What is the closest planet to Saturn?
The average distances of the planets from the sun are:
Mercury: 0.39 AU (AU = Astronomical Unit)
Venus: 0.72 AU
Earth: 1 AU (the definition of Astronomical Unit)
Mars: 1.52 AU
Jupiter: 5.2 AU
Saturn: 9.54 AU
Uranus: 19.2 AU
Neptune: 30 AU
So if the planets were all in a line, Jupiter would be the closest to Saturn. But the planets are never actually all in a line...
You have to remember that the planets are all orbiting the sun at different rates, so at any given time, any two planets may be lined up or on opposite sides of the sun, or somewhere in between. So if the question is "what planet is closest to Saturn right now ", the answer is a little different. We would need to consult real time data of the positions of the planets, such as Fourmilab's Solar System Live: https://www.fourmilab.ch/cgi-bin/Solar
For today's date, it would seem that the closest planet to Saturn (yellow, lower left) is actually Venus!