GREENFIELDBOYCE: Just being in the habitable zone, of course, does not mean a planet is actually habitable. Marc Kuchner is an astrophysicist at NASA Goddard Space Flight Center. He says the habitable zone has been a useful concept for thinking about planets in the past, but...

MARC KUCHNER: I think it may be time to discard the habitable zone. I think maybe what this discovery is telling us is it's time to move past it and start asking the next question, which is, is the planet moist and juicy like our own planet Earth?

GREENFIELDBOYCE: He says scientists are already working on powerful instruments that could someday detect signs of water on one of these alien worlds. Nell Greenfieldboyce, NPR News.