To understand and predict the nature of planetary atmospheres, it is very important to understand their host star! Different spectral types have a wide range of UV emission, changing atmospheric photochemistry. To learn more about the effects of different main sequence stars on habitable Earth-like planets, I would suggest reviewing the work of Sarah Rugheimer, a former member of the Carl Sagan Institute.
My focus is not only on how different spectral types affect planetary environments and biosignatures, but also how stars in different evolutionary phases differ from one another.
As seen above, although Sun-like stars spend the majority of their lives on the main sequence, it spends a fair bit of time in other evolutionary stages, where different types of planetary environments could occur.
White DwarfsWhite dwarfs are the end states of stars with core masses of less than 1.4 Solar masses, which comprises the majority of the stellar population. Although they start off being extremely hot, they cool down over time and will eventually have a continuous habitable zone for long enough for life to develop and thrive on a planetary surface.
It takes approximately 10 billion years for a white dwarf to cool down from 6,000 to 4,000 K, giving life enough time to develop and evolve. My work involves simulating the climate and atmospheric photochemistry of Earth-like planets orbiting in the habitable zones of white dwarfs during this period of a continuous habitable zone. Cooler white dwarf scenarios are not included because the Universe simply isn't old enough for those to exist!
Post-Main Sequence StarsHabitability throughout the post-main sequence is an interesting scenario because during this phase of evolution the habitable zone is pushed past the original frost line of the planetary system, where over 99% of the system's water is expected to exist!
Above you can see the post-main sequence evolution of our own Sun, with the red region corresponding to the red giant branch, the blue to the asymptotic giant branch, and the green the habitable zone. During this time period conditions will not be good on Earth (especially when the Sun's radius grows to our orbital distance!), but the increase in the Sun's brighteness will cause Jupiter and Saturn to enter the habitable zone! At this point in time it will be possible for icy worlds like Europa and Enceladus to melt, potentially exposing previously undetectable subsurface life.
The maximum time a planet can spend continuously in the post-main sequence habitable zone roughly corresponds to the length of the relatively stable horiztonal branch, at which point the star is fusing helium in its core. However, the length of the horizontal branch does NOT depend linearly on stellar mass, as seen in the figure above. While stars like the Sun spend less only about 1% percent of their total lifetimes on the horizontal branch, a 2.3 Solar mass star will spend nearly 30% of its life there. As a result, unlike other habitability searches around main sequence stars where smaller mass stars are prioritized, when thinking about post-main sequence habitability it is best to think of more massive stars. This is additionally convenient since stars the mass of our Sun haven't had quite enough time in our galaxy to reach the post-main sequence!