Gliese 581c is the third planet orbiting Gliese 581. It was discovered in April 2007. In their 2007 paper, Udry et al. asserted that if Gliese 581c has an Earth-type composition, it would have a radius of 1.5R⊕, which would have made it at the time "the most Earth-like of all known exoplanets". A direct measurement of the radius cannot be taken because, viewed from Earth, the planet does not transit its star. The minimum mass of the planet is 5.5 times that of Earth. The planet initially attracted attention as being potentially habitable, though this has since been discounted. The mean blackbody surface temperature has been estimated to lie between −3 °C (for a Venus-like albedo) and 40 °C (for an Earth-like albedo), however, the temperatures could be much higher (about 500 degrees Celsius) due to a runaway greenhouse effect akin to that of Venus. Some astronomers believe the system may have undergone planetary migration and Gliese 581c may have formed beyond the frost line, with a composition similar to icy bodies like Ganymede. Gliese 581c completes a full orbit in just under 13 days.