New Delhi, July 5 -- A planet twenty-five light-years from Earth is close enough, in galactic terms, that a team of astronomers is comfortable calling it a neighbor. It is not close enough to visit, and it will be decades before anyone can confirm the one fact that would actually make it habitable. That gap, between what has just been measured and what remains unknown, is the real substance of the discovery.

the Habitable-zone Planet Finder at the Hobby-Eberly Telescope in Texas, and the NEID Spectrometer at the WIYN Telescope in Arizona.

The number that has drawn attention to Gliese 3378b over other habitable-zone candidates is a comparatively mundane one: the planet receives approximately 90 percent of the stellar energy that Earth re...