Answer: Saturn

Saturn is the sixth planet from the Sun, the second largest planet in the solar system, and a gas giant like its fellow solar-system-giant Jupiter. We know a lot of things about Saturn with certainty (like the size of its rings, the volume of the planet, and so on), but there is still one matter that astronomers are both studying and debating: the planet’s rotational speed.

Because of their large volume, low density, and, most importantly, lack of concrete surface area on which to gauge speed relative to the observer, it’s tricky business to pin down the rotational speed of a gas giant. In the case of Saturn and Jupiter, there are actually three defined rotational speeds based on the movement of different areas on both planets known as System I, System II, and System III. Saturn’s System I rotational speed is defined by the movement of the Equatorial Zone and the North and South Equatorial Belts. System II covers all other regions but the polar regions (which are believed to rotate roughly at the same speed as the belts of System I). System III is defined by the rotation of Saturn’s core, which is found deep within the gaseous and liquid layers.

Even with the complexity of having three rotational systems to deal with, you would think that things would be more or less ironed out by this point in the history of astronomy. The nature of gas giants makes them difficult to observe, however, and every new probe we send near Saturn reveals new data (and often that new data contradicts or does not align neatly with existing observations and measurements).