Citation:
Howard AW, Johnson JA, Marcy GW, Fischer DA, Wright JT, Henry GW, Isaacson H, Valenti JA, Anderson J, Piskunov NE. The NASA-UC Eta-Earth Program. II. A Planet Orbiting HD 156668 with a Minimum Mass of Four Earth Masses. \apj. 2011;726 (2) :73.