How do I unlock more planets?
After finishing the initial tutorial I have Mercury, Venus, and Earth. Randomly one day the Void then showed up with a single survival mission.
I have achievements for saving Uranus(heh) and Sedna, but I can't go to those planets as they aren't on my navigation map.
I am now level 11 (on the upwards end of all planets unlocked so far), yet no more planets have showed up for me to go to? How do I unlock them?
EDIT: I should add I'm on the XB1 version, which is on a different build than PC... so rules may be different.
When you start the game, your ship is not fully functional. That includes your Navigation system, which does not have all the maps. You need to acquire them from bosses. In order to do that, you need to complete the Assassination missions on each planet, which will grant you a Nav Segment to a new planet. Almost every boss gives out a segment to a specific planet, making the unlocking path straightforward.
The Nav Segment article on the Warframe wiki has a nice image summarizing the order in which the planets unlock for new Tenno:
Some of the planets in the chart split the path, which means that the boss on that planet can give out two segments. You will likely have to defeat it a few times to get the two segments in order to unlock all the planets.
A bit beyond planets, you can access the Orokin Voids if you have Void Keys, which are obtainable from the market, daily login rewards, or as a success reward for Endless Defense, Survival, Interception and Excavation missions, as well as Capture, Exterminate and Spy missions but at a very low chance.
The last location, the Orokin Derelict, can also be access from keys. Orokin Derelict keys have to be forged from blueprints available for credits on the Market, under Equipment.