Kids have been taught since pre-school that the Sun is the center of our solar system. Wikipedia even says, “The Sun is the star at the center of the Solar System.” However, this is not exactly true. While the Sun is near the center of our solar system, it is not the “barycenter,” or the center of the solar system around which mass is evenly distributed.
The barycenter is the center of mass in our solar system, and surprisingly, the Sun doesn’t lie in the center of mass. James O’Donoghue, a planetary scientist at JAXA explains on Twitter, “Instead, everything orbits the solar system center of mass. Even the sun.”
O’Donoghue further explains that though the Sun contains about 99.8% mass of the Solar System’s total mass, Jupiter makes up most of the remaining. Due to the laws of gravitational force, Jupiter slightly pulls on the Sun the same way the Sun pulls Jupiter, so we could say that the Sun slightly orbits Jupiter!
Due to this reason, you can see in the video above that while the Sun does orbit (and sometimes pass through) the barycenter, it doesn’t stay at the barycenter due to the slight pulling force from Jupiter. “The natural thinking is that we orbit the sun’s center, but that very rarely happens,” O’Donoghue said. “It’s very rare for the solar system’s center of mass to align with the sun’s center.”
The gravitational force between the center of mass of two planetary objects is proportional to the product of the masses of the two planetary objects. That is why the gravitational force between the Sun and Jupiter causes an effect on how the barycenter is placed, and also the reason why other planets do not affect the Sun the same way. They simply do not have enough planetary mass to make a difference.
Due to the same reasons, the same effect can be seen with Earth and the Moon. Earth doesn’t coincide with the barycenter of the Moon’s orbit. Instead, Earth slightly orbits the barycenter along with the Moon.
O’Donoghue concludes saying, “The planets do orbit the Sun of course, we are just being pedantic about the situation. The natural thinking is that we orbit the Sun’s center, but that very rarely happens, i.e. it’s very rare for the solar system’s center of mass to align with the Sun’s center.”
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