Did Earth Nearly Have a Second Sun?

 It's...possible.

Scientists have been performing models to explain the Oort Cloud for years with no success.

A new model comes closest...and that model indicates that the solar system was once a binary system, with a second sun about the same size as Sol, about 1,000 AUs away.

If so, what happened to it?

Simple: It wasn't held tightly enough gravitationally (or vice versa) and wandered off to hang out somewhere else in the galaxy. This is called dissociation, and it's not uncommon for "wide" binary systems, where the stars are more than 800 AUs apart.

It may even have left something behind; if Planet Nine is real, then planetary formation theories don't explain a gas giant that far out. Capture does...and it's possible that our sun stole the planet as Sol Two left.

So, would Earth still be habitable if it had stuck around?

The answer is most likely, and perhaps surprisingly, yes. Unless the other star was much brighter than Sol (which seems unlikely at the same mass), Sol's habitable zone would be more or less the same.

But, life's evolution...and certainly the culture of any sentient species on this alternate Earth...would likely have been altered by having a second sun.

I'm now speculating. This could be quite the rabbit hole...