Humans (well, astronomers, and by extension, humans) actually know a lot about space. As a developer of a space game, I feel a responsibility to learn enough about what we know so that when I portray a faraway solar system or another galaxy, it rings true within the player (who could be an astronomer). That does not mean that EXO targets realism, however - it still needs to be a game, and with what we know, actual space would make a terrible game! Most significantly, light-years of space separates interesting parts of a galaxy, and we know no way to move significant matter at speeds faster than light; no one wants to sit and wait years as they travel between systems. Real things are the inspiration for anything, and it would be foolish not to use our knowledge of space at least some, but when, and how much exactly, should realism be applied in EXO: Perl?
As much as possible while preserving the "game" moniker, I might say. Borrowing from astronomy is like free creativity. Nature contains so many cool surprises that we would not believe if they did not exist. People did not write science fiction about black holes until they were already theorized through science - perhaps no one thought of such an object, or they decided that it would be far too outlandish, even for science fiction (which would be saying something). For a game focused on exploration, the enormous variety of objects astronomers have discovered across the universe reduces the challenge of coming up with compelling, at least believable objects and sites to discover; most of these can be taken from, or at least based on, those we have found in nature.
On the flip side, knowing what astronomers do can pose its own challenges. I wrote last week about creating terrain that has variety but still feels plausible, and that's just one example. If we knew less about the mechanics of a solar system - like how gravity determines orbits, and how the star heats up the objects - creating a system would in some ways be easier. I've had to wrestle with preventing moons from orbiting far away enough from their planet that another planet's gravity would clearly influence them, and similarly stop planets from being put in orbits too close to one another - except when their gravitational dependence is accounted for with a binary planet. I also find myself repeatedly slightly changing the way planet temperatures are determined, which depends of course on the size and heat of their star as well as the planet's distance from it, to try to keep the temperatures reasonable most of the time.
Of course, sometimes I make the intentional choice to ignore what humans know. Just as in almost all science fiction, alien, intelligent life is commonplace in EXO. It's almost certain that alien intelligent life is extremely rare, just as it is almost certain that it exists in our galaxy - it's a numbers game. But one of the most intriguing parts of exploring space is meeting and interacting with the denizens of the galaxy, so they can't be made to be so rare you're not likely to find them. In a similar vein, astronomers estimate that 85% of all stars are red dwarfs (thanks, Wikipedia), which leaves only 15% for all the rest of the types of stars to be found. It would get pretty dull if 17 of every 20 systems you went to were red dwarfs. That said, red dwarfs will still be the most common type of star (at least, in a typical galaxy of an age similar to the Milky Way), but it might be more like 33% - 50% of them, rather than a full 85%.
With these adjustments, my main tenet is Things that are rare should stay rare. Now, that may be jumping from one in a million to one in a thousand, or from astronomically rare (or at least, unknown rarity) to happening once every twenty or thirty systems - huge boosts, many orders of magnitude more common than realism - but this way they preserve a sense of "whoa, I just discovered a cool thing!", something I think is lost in many space games. I won't call them out here, but games with multicellular life or even intelligent-life settlements on every single planet just destroy all the joy in discovering those features. After going to about three planets, you expect to find life, and it would actually be cool to find a totally barren planet. Desolate worlds are part of the fascination of space, and they play foil to the lush Eden planets that should be oh-so-rare.
P.S. I leave you with a picture - look carefully! There are 4 objects: a planet, one of its moons, and two of that moon's orbiters!