Tau Ceti f

Tau Ceti f is an unconfirmed planet that may be orbiting the nearby Sun-like star Tau Ceti 11.905 light years from the Sun. It is the fifth planet from Tau Ceti in its system -the outermost planet of the system discovered as of 2012. It orbits Tau Ceti at a distance of 1.35 AU with an orbital period of 642 days and has a minimum mass of 6.6 Earth masses which classifies it as a super-Earth. Assuming thst Tau Ceti f is a terrestrial planet with an Earth-like atmosphere, the surface temperature would be approximately −40°C (233 K). With a denser atmosphere able to produce a stronger greenhouse effect it could have a much higher temperature, between 0°C and 50°C, enough for complex life to exist.

This means terraforming could easily be done by adding tons of extra greenhouse and buffer gases. But, not only would these changes make the atmosphere unfit for human breathing, the planet's higher (though as yet unknown) gravity might require humans, and any other Terrestrial life adapted to 1G to be genetically altered to live on this world (an ironic concept). Less than 12 light years away, human colonization of this system is foreseeable as soon as near-light speed propulsion systems are developed.