Through telescopes on Earth and in space, astronomers can
glimpse the far-flung reaches of the universe. And no matter how far away or
how strange the planet, at least one thing seems to hold true in space: a lot
of stuff is spherical.
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| Why are these planets and moons, illustrated here to scale next to the sun, so round? (Credit: Ron Miller via Getty Images) |
So what makes these celestial bodies round? In short,
it's gravity.
"It's pretty amazing that we know of so many things
being round in space," Anjali
Tripathi said. She is an astrophysicist at NASA's Exoplanet Exploration Program,
located at the JPL in California. Gravity's rounding effect
is a result of self-gravity, the gravity that an object — in this instance, a
celestial body — exerts on itself. Once a planet, or maybe a moon, accumulates
enough mass, its self-gravity will pull it into a sphere-like shape.
The universe's bodies formed after the Big Bang exploded
about 13.8 billion years ago. Tiny dust particles circulating in enormous
doughnut-shaped dust clouds began to collide. If the collision was gentle
enough, according to NASA, the dust particles fused. Collision after
collision created a snowball effect; The more mass a budding planet
accumulated, the more its gravity grew and the more matter it attracted.
That "gravity pulls all the matter toward the center of
gravity," said Bruno Merín, an astronomer and head of the European Space
Agency's ESAC Science Data Centre in Madrid. It's like the kitchen sink, he
said: "All water will flow through the hole in the bottom." In the
case of planets, "every piece of matter is trying to get as close as
possible to the center of gravity."
Planetary bodies will continue to shift matter around until
they find an equilibrium, a state in which every point is as close as possible
to the center. And the only shape that achieves this kind of equilibrium in
space is a sphere", he said.
Mercury and Venus are nearly perfect spheres because they
are slower-spinning rock planets. Ice planets also tend to be almost perfectly
round, as the "layer of ice distributes very evenly," Merin said.
But "round" doesn't mean that every planet is a
perfect sphere; the gas giants Jupiter and Saturn bulge at their equator
because of how fast they spin. Instead of a perfect sphere, Saturn looks like a
basketball someone is sitting on, according to NASA. Even Earth has a tiny bulge of less than 1%, due
to centrifugal
force, the outward force on a spinning object. So Earth is oblate,
or a slightly flattened sphere.
Although the universe teems with spheres, many bodies in
space aren't even remotely spherical. Asteroids
and comets can come in any shape, altered by crashes and interstellar
spinning. Mars has a potato-shaped moon called Phobos; in fact, only about 20
of the nearly 300 known moons in the solar system are
the familiar round shape we expect, the rest are more irregular. The reason for
all these nonspherical bodies: Their lower mass means they don't have enough
gravity to even out their shape, Tripathi said.