BrainStuff Classics: Why Are Stars and Planets Nearly Spherical?

are spherical. That's just the way things are, right, after all, Building model solar systems would be way more challenging if instead of using little foam balls we had to make a bunch of do decahedron shaped planet models. But have you ever wondered why planets look like this? Why are they basically spherical and not say, cylindrical or cube shaped? We should kick off this discussion by calling a spade

a spade. None of the planets in our solar system are perfect spheres, nor, for that matter, is our Sun. All those bodies could be more accurately described as oblate spheroids objects with the shape bulge slightly around the middle. To borrow an analogy from the astronomer philled plate, they look like a basketball that someone is sitting on put More technically, in a celestial body with an oblate spheroid shape, the polar circumference will be smaller than the equatorial one.

So here on Earth, if you were to travel from the north pole to the South pole and back, you'd have walked a grand total of twenty four thousand, eight hundred and twelve miles that's thirty nine thousand, nine hundred and thirty one kilometers. On the other hand, a complete trip around the equator would be a bit longer. That's because the circumference of Earth's equator is twenty four thousand,

nine hundred miles or forty thousand and seventy kilometers. As such, when you stand at sea level on the equator, you're further away from the center of our planet than you would be at either the north or South pole. On some other planets. This bulge is even more pronounced. Just look at Jupiter. Earth is only zero point three percent wider at the equator than it is from poll to poll,

but Jupiter's measurements showcase a much bigger disparity. Astronomers have found that this plus sized planet is a full seven percent wider at its equator than it is between the polls. The oblate spheroid shape is the result of two main factors, gravity and rotation. Troy Carpenter, director of Washington State's Goldendale Observatory, recently discussed the matter with us in an email exchange. They explain, everything which has mass experiences gravity, and gravity

attempts to crush an object inward in all directions. That's because all objects experience self gravity, a force which pulls their atoms toward a common center. As the mass of an object increases, so too does its self gravitational pull. After it exceeds a certain mass, the pull gets overpowering to the point where the object collapses into itself and becomes spherical. Little items, like say a banana or a lug wrench, can resist this fate because their self gravity

is relatively weak, allowing them to retain non spheroid shapes. However, in planets, suns, and other truly massive bodies, the force is so strong that they can't avoid being distorted into sphere roids. But Carpenter said gravity is not the whole story. While gravity conspires to render the planet's spherical, the speed of their rotation is simultaneously trying to flatten them. The faster a celestial body spins, the more disproportionate it's equatorial

bulge gets. Carpenter tells us this is why there are no perfect spheres in our Solar system, only oblate spheroids. The Sun is almost a perfect sphere due to its immense gravity and relatively slow rotation rate of twenty five days. A significant percentage of stars in the sky rotate much faster and bulge noticeably at their equators. One such star is all Tear, located just sixteen point eight light years away from our home planet. It's among the brightest objects

in the night sky. All Tear is also notable for spinning very very fast. It completes a full rotation on its axis every ten point four Earth hours. Accordingly, astronomers estimate that All Tears at least fourteen percent wider at the equator than it is from poll to poll. Rotational speed also explains Jupiter's bulge. After all, a day on this gas giant is a brisk nine point nine earth hours long. Other forces act upon the stars and planets

as well, altering their shapes. Although Earth is an oblate spheroid, it certainly isn't a perfect one. The gravitational pull of the Sun and Moon both influence the planet's shaped to a degree. For that matter, so do Earth's own plate tik toonics. Consequently, the mass of our home world isn't evenly distributed. In fact, it's sort of lumpy. Still, it looks a good deal rounder than Jupiter and Saturn. In turn, the planets in our universe appear way more spherical than

some of their moons do. Mars, for instance, has two small satellites, neither of which has a self gravity to be pulled into an oblate spheroid. Instead, their appearance is often described as potato shaped. In conclusion, will say this much for our home planet. It may not be flawless, but at least the place is fairly well rounded. Today's episode was originally produced by Tristan McNeil and is based on the article why our planets almost sphere on how

stuff Works dot Com, written by Mark Vancini. Brain Stuff is production of iHeart Radio in partnership with how stuff Works dot Com, and is produced by Tyler Clang. For more podcasts to my heart Radio, visit the i heart Radio app, Apple Podcasts, or wherever you listen to your favorite shows,