Currently in Warframe, there are 14 planets to choose from, and you can unlock them gradually. Here you have to complete a total of 239 missions and kill the bosses. Once you have completed them successfully, you will be benefited with rewards and resources / components with which you can assemble your Warframes and weapons.
The below table contains all previously existing planets at Warframe. In addition, you will know what level you need to unlock the planet. If you know on what planet that Boss is staying and what reward you get when you have defeated him, you can decide where you lead your journey next. Do you need, for example, components for the Rhino, then you have to take on Venus.
Planet
Boss
Reward
Resources / Components
Mercury (Lvl. 3-10) TolstoySeer components Ferrite, Polymer Bundles, Morphic, Detonite Ampules
Venus (Lvl. 8-22)Jackal on FossaRhino components Alloy Plates, Circuits, Polymer Bundles Fieldron Sample
Earth (Lvl. 6-21) Vay Hek on OroHydroid components Ferrite Rubedo, Neurodes, Detonite Ampules
Mars (Lvl. 9-24)Sgt Nef AnyoMag components Salvage, Gallium, Morphic, Fieldron Sample
Phobos (Lvl. 13-28) Lieutenant Lech Kril on IliadMiter and Gremlins components Alloy Plates, Plastids, Rubedo, Morphic
Jupiter (Lvl. 8-23)Alad V on ThemistoValkyr components Salvage, Alloy Plates, Neural Sensors Fieldron Sample
Saturn (Lvl. 9-24) Ruk on TethysEmber components Nano Spores, Plastids, Orokin Cells, Detonite Ampules
Sedna (Lvl. 12-29)Kela De Thaym on MerrowSaryn components Salvage, Alloy Plates, Rubedo, Detonite Ampules
Uranus (Lvl. 15-30) Tyl Regor on TitaniaAsh components Polymer Bundle, Plastids, Gallium, Detonite Ampules
Europe (Lvl. 12-27)Raptor on NaamahNova components Rubedo, Control Module, Morphic, Fieldron Sample
Neptune (Lvl. 20-35) Hyena on PsamatheLoki components Warframe Nano Spores, Ferrite, Control Module, Fieldron Sample
Eris (Lvl. 18-33)
-- Warframe Nano Spores, Ferrite, Control Module, Fieldron Sample
Ceres (Lvl. 20-35) Lt. Lech KrilFrost components Alloy Plates, Circuits, Orokin Cells, Detonite Ampules
Pluto (Lvl. 25-40)Ambulas - HadesFrost components Alloy Plates, Plastids, Rubedo, Morphic