12 Light Years
This feels doable in this century.
Possible habitable zone planet is a mere 12 light years away
there are several planets around the nearby star Tau Ceti, and one of them is likely to be within the star's habitable zone. ...... Once they had that in hand, the astronomers turned to the actual data from Tau Ceti without any added signals. And, this time, three signals did pop out, with each of them adding between 1010 and 1017 to the statistical fit with the real data. The authors concluded that there were three planets in this signal, orbiting with periods of 14, 35, and 94 days. ...... adding two more planets to their model increased the fit, although not by nearly the same degree. One orbited with a period of 630 days, the second at 168 days. The latter one is the object that resides in Tau Ceti's habitable zone; it has a mass that's at least four times that of Earth's.