At the time of its discovery in 2007, Gliese 581c gained interest from astronomers because it was reported to be the first potentially Earth-like planet in the habitable zone of its star, with a temperature right for liquid water on its surface, and, by extension, potentially capable of supporting extremophile forms of Earth-like life.

The team released a paper of their findings dated 27 April 2007, published in the July 2007 journal Astronomy & Astrophysics.

[1] At the time of discovery, it was reported to be the first potentially Earth-like planet in the habitable zone of its star[5][6] and the smallest-known exoplanet around a main-sequence star, but on 21 April 2009, another planet orbiting Gliese 581, Gliese 581e, with an approximate mass of 1.9 Earth masses, was announced.

[3] The radial velocity method cannot by itself determine the true mass, but it cannot be very much larger than this or the system would be dynamically unstable.

For example, if Gliese 581c is a rocky planet with a large iron core, it should have a radius approximately 50% larger than that of Earth, according to Udry's team.

However, if Gliese 581c is an icy and/or watery planet, its radius would be less than 2 times that of Earth, even with a very large outer hydrosphere, according to density models compiled by Diana Valencia and her team for Gliese 876 d.[9] Gravity on the surface of such an icy and/or watery planet would be at least 1.25 times as strong as on Earth.

Rather than evolving to a planet composed mainly of rock with a thin atmosphere, the small rocky core remains engulfed by its large hydrogen-rich envelope.

[24] The study of Gliese 581c by the von Bloh et al. team is quoted as concluding "The super-Earth Gl 581c is clearly outside the habitable zone, since it is too close to the star.

Nevertheless, this runaway greenhouse effect could be prevented by the presence of sufficient reflective cloud cover on the planet's day side.

[29] Alternatively, if the surface were covered in ice, it would have a high albedo (reflectivity), and thus could reflect enough of the incident sunlight back into space to render the planet too cold for habitability, although this situation is expected to be very unstable except for very high albedos greater than about 0.95 (i.e. ice): release of carbon dioxide by volcanic activity or of water vapor due to heating at the substellar point would trigger a runaway greenhouse effect.

Alternatively, an atmosphere large enough to be stable would circulate the heat more evenly, allowing for a wider habitable area on the surface.

A slow rotation rate approximately 117 times slower than Earth's produces prolonged days and nights.

The signal is a digital time capsule containing 501 messages that were selected through a competition on the social networking site Bebo.

[34] More than half a million people including celebrities and politicians participated in the AMFE project, which was the world's first digital time capsule where the content was selected by the public.