Speaker 1: Welcome to brain Stuff from how Stuff Works. Hi, brain Stuff,
Speaker 1: Lauren Vogel bomb here. It's something we kind of take
Speaker 1: for granted. Roses are red and planets are spherical. That's
Speaker 1: just the way things are, right after all. Building model
Speaker 1: solar systems would be way more challenging if instead of
Speaker 1: using little phone balls we had to make a bunch
Speaker 1: of dough decahedron shaped planet models. But have you ever
Speaker 1: wondered why planets look like this? Why are they basically
Speaker 1: spherical and not say cylindrical or cube shaped. We should
Speaker 1: kick off this discussion by calling a spade a spade.
Speaker 1: None of the planets in our solar system are perfect spheres, nor,
Speaker 1: for that matter, is our Sun. All those bodies could
Speaker 1: be more accurately described as oblate spheroids objects with the
Speaker 1: shape bulge slightly around the middle. To borrow an analogy
Speaker 1: from the astronomer philled plate, they look like a basketball
Speaker 1: that someone is sitting on. But more technically, in a
Speaker 1: celestial body with an oblate spheroid shape, the polar circumference
Speaker 1: will be smaller than the equatorial one. So here on Earth,
Speaker 1: if you were to travel from the north pole to
Speaker 1: the South pole and back, you'd have walked a grand
Speaker 1: total of twenty four thousand, eight hundred and twelve miles
Speaker 1: that's thirty nine thousand, nine hundred and thirty one kilometers.
Speaker 1: On the other hand, a complete trip around the equator
Speaker 1: would be a bit longer. That's because the circumference of
Speaker 1: Earth's equator is twenty four thousand, nine hundred miles or
Speaker 1: forty thousand and seventy kilometers. As such, when you stand
Speaker 1: at sea level on the equator, you're further away from
Speaker 1: the center of our planet than you would be at
Speaker 1: either the north or south pole on some other planets.
Speaker 1: This bulge is even more pronounced. Just look at Jupiter.
Speaker 1: Earth is only zero point three percent wider at the
Speaker 1: equator than it is from poll to poll, but Jupiter's
Speaker 1: measurements showcase a much bigger disparity. Astronomers have found that
Speaker 1: this plus sized planet is a full seven percent wider
Speaker 1: at its equator than it is between the polls. The
Speaker 1: oblate spheroid shape is the result of two main factors,
Speaker 1: gravity and rotation. Troy Carpenter, director of Washington State's Goldendale Observatory,
Speaker 1: recently discussed the matter with us in an email exchange
Speaker 1: they explain, everything which has mass experiences gravity, and gravity
Speaker 1: attempts to crush an object inward in all directions. That's
Speaker 1: because all objects experience self gravity, a force which pulls
Speaker 1: their atoms toward a common center. As the massive an
Speaker 1: object increases, so too does its self gravitational pull. After
Speaker 1: it exceeds a certain mass, the pull gets overpowering to
Speaker 1: the point where the object collapses into itself and becomes spherical.
Speaker 1: Little items, like say a banana or a lug wrench,
Speaker 1: can resist this fate because their self gravity is relatively weak,
Speaker 1: allowing them to retain non spheroid shapes. However, in planets, suns,
Speaker 1: and other truly massive bodies, the force is so strong
Speaker 1: that they can't avoid being distorted into spheroids. But Carpenter
Speaker 1: said gravity is not the whole story. While gravity conspires
Speaker 1: to render the planet's spherical, the speed of their rotation
Speaker 1: is simultaneously trying to flatten them. The faster celestial body spins,
Speaker 1: the more disproportionate its equatorial bulge gets. Carpenter tells us
Speaker 1: this is why there are no perfect spheres in our
Speaker 1: Solar system, only oblate spheroids. The Sun is almost a
Speaker 1: perfect sphere. Due to its immense gravity and relatively slow
Speaker 1: rotation rate of twenty five days, A significant percentage of
Speaker 1: stars in the sky rotate much faster and bulge noticeably
Speaker 1: at their equators. One such star is all Tear, located
Speaker 1: just sixteen point eight light years away from our home planet.
Speaker 1: It's among the brightest objects in the night sky. All
Speaker 1: Tear is also notable for spinning very very fast. It
Speaker 1: completes a full rotation on its axis every ten point
Speaker 1: four Earth hours. Accordingly, astronomers estimate that all Tears at
Speaker 1: least fourteen percent wider at the equator than it is
Speaker 1: from pole to pole. Rotational speed also explains Jupiter's bulge.
Speaker 1: After all, a day on this gas giant is a
Speaker 1: brisk nine point nine Earth hours long. Other forces act
Speaker 1: upon the stars and planets as well, altering their shapes.
Speaker 1: Although Earth is an oblate spheroid, it certainly isn't a
Speaker 1: perfect one. The gravitational pull of the Sun and Moon
Speaker 1: both influence the planet's shape to a degree. For that matter,
Speaker 1: so do Earth's own plate to topics. Consequently, the mass
Speaker 1: of our homeworld isn't evenly distributed. In fact, it's sort
Speaker 1: of lumpy. Still, it looks a good deal rounder than
Speaker 1: Jupiter and Saturn. In turn, the planets in our universe
Speaker 1: appear way more spherical than some of their moons do. Mars,
Speaker 1: for instance, has two small satellites, neither of which has
Speaker 1: a self gravity to be pulled into an oblate spheroid. Instead,
Speaker 1: their appearance is often described as potato shaped. In conclusion,
Speaker 1: will say this much for our home planet. It may
Speaker 1: not be flawless, but at least the place is fairly
Speaker 1: well rounded. Today's episode was written by Mark Mancini and
Speaker 1: produced by Tristan McNeil. For more on this and lots
Speaker 1: of other interplanetary topics, visit our home planet how stuff
Speaker 1: works dot com