What planets to colonize?

Prioritize the green (breathable) ones, but colonize every planet you can...

Colony ships are cheap and every planet has some reason to be part of your empire

I think later on you can change the atmosphere of the planet (turning it to green) but im not sure if anything can be done about the size of a planet

That may have been the case with SE4 but it doesn't necessarily hold true for SE5. Why? Because every facility now has maintenance costs. Colonizing a bunch of really low-value planets can become prohibitively expensive if you don't have enough worthwhile mining worlds to pay for them all. Admittedly I haven't crunched the numbers to see how many facilities a 100% Mineral Miner can support, but the number certainly isn't infinite.

A level 1 mineral miner costs 1500 minerals and produces 3000 minerals per turn. That's nearly four times what it produced in SE4, but then there are those pesky facility and unit maintenance costs - still quite a nice ROI though Facility maintenance costs are 10%, so the miner would cost 150 minerals per turn in maintenance. Thus, 20 miners would cost 3000 minerals in maintenance, so one miner can effectively support 20 miners. That's not a very useful calculation, of course, since the other miners are also producing ridiculous amounts of minerals, and miners are on the cheap end for facilities! Let's take a cargo facility instead - that's 5000 minerals, or 500 per turn in maintenance. Then each miner you have can support six cargo facilities. That seems like a good enough average. Of course, some other facilities cost other resources, too - especially the religious shrines; they are about even in terms of resource usage. But fortunately farms and refineries cost the same and produce at the same rate as miners, so you can just add up the resource costs, take 10% of the total, and divide into 3000 to find out how many facilities you can support for each (level 1) resource facility.

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Mr. Flibble says...
Game over, boys!

It isn't as cut and dry as the 3000 rate indicates. The actual rate depends on resource percentage of the planet, population of the planet, and I think happiness also plays a part.

On a home planet with minerals at 124% and 4000 pop, the rate of a single miner was around 5900. Jettison the population and the rate drops to 200. So I'm guessing newly colonised planets would only mine a small amount until the population increases.

I colonize every single planet I can!

Not right away; of course; unless I absolutely had no other planets to use. It can be a severe drain on your economy when in the fragile early stages of developement. But sometimes you have no choice.

I colonize the greens first, follow by the reds with some valuable commodity of minerals, organics, radioactives based on size and planet conditions. In other words huge/optimal first and tiny/deadly last.

When it comes to tiny/deadly's I try to wait until I have some decent planet utilizations research done and then plop down a colony with the sole intention of making the conditions better if I have time or rush it into service in some valuable capacity if I don't have time.

Once the planet is more hospitable I'll usually scrap that facility for something more useful like a fleet training facility or the medical facility to prevent plagues or maybe just another resupply facility or spaceyard. Well, you get the picture.

Put something on those tiny's that you don't have room for on your more productive planets. Plus they can always build extra units for you like drones and fighters and troops and satellites!

I would never pass up the chance to convert 4 or 5 tiny/deadly's into 4 or 5 more reaserch outposts or intelligence outposts or resupply bases, and so on.

As has been said before, every planet can have some value to your empire!

just my 2 cents.

I Want The 58th In My Sky And Raining Fire!!

I personally will have to rethink my SE4 strategy of "a spaceyard on every planet" - with facility maintenance, those things suck down 1000 minerals a turn even without building anything! 8O
I think that when I make a mod, I will have several sizes of spaceyards...

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Mr. Flibble says...
Game over, boys!

I personally colonize every world I find, with priority on the largest and breatheable. The tiny planets tend to be research and intelligence, and the biggest are production and resource gathering, or massive research facilities.

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With all these people, you would expect one of them to carry an iguana around...

I tend to play with the racial trait that eliminates the need for spaceports, so even if it isnt breathable, rich tiny planets can be nice with a monolith gathering thingy (correct name escapes me)

Ancient Race gives you a full view of where all the planets are, and what kind they are from the start of the game. Great way to find that next high-mineral green planet nearby.

Also cuts down on wasted trips through long nebula clouds, asteroid belts, dead ends, and the occasional black hole.