(also known as planet "Ranks")
Each stat can be upgraded a maximum of 4 times on a planet.
Planets of different space types may have 3/4/5/5 (Nebula/Space/Deep Space/Dead Space) upgrades total. Upgrades will also increase the energy capacity and energy regen of a planet with each upgrade, increasing both by a factor of 1.2 per upgrade, for a total of 2.48832x (1.2^5) base energy capacity and regen at rank 5.
Each rank upgrade of a planet requires a linearly increasing amount of silver for each subsequent rank: 20%/40%/60%/80%/100% (of planet silver cap)
The way in which upgrade rings around a planet arrange themselves is always in the order (from centre, outwards): Defense, Range, Speed.