- Our solar system orbits an invisible point at its centre called the barycenter, from which its mass is evenly distributed.
- Even the sun orbits this point – so the centre of the solar system doesn’t always align with the centre of our star.
- Planetary scientist James O’Donoghue made an animation to show how the sun, Jupiter, and Saturn play tug of war around the solar system’s barycenter.
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We all know that the sun is the centre of the solar system, and the planets orbit around it – along with a heap of asteroids, some meteor fields, and a few far-flung comets.
But that’s not quite the entire story.
“Instead, everything orbits the solar system centre of mass,” James O’Donoghue, a planetary scientist at the Japanese space agency, JAXA, recently explained on Twitter. “Even the sun.”
That centre of mass is called the barycentre and it’s the point of an object at which it can be balanced perfectly, with all its mass distributed evenly on all sides. But in our solar system, that point rarely lines up with the centre of the sun. Mind, blown.
To explain it all, O’Donoghue created this animation which shows how the sun, Saturn, and Jupiter play a kind of tug-of-war around the barycentre that pulls our star in looping mini-orbits:
O’Donoghue uses his free time to create animations that show how the physics of planets, stars, and the speed of light work.
“The natural thinking is that we orbit the sun’s centre, but that very rarely happens,” he said. “It’s very rare for the solar system’s centre of mass to align with the sun’s centre.”
While O’Donoghue has exaggerated the sun’s movement in the video above to make it more visible, our star does circle millions of kilometers around the barycentre – sometimes passing over it, sometimes away from it.
Jupiter’s gravity creates much of that movement. The sun makes up 99.8% of the solar system’s mass, but Jupiter contains most of the remaining 1.2%, and that mass pulls on the sun ever so gently.
“The sun actually orbits Jupiter slightly,” O’Donoghue said.
The planets and their moons have their own barycentre within the solar system. Earth and the moon do a simpler dance, with the barycentre remaining inside Earth. As you can see in O’Donoghue’s video:
In the animation, you can see how the Earth and moon will move over the next three years, all in 3D. (Keep in mind the distance between Earth and the moon is not to scale.)
Pluto and its moon, Charon, do something similar, but unlike with Earth, the barycentre is always outside of Pluto.
So, every planetary system orbits an invisible point, including the star or planet that appears to be at the centre. Barycentres can also help astronomers find hidden planets circling other stars, because they can calculate that the system contains mass they can’t see.
“The planets do orbit the sun of course,” O’Donoghue said. “We are just being pedantic about the situation.”
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