Jupiter,
the fifth planet from the sun, gas giant, and subject of the Juno mission, is
huge. Huge. It's so huge, in fact, that it doesn't actually orbit the sun. Not
exactly. With 2.5 times the mass of all the other planets in the solar system
combined, it's big enough that the center of gravity between Jupiter and the
sun doesn't actually reside inside the sun — rather, at a point in space just
above the sun's surface.
Here's
how that works. When a small object orbits a big object in space, the less
massive one doesn't really travel in a perfect circle around the larger one.
Rather, both objects orbit a combined center of gravity.
In
situations we're familiar with — like Earth orbiting the much-larger sun — the
center of gravity resides so close to the center of the larger object that the
impact of this phenomenon is negligible. The bigger object doesn't seem to move,
and the smaller one draws a circle around it. But reality is always more
complicated.
For
example: When the International Space Station (ISS) orbits the Earth, both the
Earth and the space station orbit their combined center of gravity. But that
center of gravity is so absurdly close to the center of the Earth that the
planet's motion around the point is impossible to spot — and the ISS describes
a near-perfect circle around the whole planet.
The
same truth holds when most planets orbit the sun. Sol is just so much larger
than Earth, Venus, Mercury, or even Saturn that their centers of mass with the
sun all lie deep within the star itself. Not so with Jupiter.
The
gas giant is so big that its center of mass with the sun, or barycenter,
actually lies 1.07 solar radii from the middle of the sun — or 7% of a
sun-radius above the sun's surface. Both the Sun and Jupiter orbit around that
point in space.
That is, in essence, how Jupiter and the sun
move through space together — though the distances and sizes are far different.
Jupiter is still only a fraction of the sun's size.
But
next time someone asks you for a crazy space fact you'll know: Jupiter is so
massive, it doesn't orbit the sun.
What about the earth and the moon? Does the moon orbit the earth? Is the distance between them too small?
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