Why Are Stars and Planets Nearly Spherical?

Planets and stars are basically spheres, but why? We explore the science in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:02):
Welcome to brain Stuff from how Stuff Works. Hi, brain Stuff,
Lauren Vogel bomb here. It's something we kind of take
for granted. Roses are red and planets 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 phone balls we had to make a bunch
of dough decahedron shaped planet models. But have you ever

(00:22):
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

(00:46):
from the astronomer philled plate, they look like a basketball
that someone is sitting on. But 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.

(01:09):
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.

(01:29):
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,

(01:50):
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

(02:11):
their atoms toward a common center. As the massive 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,

(02:31):
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 spheroids. 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 celestial body spins,

(02:55):
the more disproportionate its 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

(03:18):
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 pole to pole. Rotational speed also explains Jupiter's bulge.

(03:41):
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 shape to a degree. For that matter,
so do Earth's own plate to topics. Consequently, the mass

(04:01):
of our homeworld 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,

(04:23):
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 written by Mark Mancini and
produced by Tristan McNeil. For more on this and lots
of other interplanetary topics, visit our home planet how stuff
works dot com