Seriously, though, people have been making up worlds for as long as we've been telling stories to one another. If anything about the setting of a story is supplemental or contradictory to the experience of the audience, the world is a built one. Places like More's Utopia, Swift's Land of the Hhouyhnhnms, and the Moon on which Méliès' adventurers tromp around, are all built worlds, to one degree or another.
There is a spectrum, of course.
High fantasy stories like The Lord of The Rings postulate wildly different creatures and environments, often without any great concern for the scientific plausibility of their settings. Science fiction writers, such as Larry Niven (Ringworld), Hal Clement (Mission of Gravity), and Frank Herbert (Dune) have tried (with varying degrees of success) to design settings for their stories that are unerringly true to the physics of the universe as we understand them.
There is even a category of so-called hard fantasy, whose authors strive "...to present stories set in (and often centered on) a rational and knowable world [and] build their respective worlds in a rigorous and logical manner." 
I suppose that's why I started collecting the data, information, and equations and built the companion site; while my own interests run more to fantasy than SF (in terms of my own writing—I read it all!), I have always felt that it was important to know that the worlds I was describing were at least reasonably plausible.
That being said, I am not a physicist. I am not an astronomer. I am not a mathematician, nor a cosmologist, nor a geologist, nor a meteorologist, nor a metallurgist....
I am a scholar; I have a B. A. in Art History and an M. A. in History, and as such I am driven to practice good scholarship and engage in deep and proper research.
And, as I said, in dabbling with fiction writing, I've always had a niggling fear about someone reading something I've written and commenting, "You know, it wouldn't really work that way, because...."
Yeah, yeah, I know: The Omega Argument  says that "It is so because I say it is," and that's really all an author needs in the way of justification should a reader choose to pick the nit.
But I prefer slightly thicker armor, okay?
I'd consulted published books on the subject for decades, and when the Web came along, I began to trawl the internet for help, assistance, and guidance.
But, I found that while there are a (growing) number of publications, websites, and YouTube™ channels dedicated to the maturing art of Worldbuilding, I had yet to find a "one-stop shop" that had everything I needed and wanted all in one place for easy reference and consultation.
"Find a niche and fill it" right?
Still, my collection of worldbuilding tools and practices is not exhaustive; I'll mention seasons, but I won't covere climate and meteorology. I'll touch on density and composition, but I won't scratch the surface (pun intended) of geology. Cartography? Perhaps at some point, but not now.
And let's not even open that can of worms over there labeled "Culture and Society".
But, if you want to slap together a star system that has some kind of chance of being realistic and be able to say with confidence things like, "The Eye of Dorámak appears in the northern skies only once every 3300 years," and be able to prove it should anyone ask, perhaps you'll find something here and at the companion site to help you get a good start on that.
2. Also, rationale ultimum