The six planets around HD 219134 were detected by precisely measuring the tiny wobbles the planets induce on their host star. The 5 inner planets are orbiting too close, and hence, too hot to be habitable. Nevertheless, HD 219134 is 0.31 times as luminous as the Sun and a planet orbiting it at 0.56 AU, corresponding to an orbital period of 167 days, would receive the same intensity of stellar radiation Earth receives from the Sun. If this hypothetical planet has a mass equal to that of Earth, the amount of wobbling it induces on its host star would be challenging but not impossible to detect.
Vogt et al. (2015), “A Six-Planet System Orbiting HD 219134”, arXiv:1509.07912 [astro-ph.EP]