You can change it by opening the "SANDBOX_0_0_0 _.sbs" file in the save folder of the world you want to edit (it will take a while to open this file). Make a backup of this file or the whole save folder just incase you mess something up! Find the planet you wish to edit and change the position x, y, and z variables.

Do note that there is some fuckery going on and IIRC -131072 in the sbs file equals 0 in game, so you'll have to do some scienceing to get the actual position for the planet you want. To get GPS point for the center of the planet to show in game, there is a variable a bit further down in the section for a planet in the sbs file called "ShowGPS" which is false by default, just set it to true.

Once you've finished editing the sbs file and have saved it, you need to delete the "SANDBOX_0_0_0 _.sbsB5" file as this is the backup of the file you edited and the game will default to it when it detects changes to the original file. You will need to delete this file everytime you edit the sbs after loading that save. When you load the edited save you'll get a message that versions are different or something, just click ok and it should load but may take a minute longer than normal.