Tectonic bombs will do the trick
But seriously, you want to reduce the number of planets?
1) Create a new folder in your se4 directory. Name it "Maxxmod"
2) Create a new folder inside the "Maxxmod" folder. Call it "Data"
3) Copy "SystemTypes.txt" from the SE4\data folder to your new SE4\Maxxmod\data folder.
4) Open up the copy of "Systemtypes.txt" with your favorite text document editor and romp through the file deleting planet entries.
Be sure to update the
"Number of System Objs := "
line to reflect your changes, and make sure the
"Obj 1 ..."
lines do not skip any numbers.
5) In your se4\ folder, there is a file called "path.txt". Open it up and find the only line in there:
"path := none"
Change it to
"path := Maxxmod"
6) Run SE4, and it will use your modded files instead of the originals, leaving you with less planets per system next time you generate a new map.
7) To restore the game to its original state, for multiplayer action, simply change the path.txt to read "path := none"