Past Blog Tie-In
• Star Systems, Part 1: Fundamental Orbits
The semi-major axes of the inner planets of the Solar System are:
However, the Sun's luminosity has varied over its lifetime; shortly after its formation, its luminosity was about 70% of its current value, resulting in an average increase of about 6.7% per billion years. Thus, when the Sun was 1 billion years old, its luminosity was about 76.7% of its current value. Doing the calculations for the nucleal orbit and optimistic Habitable Zone based on this luminosity:
Thus, while Venus is demonstrably uninhabitable now, it is possible (perhaps likely) that Venus was far more habitable 3 billion years ago, especially if its rotational period were more-or-less what it is now, as is discussed in a Scientific American article from August 2016 titled Hellish Venus Might Have Been Habitable for Billions of Years .
The Sun's Future Habitable Zone
Shortly thereafter, the Sun will enter its red giant phase and its luminosity will shoot up dramatically, as will its diameter, utterly engulfing Mercury and rendering Venus and Earth into hellish, lifeless balls of mostly molten rock. 
Stars Unlike the Sun
Aldebaran's radius is 44.2 times that of the Sun, or 695,700 ⨉ 44.2 = 30,749,940 km. One astronomical unit is 150,000,000 km, which means that Aldebaran's radius measures:
0.387 - 0.205 = 0.182 AU inside Mercury's orbit.
The calculated nucleal orbit is 22.76 AU. An Earth-mass planet at that orbit would have an orbital period of:
The Innermost Stable Orbit for a star of 518 times the luminosity of the Sun is calculated by:
Planets which were habitable in a star's youth may be uninhabitable in its later life (or destroyed altogether), while planets uninhabitable when the star is young may become habitable (for a time, at least) in its waning years.