If you didn’t have time to complete an area, you can simply leave and return, the Reapers will respawn instantly but will all come from the edge, giving you time to throw off a couple of scans or a quick probe. You don’t have to fly to an exit arrow to leave the area, just go to edge and you’ll escape. If you’ve discovered something, if you go into orbit to scan and probe, the Reapers will freeze in place, even if they’re right next to you, as long as you move the opposite direction as soon as you’re out, you should be fine. Try to catch several planets in one scan if possible, to minimize the Reaper interest. The radius is slightly smaller than then animation makes out so be sure to scan every inch. You’re able to scan simply by flying around space and hitting the appropriate key (displayed in the lower left of the screen).

This Mass Effect 3 Planet Salvage And War Assets Guide will list all the available planets, as well as the things you may discover on each planet. Mass Effect 3 is filled with planets waiting to be discovered, some contain rare salvage such as credits and technology, others contain assets that are vital to your war effort.