sentences:  Although unconfirmed, there is 98.6% probability that this planet does exist.The National Science Foundation announced on 29 September the discovery of a fourth super-Earth (Gliese 581g) orbiting within the Gliese 581 planetary system.The planet has a minimum mass 3.1 times that of Earth and a nearly circular orbit at 0.146 AU with a period of 36.6 days, placing it in the middle of the habitable zone where liquid water could exist and midway between the planets c and d. It was discovered using the radial velocity method by scientists at the University of California at Santa Cruz and the Carnegie Institution of Washington.