Do it the same way you'd do it for Pathfinder. Generate a list of places for them to visit, choose the most interesting ones, and arrange and stat appropriately.
So for your situation, if all the exploration is localized to that star system, then you just need to fill it with stuff. I would aim to have at least 3 things going on. For example, I could choose to have
1) the ruins of an ancient hyper advanced civilization
2) a group of space pirates hiding out in an asteroid belt
3) the corpse of a godlike eldritch abomination floating in space
Those would form my main plot thread for the system. I would then seed the system with encounters based on those. Examples:
1) I would scatter an alien city somewhere, and maybe some temples or other structures. I would also work out their lore, description, and what killed them. Perhaps what killed them is still around...
2) Random encounter table is mostly pirates. They get attacked several times, and the pirates have a base somewhere. I would consider why the pirates are there, perhaps hunting treasure. What's the treasure and where is it?
3) A cult shows up to try and revive it. Creatures spawn from it's corpse. It radiates terrible energy and causes damage. It isn't actually dead.
And right there, I have the basics for a system to be explored. Now, the hard part is statting it. Here's a neat time saving trick that if you're good, the players will never notice (might be too railroady for you, but it is a good time saver): no matter where they go, they find the same stuff. So lets say I stat a temple on a planet. No matter where they land on the planet, they find that temple. If that's not your style, create a few things and let them scan the planet and find them. That way, they get a choice. Space encounters (attacked by pirates) they really shouldn't get a choice in.
That's how I'm approaching it. I hope that helps!
It can be extra difficult building a sandbox, having to prep so many adventures at once to suit the PCs, only to have them go off course or level faster than anticipated.
So what do you do?
Lie, Cheat, & Steal!
1. Okay, maybe don't lie lie, but do keep up the GM demeanor that there's a huge, lush universe out there chock full of vibrant NPCs. And then have a staple of pregen NPCs (especially their personalities) & locations on call for said occasions.
2. Have a cheat sheet of interesting encounters, traps, & adventures, but place them as desired. The map grows as the players move. Since your goal as a GM is often to design a map that organically leads to a natural flow, this fulfills that and relieves much of the burden. Also, this alleviates much of the trouble when PCs go off course.
3. More borrow actually. Look at some of your favorite adventures, or ones that are famous (i.e. Isle of Dread, one of the first exploration themed modules). Reskin & remap them them to suit SF, so maybe in the case of the Isle, there's an unexplored planet renowned for its lushness and which is rumored to hold a unique gem or crystal (huge black pearl in the original). The islands around the main island could be moons (w/ people, maybe even spacefarers w/o drift), and the various spots and general nature of the planet could be on a continent. It might be difficult both to land and leave, sensors might have difficulty, etc., etc. Add spaceships as needed/
Even a module as simple as Keep on the Borderlands could be the framework for a whole solar system if desired. The Keep on one planet, various other planets to explore, and one massive gas giant (of Chaos!) with lots of moons which raiders launch from.
Hope that aids.
This thread may help.