I see you are a multi directional artist, drawing anime style art style and Sci-Fi style as well
But regarding planets. It is basic knowledge that one day is the rotation period of a planet around it's axis, and a year is the rotation around it's sun (or mother planet?). But how you call a moon's rotation around it's mother planet? For example the Luna (our Moon) because of it's slow rotation speed, one moon day is equivalent roughly one Earth month while it's year is the same as well as it needs this one moth to rotate around us.
But let's say there is an Earth type planet which rotates around a Gas Giant in 100 days, while the Gas giant rotates around the Sun in 4 years. How would such planet use time? Short and Long Years?
The orbit of a planet around a star is called a year, and the orbit of a moon around a planet is called a month. Luna is unusual in that it is tidelocked to Earth (the same face always points to Earth), so Luna's month is the same length as its day.
In your example, the Earth type planet is a moon, so you could say that its month is 100 Earth days and its year is 4 Earth years. The planet's seasons won't change significantly as it orbits the gas giant, as it will still have the same orientation towards the sun.
One thing that stuck in my mind from some of the RTS I've played - Sins of a Solar Empire, Hegemonia, Homeworld - were the spacescapes, and the planets in them. If I've got to spend hours building an empire in space, the planets and space around them had better look gorgeous. I think it was Hegemonia that did a really good job varying up the light of the stars, and the kinds of planets. (Their desert planets were gorgeous -drools- ) Sins really went the extra step - I seem to recall nebula backlighting? Been a while since I've played either so...
So I think you're on track as far as designing an RTS space environment I'd want to putter around in, looking at all the pretty swirly planets.
One thing I really like is your corner illustration - planets developed for agriculture vs planets developed for industry. The idea that they can visually change depending on what tech tracks you take your colonies on is a very cool one. I'm also really liking the variations of planets you've got - and all the connections between them.
The planets are rendered in-game as a shaded globe with several texture layers for night-side lights, clouds, and various forms of visible (exaggerated) exploitation, so I'm trying to get as much interesting visual variety that also conveys game information.
It will be a challenge to make things look right with a sort of hand-painted style that matches the rest of the 2D graphics.
Looks like one you'll be able to meet though. I think your concept art here does a good job of working on that variety. And I see a few planet varieties I don't think I've seen in either Sins or Hegemonia, so you've got that going for you too.