Kepler-155 c is a planet about twice Earth's width, probably in the habitable zone of a cool star about 950 light-years away. The habitable zone is the range of distances where a planet could keep liquid water on its surface. It crosses in front of its star every 52.7 days and receives almost exactly as much light as Earth gets from the Sun, about 1.03 times as much, according to a 2017 analysis.
It is the outer of two planets in the system, after Kepler-155 b, both found by NASA's Kepler space telescope from the small dips in starlight as they pass across their star. They were confirmed in 2014 in a study led by Jason Rowe that validated hundreds of Kepler planets in multi-planet systems, which are rarely false alarms. In 2017 a team led by Guillermo Torres re-examined Kepler-155 c, listed as KOI-438.02, with a separate statistical method called BLENDER, and validated it at about 99.91 percent confidence.
Torres's team was looking for small planets most likely to be both rocky-sized and in the habitable zone, allowing for the possibility that a planet circles a faint star hidden behind its target and is larger than it seems. Kepler-155 c was one of seven candidates in their study with a better than 50 percent chance of being smaller than twice Earth's width and in the habitable zone. They estimated its equilibrium temperature, set by starlight alone, at about 286 kelvin. Its mass has never been measured, so whether it is rocky or has a thick atmosphere is unknown.