Remember you are basing this on the current scientific theories that we can not travel faster than the speed of light, the traveller brought about a golden age of scientific advancements, we could be manipulating black holes, bending time and space back on itself like the Vex, or developed warp bubbles that nasa are currently working on or even an enhancement on the ion propulsion which might be why guardians have not travelled to the outer planets
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Technically Saturn is one of the outer planets. We are taught about the 8 major planets that we know of. Sure we hear of a new discovery from time to time but who knows what those planets are like now because what we see is from some time ago. Think of a light year like distance and time (even though it isn't a time) if you put a mirror in space 100 light years away and look at it through a telescope and you see what's reflected off it, you're looking 100 years into the past because your seeing what the light that took 100 years to get to you was showing. Planets and other things far away could potentially not be there anymore because we're looking into the past when we look at the stars. What's there today with our eyes might not be there anymore.