The Red Planet is wiggling and wobbling as it spins, research in the journal Geophysical Research Letters confirms, and astronomers have no idea why.

Like a toy top that teeters as it loses speed, the poles of Mars are wandering ever-so-slightly away from the planet’s axis of rotation, moving about 4 inches (10 centimeters) off-center every 200 days or so, researchers reported in a study published Oct. 13, 2020. That makes Mars only the second known planet in the universe to exhibit this phenomenon — known as the Chandler wobble — with Earth being the first, according to the American Geophysical Union’s (AGU) news blog, Eos.org.