The biggest number being thrown about in NASA’s big exoplanet announcement on Tuesday wasn’t 1,284 (the number of new exoplanets confirmed by the agency’s scientists). It was nine — the number of new planets found to be located in the Goldilocks zone of their respective star systems.

You might be thinking: “Goldilocks zone? Is that for real?” Fair question, and the answer is yes — it is real. The Goldilocks zone is another word for the habitable zone around a star’s orbital space, where a planet is thought to have the potential to develop the types of global environmental conditions necessary for evolving and/or fostering life. As the story goes, it’s just right — for humans.

How did it get that name? The answer requires a bit of a primer on the nuances of what scientists are looking at and referring to when it comes to extraterrestrial habitability.

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