Jupiter doesn't orbit the sun!?!

Jupiter, the fifth planet from the Sun, a gas giant, and also subject of the Juno mission, is huge. It's so huge, in fact, that it doesn't actually orbit the Sun. 

Not exactly. With 2.5x the mass of all the other planets in the Solar System combined, it's big enough that the centre 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.

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 centre of gravity.
In situations we're familiar with – like Earth orbiting the much-larger Sun – the centre of gravity resides so close to the centre 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 Earth, both Earth and the space station orbit their combined centre of gravity. But that centre of gravity is so absurdly close to the centre of Earth that the planet's motion around the point is impossible to spot – and the ISS follows a near-perfect circle around the whole planet.

The same truth holds when most planets orbit the Sun. The Sun is just so much larger than Earth, Venus, Mercury, or even Saturn that their centres of mass with the Sun all lie deep within the star itself.
Not so with Jupiter.

The gas giant is so big that its centre of mass with the Sun, or barycentre, 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.
This not-to-scale video from NASA illustrates the effect:

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.
So next time someone asks you for a crazy space fact, you'll know: Jupiter is so massive, it doesn't orbit the Sun.


Want to know more about Jupiter? A new article will be coming in two weeks about its Great Red Spot. See more tomorrow in another article about this year's perseid meteor shower.

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