Other Factors
As was clear from our study of the Solar System,
factors other than distance from the parent star
influence the temperature of a planet or moon. For example,
Venus and Earth differ by only 30 percent in distance from the Sun,
but have very different surface temperatures because of
greenhouse heating. Recall also that Jupiter's moon Europa
might have liquid subsurface oceans because of tidal heating,
even though it lies far outside the habitable zone.
Thus, we should take the habitable
zones marked in the figure of the left panel as only a rough
guide to whether a planet at that distance has the
right surface temperature to support life.