Ok so flat bonus kinda suck in that ship growth is unlimited BUT planet defense is fixed, so I suggest this

New defense structure

Turbo energy device

By absorbing the ambient energy on a planet, this building can slowly build.
Space 2,
Adds 100 Defense for each day the the planet has been in your possession. Max 2. (the defense value is of the building does not add passive)

Turbo Blasting Device

By releaseing ambient energy on a planet this building can create huge charges
Space 2
Adds 100 Defense for each day the planet has been in your possession, Max 2. (the attack vlaue is of the building does not add passive)

-1
problem is simply this
at low rank, it is quite hard to add 400 attack pr day

so lets presume a low rank player (rank 10 to 200) colonize a planet and puts those structures on it, it will gain +200 attack and + 200 defence pr day
= 1400 attack/ 1400 defence pr week, = 6000 attack / 6000 defence pr month

so this would make it extreamly hard for players of the same rank to invade any planets that have been owned for more then 2 weeks, if the owner have acces to those structures
and for high rank players with 100K Tactical Officers it's not going to make mutch difrence


so unless you limit the number of them to 5, 10 or 15 total, so players can only use them to protect a limited number of planets
it will kill planet stealing at low rank

perhaps it would become available at a much higher rank?.

We won't use it though, at least not if it stays as a size 2 structure... and then we'd need to weigh up if it's worth the space on a planet, if it doesn't add artifact production too.

Everyone in EoTS knows that no matter the defense/attack of a planet, throw enough people at it, someone will get it. So it would need to have some sort of production element to it, if it was available at a "higher" rank.

as it is not stated in the sugestion, I'll presume that they are avalible from rank 1 to rank 9999
proberly even a TC lab artifact, so you can make as many of them as you want


and making them rank 500+ wont make mutch difrence
they will still be to powerfull at low rank and useless at high rank