This is a quite different adventure setting for Traveller, utilising the entirity of a solar system (which one doesn't really matter, just pick a suitable system in your universe) rather than seeing it as virtually empty space through which you travel merely to get to the 100D distance to turn on your Jump drive. The Introduction explains this, along with notes on what the party will need to get around there - any small vessel, which doesn't need to be Jump-capable, or even friends who have one and will give them a ride. The idea is that there's a fair bit of 'local' in-system traffic and characters will be able to hitch or pay for transportation if they do not have a suitable ship of their own.
The next section, Background Data, is a mix of some basic stuff about the Third Imperium (genuine background this, it does not impinge on what's going on here but is more of a vague backdrop) and a wealth of information about what is likely going on across a reasonably well-developed system from settlements on planets other than the mainworld, moons or indeed in space stations to more transient folk engaged in exploration, mining or salvage operations. There are also notes on what's to be found there: gas giants, asteroids, moons, and smaller rocky but airless planets.
Then there's a developed example, the Kendelsei Outsystem. Kendelsei itself is a gas giant around which a fair bit of spacefaring civilisation has built up separate from the main world and those who generally jump in to visit it. It's got quite a few moons, as gas giants tend to, as well as other planetoids clustered in its Trojan points. A neat thing is that everything here as astronomically sound as well as working in a game context.
The next section is Space Travel in the Outsystem which talks about the different sorts of vessels likely to be found there. This is followed by Adventuring in the Outsystem, a short section that crams quite a few ideas into a few short paragraphs.
Many people take the trouble to generate an entire system rather than just the main world, but then concentrate all their efforts and plots on that main world (with perhaps a refuelling stop at the gas giant). Now here are some ideas and resources to help you bring the rest of the system to life. As it is likely that people are not going to stay only on the main world, it adds added realism to your alternate reality - and there is plenty of scope for adventures in the outsystem as well.