Neither Mars nor Gliese 581g nor any other exoplanet could become Planet B

A habitable exoplanet –For real this time

“After years of saying habitable exoplanets are just around the corner, planet hunters have finally found one. Gliese 581g is the first planet found to lie squarely in its star's habitable zone, where the conditions are right for liquid water (…) The new planet is one of six orbiting the star Gliese 581, a red-dwarf 20 light-years from Earth. Two of the planet's siblings, dubbed planets C and D, have also been hailed as potentially habitable worlds. The two planets straddle the region around the star where liquid water could exist – 581c is too hot, and 581d is too cold. But 581g is just right.”

September 2010, Wired Magazine