This page will give you an understanding of the uses and some strategies of planets.
For further information on planets, please check the Dark Forest Wiki.
The main function of planets is to be the war force of your empire. They are the only celestial bodies which can be upgraded, increasing their base stats (which stat is dependent upon the upgrade tree you choose).
Since planets are the only celestial body type which can upgrade, making their power comparative to a non-upgraded celestial body of a level higher, they can become incredibly strong, reaching further, faster or being able to absorb more damage than other celestial bodies. Planets are always useful to have, particularly planets with x2 buff moons and upgrades. You will find that you will capture ‘enough’ of other celestial bodies, but you can never have ‘enough’ planets.
Planets have a multiplier of 1x for all of the default stats.
Planets are the most complex celestial body within the game and the optimal strategy will often be very difficult to determine. The most obvious advice is to aim for planets which have buffs (energy regen is great on lower level planets, and range is amazing on any planet). The important thing to note about planets is that while their energy capacity grows exponentially (not exactly, but close) the higher their level, their energy regen only grows linearly. This, coupled with the regeneration s-curve means that high level planets need a lot of support. This support is generation found through many ‘feeder’ planets - planets of a lower level nearby to the high level planet - which can donate their energy by sending it as reinforcements to your higher level planets, keeping them full of energy and ready to attack or defend.