Any basic study of the solar system eventually finds an apparent paradox: very large planets spin faster than very small ones. Venus has a stunning 243-day-long rotation (it takes 243 days on Earth for just one day to pass on Venus), whereas Jupiter, by far the largest planet in the solar system, zips around its axis once every 10 hours.
– Short Days –
More specifically, the average small rocky planet has a day that lasts longer than the average large gas giant. Circling on Jupiter or Saturn, then, you would circle the planet in less time than it takes Earth to rotate once, even though the distance travelled is far larger. The reason for this lies in the physics of planetary composition: large planets are held together by far more powerful gravitational forces and spin around their axis extremely quickly due to centripetal forces. For this reason, the most distant planet of all, rocky Pluto, returns to having relatively long days, in its case about one Earth-week.
In contrast to Earth’s and Mars’s roughly 24-hour days (Venus’s extraordinarily long day is probably due to a cataclysmic ancient impact), Jupiter makes one rotation every 10 hours. One day on Saturn takes 11 hours, a day on Uranus is 17 hours, and a day on Neptune is 16 hours. Note that it is complex to measure a day on a gas giant: there is no solid surface to stand on and count the time on, as there is on Earth. For this reason, astronomers measure gas-giant days by timing how long it takes for the planet’s magnetic field to rotate once.
– Slow Orbits –
At the same time, because all of the gas giants in the solar system are far away, they orbit the Sun extremely slowly: Jupiter’s day lasts just 10 hours, for example, but its year lasts for eleven Earth-years. The slower orbits of the gas giants are actually due to their position in the solar system, as opposed to their physical composition: distant objects orbit more slowly, because the Sun’s gravity is weaker at such large distances. (Objects travelling too quickly escape orbit and leave the solar system.) As a result, gas giants in other solar systems may be closer to their Sun and have much shorter years, some measured in mere weeks or even days.
Here in the solar system, Jupiter orbits the Sun once for every eleven times that Earth makes the same circuit. Saturn’s year lasts 29.5 years, Uranus’s year lasts 184 years, and Neptune’s year lasts a surprising 165 years on Earth. Incidentally, the longest years in the solar system belong to the even more distant but far smaller dwarf planets Pluto and Eris: respectively, 248 years and a stunning 557 years.
