As others have said, it's DLC. It might as well be a new game though because it changes so much. It also released with a free update that's pretty big too.
This is DLC in the way that Concerned Ape's updates to Stardew are DLC. This is an update that add additional content but also fundamentally changes parts of the base game.
I like factorio in the same way I like cocaine. The first time I played factorio (without any stimulants involved) I played for 72 hours without food, water, or sleep.
Holy fuck what a great game. Great for independently wealthy people with no family or responsibilities.
I'm assuming you've played it, but for others reading this: It's more than just bonus content, the base game has been reworked to integrate the planets into the progression.
A Space Age playthrough is different than a base game playthrough basically from the get go.
The creator of Space Exploration was hired before they started the Space Age DLC, so a lot of lessons learned from it are implemented in Space Age.
There are tons of great overhaul mods to play though. I'd probably play something else before doing a Space Exploration run because they do share so many similarities it might be better to leave some space between playing them.
And also important is, that space exploration is insanely big. You can prepare to spend about 300h with it(of course depending on how much experience you have).
Yeah this is why I've never tried it. When I was still on reddit the way people talked about space age turned me off. It seemed like a massive time sink that was more about upping the logistics to the nth degree than about new mechanics.
I really like space age so far because of how differently I've had to play on the first planet I landed on.