 Two previously discovered planets in the same system , Gliese 581 c and d ( inward and outward from planet g , respectively ) , were also regarded as potentially habitable following their discovery . Both were later evaluated as being outside the conservatively defined habitable zone , leading Vogt et al. to remark that " The GJ 581 system has a somewhat checkered history of habitable planet claims " . However , a subsequent downward revision of the period of planet d from 83 to 67 days has bolstered its habitability prospects , although a large greenhouse effect would be needed . 
