to answer AdAstraPerAspera`s question..
i really don`t think it`s possible to form a mathmatical `average` on the rate of explored planets
there`s just too many variables in the code that make it completely random
though, there are certain things you can do to increase your chances
for instance, it is known that it`s a little bit easier to find better planets at certain times of the day.
i think exploring during rare dig times is one of them.
also, having a paid account can increase your chances. although having a paid doesn`t neccessarily directly increase the land sizes, you are able to find barrens that have higher then 950 land, so it indirectly increases the chances for finding higher land planets by a small percentage
other then that, it`s just too random. i remember times where i wanted a U.Spaz, and spent 15 straight hours exploring all day and never found one until i finally gave up.
the next day, i was just exploring for barrens, and i ended up getting 3-4 U.spaz`s within 40 minutes
though i guess you can explore over a very long period of time (like a month) write down how many you get, and find a ballpark `average` that way.
but even doing that, i`m pretty sure if you did the same thing another month, your outcome would (or could) be a lot different then the first time