They are complete separate planets / games.
You can link them to Prime if you travel to Humble or Toxic via the interplanetary travel Rocket then the saves are merged into Prime.
You also have the 2 free moons , 5 new games in all. Great game.

So you can play them sequentially as part of the same save? Does any research or progression carry over?

The above post is misleading. They are the same game and same saves. You can complete terraforming on Humble or Prime, build an interplanetary travel rocket, and go visit the moons or the other planets (Toxicity and either humble or prime depending on where you started). You can do this all in a single playthrough and travel back and forth between the areas.

Each area continues progressing even after you leave it. Automated mining and crafting stays producing, drones continue working - gathering, distributing and delivering - terraformation progress keeps going up. Any crafting you unlock on one planet "carries over" to the other planet, so long as you can find the resources to make the things.

You can even carry items with you to make starting on a new planet easier...and there's both manual and automated delivery between planets (for most planets, again if you have the resources) for items you need but can't find.

All in all, nothing is a 'new game' here. Even the 'story' continues...

The only requirement is that you complete one of the starting planets to unlock the ability to leave that planet. Once you have the ability to leave the planet you have the option to end your game via one of the endings or to travel to another moon - one of the DLCs or one of the free moons - to continue your game.

Thanks for the detailed explanation, very helpful. I'll likely buy the DLC's later.

引用自 Steve：

Thanks for the detailed explanation, very helpful. I'll likely buy the DLC's later.

You can do the endings and still keep traveling. The endings will just put you back where you were after the cutscenes.
The main game includes two moons to travel and terraform.
The other two planets are dlc.

You can start on Toxicity on a fresh, new save and can eventually still travel to the other planets from that same Toxicity save once you get the extraction platform unlocked. If you start on a new save file then no, the progression on the other planets will NOT be unlocked. You of course can travel to Toxicity from Humble or Prime if you have those terraformed on an existing save (one made prior to Toxicity release). I wanted the challenge so I started Toxicity with a new save file.

Whether they are a continuation of "the same game" is really up to the player choice and if you want to play them in separate save files vs using the extraction platform to move around planets.

引用自 Illydth：

The above post is misleading. They are the same game and same saves. You can complete terraforming on Humble or Prime, build an interplanetary travel rocket, and go visit the moons or the other planets (Toxicity and either humble or prime depending on where you started). You can do this all in a single playthrough and travel back and forth between the areas.

Each area continues progressing even after you leave it. Automated mining and crafting stays producing, drones continue working - gathering, distributing and delivering - terraformation progress keeps going up. Any crafting you unlock on one planet "carries over" to the other planet, so long as you can find the resources to make the things.

You can even carry items with you to make starting on a new planet easier...and there's both manual and automated delivery between planets (for most planets, again if you have the resources) for items you need but can't find.

All in all, nothing is a 'new game' here. Even the 'story' continues...

The only requirement is that you complete one of the starting planets to unlock the ability to leave that planet. Once you have the ability to leave the planet you have the option to end your game via one of the endings or to travel to another moon - one of the DLCs or one of the free moons - to continue your game.

Wait, so, if I go to Humble, does my terra tokens grind continue going on Prime?

Yes

Oh wow, I've been afking for hours because I didn't want to do portal runs LOL

Finally, I can just leave and let them do their thing!

Have a good trip, don't forget to pack your toothbrush :)