Ask Astro: Why do the planets orbit the Sun counterclockwise? – Astronomy Magazine

Posted: October 29, 2020 at 6:24 pm

Q: The Sun orbits the center of our galaxy in a clockwise direction, but the planets in the solar system orbit the Sun in a counterclockwise direction. Why is this?

Rich Zaykoski

Hampstead, Maryland

A: The planets of our solar system orbit the Sun in a counterclockwise direction (when viewed from above the Suns north pole) because of the way our solar system formed. Our Sun was born from a cloud of dust and gas, the remnants of which called the solar nebula became the planets. As that cloud collapsed into the Sun, it also began to spin. Its a matter of chance that it ended up spinning in a counterclockwise direction when viewed from the top down.

But lets step back for a moment. When we say that the Sun formed from a cloud of dust and gas, that cloud was actually a very small sub-region of a much larger, so-called giant molecular cloud. Giant molecular clouds dont just form one star, but many hundreds or thousands. And, in general, the angular momentum and axis of rotation of these giant molecular clouds does tend to be oriented either with (prograde) or against (retrograde) the Milky Ways rotation. Within these larger clouds, however, things are different. Factors such as turbulence caused by supernova shock waves and magnetic effects that occur when portions of the cloud start collapsing into stars affect the final angular momentum and spin orientation of newborn stars. These interactions can be quite complex, which is likely why stars dont spin in any one preferred orientation or direction. While a stars planets will likely all rotate either clockwise or counterclockwise around it, the outcome is more affected by local conditions during the stars birth than by the rotation of the Milky Way, or even the larger cloud from which the star formed.

Alison Klesman

Senior Associate Editor

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Ask Astro: Why do the planets orbit the Sun counterclockwise? - Astronomy Magazine

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