Why are Planets ROUND and Galaxies FLAT?

 

This is our earth it's a sphere like a football no not the American football the proper football the one that is actually shaped like a ball our earth has a moon that is also shaped like a ball and so is our sun so then how come our galaxy the milky way is flat like a pancake welcome the Science Reads and today let's find out why galaxies are flat when stars and planets around when we look at our star system we see objects of all sizes we have tiny grains of dust rocky planets of smaller size gas giants of a

 larger size and of course the sun which

 isn't even too big for its own kind just the perfect size to nurture life on the earth the most obvious thing that separates these cosmic entities from one another is their shape while smaller objects like asteroids and dwarf planets like Haumea come in irregular shapes planets and moons of the larger size form distinct spherical shapes along with the sun if you are wondering why that is well the answer boils down to gravity an object's gravitational pull will always point towards the center of its mass the bigger something is the more massive it is and the larger its gravitational pull for solid planets and moons like the earth and well the moon the gravitational force is contested

 by the strength of the object itself

 the downforce form of the gravity that

 we experience on the earth is an example of this phenomenon as gravity doesn't pull us to the center of the earth because the ground pushes us back upwards the two forces counter each other and balance one another and that's how we manage to not get swallowed up by the earth or get jettisoned into the atmosphere and manage to keep our feet to the ground but earth's strengths has its limits for instance mount Everest manages to get larger and larger as the planet's plates push against each other when Everest gets taller it also gets bigger mass-wise and the more weight it gathers the more gravity pulls it back toward the center of the earth making it sink back capping its height to a maximum point if the earth was made of pure water then mount Everest would just sink down all the way to the center of the earth

 displacing any water it passed through

 any areas where the water was unusually high would sink pulled down by earth's gravity areas where the water was unusually low would be filled up by water displaced from elsewhere resulting in that this imaginary ocean earth would become perfectly spherical interestingly the force that binds our universe together called gravity isn't as strong as people like to think an object has to be really big before it could exert a strong enough gravitational pull to overcome the strength of the material from which it's made smaller solid objects which are only a few meters or kilometers in diameter have a significantly weak gravitational pull and that hinders them from forming a spherical shape funny enough this is the very reason we humans even animals like blue whales or any other living being doesn't curl up into the shape of a ball under our own body's gravitational pull because our body is too strong to give into gravity except for armadillos they can turn into a ball and back whenever they can because magic it's interesting that the smaller objects have strength winning but as the object gets bigger and bigger gravity starts taking hold of it overcoming the strength of the material from which the object is made it will tend

 to pull all of the object's material

 into a spherical shape like an expert

 blacksmith it will level it down

 pulling down the bits that are too high and pushing the areas that are too low outwards achieving the spherical shape is known as being in the state of hydrostatic equilibrium we can see this phenomenon all around us as well the more a person gains weight the rounder they appear but all jokes aside the concept of hydrostatic equilibrium poses a new question how big an object must be to reach the state of hydrostatic equilibrium and become near perfect spherical well to answer that simply it depends on the ingredients that make up the object and how much resistance they can pose the gravity an object made of just liquid water would manage it really easily as it would essentially have no strength as water molecules move around quite easily you can see it in your own home just go to a leaky tap and watch the droplets of water forming a perfectly spherical shape the same applies to objects made up of nothing but pure gases but throw in some iron and the size requirement for the object to achieve hydrostatic equilibrium suddenly shoots up through the roof

 in our solar system it is estimated

 that the threshold diameter required for an ic object to become spherical is at least 400 kilometers for objects made primarily of stronger material obviously the threshold gets exponentially larger the smallest spherical object that we are aware of in our solar system is memos Saturn's moon that very much resembles the death star it has a diameter of 396 kilometers however there is more to this contest between gravity and the strength of the objects

