecting an object and just as importantly multiple "sources" of gravity can be affecting the same object; For example: earth, is not perfectly round. Earth's spin causes it to bulge slightly at its middle. I'm also pretty sure the moon or if not our moon at least some of the moons orbittng gass giants aren't perfectly round because the gravity of the gas giant is pulling hard on the side of that moon facing the gass giant. So gravity causes things to ultimately WANT to be round; But that doesn't actually mean everything is round. Mark, Sun, 11th Apr 2010

Planets are things large enough to be rounded out by their own gravity. There was a contest to see if the world was prolate (stretched out) or oblate (squashed down). Newtownians said oblate, while Cartesians said prolate. Measurements showed that the earth was indeed oblate. To learn more about this look up Pierre-Louis Moreau de Maupertuis and his 1736 trip to Lapland to measure the shape of the earth. stereologist, Tue, 13th Apr 2010

the gravitational attraction between each and every particle in an asteroid planet or star is basically what holds them together.  as a body gets more massive this gravitational effect gets stronger.  the gravitational field at the surface of the moon is only one sixth of that of the earth while the gravitational field at the surface of the sun is twenty seven times that of the earth.

Now think of the earth as being made out of water if a bit of it gets higher than another bit it will rapidly find its own level and sink back like a wave.  so it is reasonably clear that a non rotating  planet made out of water would be spherical because the gravity is acting in all directions to hold it in evenly and the surface of a sphere is a group of points equididtant from the centre.

Now asteroids are small and have only very weak gravitational fields and the solid lumps that make them up can stick together pretty much as they like so they can have pretty irregular shapes with lumps and bumps on them and spaces between larger lumps inside them.  however as they get bigger and the gravity gets stronger this tends to crush things up and eventually the material in the middle gets squashed so strongly that it tends to collapse and behave more like a liquid so the asteroid eventually becomes spherical and we call it a dwarf planet and eventually a full planet.  This tends to happen with objects about a few hundred miles across when they are made out of rocks and ice.

The amount of surface irregularity depends on the strength of the rock and the size of the object.  As you might expect the smaller objects can be more irregular and the bigger a solid planet is the smoother its surface is likely to be.

Most objects much larger than the earth like the giant planets Jupiter and Saturn  and stars  do not have solid surfaces and are balls of gas and maybe liquid inside although there may be a solid core of metallic hydrogen in the middle of planets like jupiter.

If an object is rotating the most stable surface is distorted slightly to become broader at the equator than the poles because of the effect of centrifugal forces as well as gravity.  this is measurable on the earth and visible on the giant planer Jupiter which in spite of its size rotates on its axis in just under ten hours and causes it to bulge very noticeably around its equator.

Soul Surfer, Tue, 13th Apr 2010

Perfect Answer! Thanks absfrm, Fri, 18th Jul 2014

why larger objects like whole solar system, or galaxies are disk like? Enam, Sat, 20th Jun 2015

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