NASA is particularly interested in identifying planets one half to twice the size of Earth —
terrestrial planets rather than the gas or ice giants or hot - super-Earths in short period orbits that evidence suggests exist in large numbers — especially ones that are located in the habitable zone of their stars.
Based on humankind's admittedly limited experience, habitability seems to mean a small world —
a terrestrial planet rather than a gas giant like Jupiter or Saturn — orbiting its star at a comfortable «Goldilocks» distance that allows water to persist in liquid form.