 the external gravity of other objects forces cosmic beings to spin and tumble through space and when an object is spinning locations at its equator that is the halfway point between the two poles effectively feel a slightly less gravitational pull in comparison to the gravity near the poles this is why the perfectly spherical shape you'd expect in hydrostatic equilibrium is shifted to what we call an oblate spheroid that means that the object is wider at its equator than it is at its poles it can be observed on our own earth which has an equatorial diameter of twelve thousand seven hundred fifty six kilometers and a pole to pole diameter of twelve thousand seven hundred 712 kilometers 7899 miles the faster an object in space spins the more dramatic this effect is Saturn which is less dense than water spins on its axis every 10 and a half hours as a result it is much less spherical than earth this is visible more prominently with stars like Altair which spins once every nine hours or so this is insanely fast for a star and it causes its equatorial diameter to be 25 larger than the distance between its poles making it look a little flatter why is this important you will find out soon now that we have established why stars and planets around let's look at why galaxies are flat like a disk but before we get there we are leaping over an important building block of galaxies the star systems look at our solar system for instance the planets orbit the sun in the same plane instead of being distributed in a spherical arrangement similar to galaxies star systems are flat too from nanoscale to macro scale everything in the universe pretty much follows the laws of gravitational force in the same way just like smaller objects orbit around the nucleus smaller cosmic objects like planets and moons orbit around stars Saturn's rings are arranged in a disk shape in the same plane with the planet's center of mass as well asteroid belts are also flat and so are most galaxies this pattern can be explained by a fundamental law of physics called the conservation of angular momentum angular momentum is

 the rotational analog of linear momentum and is defined as the product of its

 moment of inertia and its angular velocity so when no external torque acts on an object no change of angular momentum will occur if the net torque is zero then angular

 momentum is constant or conserved

 the conservation of angular momentum

 explains why an ice skater can increase

 the angular acceleration by bringing their arms and legs close to the vertical axis of rotation because the moment is conserved when the skater decreases the rotational inertia the rotation rate must increase around 4.6 billion years ago there was no planet earth and the sun had yet to form instead a huge blob of gas and dust floated about for millions of square miles then gravity came into play and it caused particles of matter to gradually move closer and eventually resulting in collisions the collision caused new random trajectories but the angular momentum must have remained constant in this isolated system because the local gravitational effect of other bodies was negligible as angular momentum is a physical quantity that's constant around a fixed axis that would be a point in 2d space and a line in 3d space so the system must have rotated along a plane that's perpendicular to its axis now when all the primordial gases and dust began colliding the particles would have moved in every direction but the up and down motions phased each other out eventually the particles lose their liberty to move anywhere but the selected 2d plane as a result the matter formed a disc on a single plane this very matter then evolved into planets and other entities which remain fixed to this proverbial plane so the very angular momentum that helped the sun planets and moons to form was the reason why they together formed a disk solar systems are made of planets dwarf planets asteroids moons and stars often more than one galaxies two are composed of star systems stars gas planets and other objects swirling around the black hole located at the center at the basic level galaxies and star systems are simply the same system working at different levels with a group of objects bound by gravity to a more massive object so the laws of physics that apply to the solar system apply similarly to a galaxy ii galaxies can take different shapes as they evolve some organize in the shape of a spiral others have an elliptical shape and a few rare one group up has a ring around their black hole but no matter what shape they take they

 all end up becoming flat when looked at

 from the side and it's the same reason why the solar system looks flat the angular momentum similar to star systems as galaxies form around a selected 2d plane and form a disk the up and down collision's forces would cancel each other out due to conservation of momentum they will keep moving in the same direction as the rest of the galaxy but they will not move much in other directions galaxies do not end up in a spherical shape like planets because the distance between the center and the arms keeps the objects in a certain balance where the stars are orbiting or swirling around the black hole but they are not being completely brought into the core some galaxies with big enough black holes will eventually die as a result of the

 Hawking radiation but that process would take hundreds of millions or even billions of years so while that happens the galaxies will still keep their pancake-like shape tell us in the comments which other space questions keep you up at night