March 20, 2021 • 5 mins
Planets and stars are basically spheres, but why? We explore the science in this classic episode of BrainStuff, based on this article: https://science.howstuffworks.com/why-are-planets-almost-spherical.htm
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Episode Transcript
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Speaker 1 (00:01):
Welcome to brain Stuff production of I Heart Radio. Hey
brain Stuff, Lauren Bogle bomb here with another classic episode
from our archives. This one deals with one of the
most simple sounding yet cosmically impressive concepts that we humans
can contemplate. Why are stars and planets all nearly spherical? Hi,
(00:24):
brain Stuff, Lauren bog O 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 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?
(00:47):
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
(01:08):
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
(01:28):
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
(01:49):
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,
(02:14):
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.
(02:36):
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
(02:56):
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
(03:18):
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
(03:40):
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
(04:02):
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
(04:24):
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,
(04:45):
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
(05:06):
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,
Welcome to brain Stuff production of I Heart Radio. Hey
brain Stuff, Lauren Bogle bomb here with another classic episode
from our archives. This one deals with one of the
most simple sounding yet cosmically impressive concepts that we humans
can contemplate. Why are stars and planets all nearly spherical? Hi,
(00:24):
brain Stuff, Lauren bog O 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 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?
(00:47):
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
(01:08):
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
(01:28):
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
(01:49):
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,
(02:14):
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.
(02:36):
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
(02:56):
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
(03:18):
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
(03:40):
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
(04:02):
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
(04:24):
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,
(04:45):
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
(05:06):
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,
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