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basstard [soylentnews.org] writes:

BBC has an article by Christopher Watson on what makes a planet habitable. [bbc.com]

What if the planet sports a blanket of white clouds? Clouds are reflective and therefore will cool the planet, acting to push the habitable zone closer to the star. Amusingly, if we calculate this "equilibrium temperature" for the Earth, taking into account its beautifully reflective clouds, then it turns out that we live outside the classical habitable zone! The same calculation for Venus gives an expected equilibrium temperature of about -10°C, but in reality it is more like 450°C.

Both these planets have greenhouse gases present in their atmospheres, warming the planet up and driving the outer-boundary of the habitable zone further away from the star (while clouds drive the inner-boundary closer to the star). The very latest habitable zone definitions use simulations of these cloud and greenhouse effects - widening and blurring the crude classical definition. Throw into the mix that we currently can't study the atmospheres of rocky terrestrial exoplanets (and therefore have no idea whether they have clouds, greenhouse gases, or even an atmosphere at all!) - then to say "that planet is habitable" is impossible, for the time-being at least.

Just to complicate matters, the habitable zone also depends on the type of star the planet orbits. The more massive and hotter the star, the further out the habitable zone will lie. Conversely, small cool stars will have a habitable zone that is much closer in. Indeed, "red dwarf" stars are so cool and dim that a planet in the habitable zone might have a "year" that lasts only a few days, so feeble is the red dwarf's light.

This would raise other problems for life on such a planet. Red dwarfs like to chuck out large flares, stellar eruptions that release charged particles and X-rays. Given the close proximity of the planet, this might cause substantial atmospheric losses. High doses of radiation also tend to be harmful to biological material, and X-rays are capable of dissociating water - thereby depleting any water supply. Not ideal. Maybe things are better around hotter stars, where a habitable planet would lie further way from any nasty stellar blasts? Well, now we run into another problem, that of the lifetime of the star.

Massive, hot stars are real gas-guzzlers. Yes, they may have far larger "fuel tanks" (they have a lot more mass to "burn"), but they gobble that fuel much, much faster, and die much younger than small, frugal cool stars. For example, some of the most massive stars may live for only a few million years, while our Sun will hang around for about eight billion years. Based on our knowledge of how life evolved on Earth, it is unlikely that even simple life would have time to evolve around stars that are all that much hotter than our Sun. Returning to the diminutive red dwarf stars at the other end of the scale, these can hang around for about 100 billion years. Perhaps if a planet DID hang on to its atmosphere, then over such a long time life might evolve to cope with being frequently doused in radiation?

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