WEBVTT - Why Are Stars and Planets Nearly Spherical?

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<v Speaker 1>Welcome to brain Stuff from how Stuff Works. Hi, brain Stuff,

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<v Speaker 1>Lauren Vogel bomb here. It's something we kind of take

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<v Speaker 1>for granted. Roses are red and planets are spherical. That's

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<v Speaker 1>just the way things are, right after all. Building model

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<v Speaker 1>solar systems would be way more challenging if instead of

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<v Speaker 1>using little phone balls we had to make a bunch

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<v Speaker 1>of dough decahedron shaped planet models. But have you ever

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<v Speaker 1>wondered why planets look like this? Why are they basically

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<v Speaker 1>spherical and not say cylindrical or cube shaped. We should

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<v Speaker 1>kick off this discussion by calling a spade a spade.

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<v Speaker 1>None of the planets in our solar system are perfect spheres, nor,

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<v Speaker 1>for that matter, is our Sun. All those bodies could

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<v Speaker 1>be more accurately described as oblate spheroids objects with the

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<v Speaker 1>shape bulge slightly around the middle. To borrow an analogy

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<v Speaker 1>from the astronomer philled plate, they look like a basketball

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<v Speaker 1>that someone is sitting on. But more technically, in a

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<v Speaker 1>celestial body with an oblate spheroid shape, the polar circumference

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<v Speaker 1>will be smaller than the equatorial one. So here on Earth,

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<v Speaker 1>if you were to travel from the north pole to

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<v Speaker 1>the South pole and back, you'd have walked a grand

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<v Speaker 1>total of twenty four thousand, eight hundred and twelve miles

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<v Speaker 1>that's thirty nine thousand, nine hundred and thirty one kilometers.

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<v Speaker 1>On the other hand, a complete trip around the equator

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<v Speaker 1>would be a bit longer. That's because the circumference of

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<v Speaker 1>Earth's equator is twenty four thousand, nine hundred miles or

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<v Speaker 1>forty thousand and seventy kilometers. As such, when you stand

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<v Speaker 1>at sea level on the equator, you're further away from

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<v Speaker 1>the center of our planet than you would be at

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<v Speaker 1>either the north or south pole on some other planets.

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<v Speaker 1>This bulge is even more pronounced. Just look at Jupiter.

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<v Speaker 1>Earth is only zero point three percent wider at the

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<v Speaker 1>equator than it is from poll to poll, but Jupiter's

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<v Speaker 1>measurements showcase a much bigger disparity. Astronomers have found that

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<v Speaker 1>this plus sized planet is a full seven percent wider

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<v Speaker 1>at its equator than it is between the polls. The

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<v Speaker 1>oblate spheroid shape is the result of two main factors,

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<v Speaker 1>gravity and rotation. Troy Carpenter, director of Washington State's Goldendale Observatory,

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<v Speaker 1>recently discussed the matter with us in an email exchange

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<v Speaker 1>they explain, everything which has mass experiences gravity, and gravity

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<v Speaker 1>attempts to crush an object inward in all directions. That's

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<v Speaker 1>because all objects experience self gravity, a force which pulls

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<v Speaker 1>their atoms toward a common center. As the massive an

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<v Speaker 1>object increases, so too does its self gravitational pull. After

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<v Speaker 1>it exceeds a certain mass, the pull gets overpowering to

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<v Speaker 1>the point where the object collapses into itself and becomes spherical.

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<v Speaker 1>Little items, like say a banana or a lug wrench,

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<v Speaker 1>can resist this fate because their self gravity is relatively weak,

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<v Speaker 1>allowing them to retain non spheroid shapes. However, in planets, suns,

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<v Speaker 1>and other truly massive bodies, the force is so strong

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<v Speaker 1>that they can't avoid being distorted into spheroids. But Carpenter

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<v Speaker 1>said gravity is not the whole story. While gravity conspires

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<v Speaker 1>to render the planet's spherical, the speed of their rotation

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<v Speaker 1>is simultaneously trying to flatten them. The faster celestial body spins,

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<v Speaker 1>the more disproportionate its equatorial bulge gets. Carpenter tells us

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<v Speaker 1>this is why there are no perfect spheres in our

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<v Speaker 1>Solar system, only oblate spheroids. The Sun is almost a

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<v Speaker 1>perfect sphere. Due to its immense gravity and relatively slow

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<v Speaker 1>rotation rate of twenty five days, A significant percentage of

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<v Speaker 1>stars in the sky rotate much faster and bulge noticeably

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<v Speaker 1>at their equators. One such star is all Tear, located

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<v Speaker 1>just sixteen point eight light years away from our home planet.

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<v Speaker 1>It's among the brightest objects in the night sky. All

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<v Speaker 1>Tear is also notable for spinning very very fast. It

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<v Speaker 1>completes a full rotation on its axis every ten point

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<v Speaker 1>four Earth hours. Accordingly, astronomers estimate that all Tears at

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<v Speaker 1>least fourteen percent wider at the equator than it is

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<v Speaker 1>from pole to pole. Rotational speed also explains Jupiter's bulge.

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<v Speaker 1>After all, a day on this gas giant is a

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<v Speaker 1>brisk nine point nine Earth hours long. Other forces act

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<v Speaker 1>upon the stars and planets as well, altering their shapes.

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<v Speaker 1>Although Earth is an oblate spheroid, it certainly isn't a

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<v Speaker 1>perfect one. The gravitational pull of the Sun and Moon

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<v Speaker 1>both influence the planet's shape to a degree. For that matter,

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<v Speaker 1>so do Earth's own plate to topics. Consequently, the mass

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<v Speaker 1>of our homeworld isn't evenly distributed. In fact, it's sort

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<v Speaker 1>of lumpy. Still, it looks a good deal rounder than

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<v Speaker 1>Jupiter and Saturn. In turn, the planets in our universe

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<v Speaker 1>appear way more spherical than some of their moons do. Mars,

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<v Speaker 1>for instance, has two small satellites, neither of which has

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<v Speaker 1>a self gravity to be pulled into an oblate spheroid. Instead,

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<v Speaker 1>their appearance is often described as potato shaped. In conclusion,

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<v Speaker 1>will say this much for our home planet. It may

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<v Speaker 1>not be flawless, but at least the place is fairly

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<v Speaker 1>well rounded. Today's episode was written by Mark Mancini and

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<v Speaker 1>produced by Tristan McNeil. For more on this and lots

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<v Speaker 1>of other interplanetary topics, visit our home planet how stuff

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<v Speaker 1>works dot com