Gliese 581 c

When its discovery was announced in 2007 by Stéphane Udry's team using the HARPS spectrograph at ESO's La Silla Observatory, Gliese 581 c appeared to orbit within the habitable zone of its red dwarf host star. With an estimated mass of roughly five Earth masses, it was among the first exoplanets seriously discussed as a possible abode for liquid water.

Subsequent climate modeling suggested that Gliese 581 c likely underwent a runaway greenhouse effect, trapping so much heat in its atmosphere that any surface water would have boiled away. The planet is now considered more analogous to Venus than to Earth, with surface conditions far too extreme for life.

Gliese 581 is a red dwarf star located approximately 20 light-years from Earth in the constellation Libra. Its relative proximity made it one of the highest-priority targets for early exoplanet habitability studies.

The system has confirmed planets including Gliese 581 b, c, and e, with additional proposed candidates—notably Gliese 581 d and g—remaining scientifically disputed. Some of these candidates were later attributed to stellar activity rather than actual planetary signals, and the exact count remains a topic of active debate in the literature.