Context : Stellar insolation has been used as the main constraint on a planet ’ s potential habitability .
However , as more Earth-like planets are discovered around low-mass stars ( LMSs ) , a re-examination of the role of tides on the habitability of exoplanets has begun .
Those studies have yet to consider the misalignment between a planet ’ s rotational axis and the orbital plane normal , i.e .
the planetary obliquity .

Aims : This paper considers the constraints on habitability arising from tidal processes due to the planet ’ s spin orientation and rate .
Since tidal processes are far from understood we seek to understand differences between commonly used tidal models .

Methods : We apply two equilibrium tide theories – a constant-phase-lag model and a constant-time-lag model – to compute the obliquity evolution of terrestrial planets orbiting in the habitable zones around LMSs .
The time for the obliquity to decrease from an Earth-like obliquity of 23.5 ^ { \circ } to 5 ^ { \circ } , the ‘ tilt erosion time ’ , is compared to the traditional insolation habitable zone ( IHZ ) in the parameter space spanned by the semi-major axis a , the eccentricity e , and the stellar mass M _ { \mathrm { s } } .
We also compute tidal heating and equilibrium rotation caused by obliquity tides as further constraints on habitability .
The Super-Earth Gl581 d and the planet candidate Gl581 g are studied as examples for these tidal processes .

Results : Earth-like obliquities of terrestrial planets in the IHZ around stars with masses \lesssim~ { } 0.25 M _ { \odot } are eroded in less than 0.1 Gyr .
Only terrestrial planets orbiting stars with masses \gtrsim~ { } 0.9 M _ { \odot } experience tilt erosion times larger than 1 Gyr throughout the IHZ .
Tilt erosion times for terrestrial planets in highly eccentric orbits inside the IHZ of solar-like stars can be \lesssim~ { } 10 Gyr .
Terrestrial planets in the IHZ of stars with masses \lesssim~ { } 0.25 M _ { \odot } undergo significant tidal heating due to obliquity tides , whereas in the IHZ of stars with masses \gtrsim~ { } 0.5 M _ { \odot } they require additional sources of heat to drive tectonic activity .
The predictions of the two tidal models diverge significantly for e~ { } \gtrsim~ { } 0.3 .
In our two-body simulations , Gl581 d ’ s obliquity is eroded to 0 ^ { \circ } and its rotation period reached its equilibrium state of half its orbital period in < ~ { } 0.1 Gyr .
Tidal surface heating on the putative Gl581 g is \lesssim~ { } 150 mW/m ^ { 2 } as long as its eccentricity is smaller than 0.3 .

Conclusions : Obliquity tides modify the concept of the habitable zone .
Tilt erosion of terrestrial planets orbiting LMSs should be included by atmospheric modelers .
Tidal heating needs to be considered by geologists .