Sunday, July 12, 2009

Life In the Universe: My Own Amateur Calculations (Part 3)

In the last post, we delved into the basics of star formation as it pertains to the potential for complex life to develop on an Earth-massed planet orbiting with that star’s habitable zone. We also explored the difference between low massed main sequence stars (red) and the corresponding high massed ones (blue). I already implied the advantages of low-massed stars vis-à-vis high-massed ones concerning the presence of complex life and highly developed ecosystems. Now we look more in-depth at the pros and cons of each:



Enormously Long Expected Lifetimes. This is their biggest advantage. Simply put, longer lifetimes on the main sequence allow more time for life to form and develop; thereby increasing the odds of life actually existing around such stars – whether at present or in the future (particularly complex life). Life on Earth started around 4 billion years ago, shortly after our planet finished forming. Yet, only in the past half-billion years has surface-dwelling animal life existed on our planet – barely more than 10% of our planet’s existence!

As for species even potentially capable of developing technology Homo Sapiens Sapiens emerged only 200,000 or so years ago (the last 1/22,500 of Earth’s existence). Compare that to how long Television existed!!! Now you should appreciate how tiny a fraction of a planet’s lifetime is occupied by technically advanced lifeforms and therefore also appreciate the value of a long-lived star for increasing the probability life will arise on a planet around it.

Low radiation output,providing that red star has no more tendency to flare or suddenly vary in brightness as our sun(which unfortunately is not the case for many red stars, as explained next). If the red star is neither prone to flare NOR suddenly brightens at random or even frequent regular intervals (what astronomers call Variable Star), then life has all but infinitely better odds of surviving and thriving around such a red star – all other things being equal.


Tendency to Flare or Vary in Brightness Suddenly. Still, red stars have their share of disadvantages, especially the lower massed red ones. As mentioned above, even if their radiation output is much less than larger and hotter stars, many red stars still tend to flare periodically (i.e. send jets or erupting gases, which emit high levels of radiation, heat, and light). In short, the smaller the star stars, the more likely they are to vary greatly in brightness. Clearly, any otherwise potentially life-friendly planet orbiting it will face irradiation and sudden increases in temperature, potentially killing what life evolved so far (and perhaps boiling off its oceans as well).

Narrower Habitable Zone. Just as small campfires on a cold night can’t provide nearly as wide a comfort zone as a large bonfire, so a cooler star can’t provides as much area suitable for life and/or liquid water that a hot star can (and light and radiation in general, too, for that matter). In other words the cooler the star, the narrower the life zone. For small stars, this significantly lowers the probability that a planet of any sort will be in the habitable zone. In our solar system, our habitable zone for an planet just like Earth ranges from just inside our orbit to near Mars’ orbit (more specifically from 0.95 to 1.4 Astronomical Units, where 1 AU = Earth to sun distance). For small stars, the problem is potentially even worse; which brings us to the next disadvantage of small red stars.

Rotation Lock Within The Habitable Zone. Rotation Lock is when a smaller body orbiting a larger body always shows the same side towards the body it orbits. This is the case with the Moon. You see only one face of the moon because it is so close to the Earth that the difference in Earth’s gravitational pull between one face of the Moon and the other is great enough to hold one side of the Moon toward Earth. The same thing can happen regarding a planet near its star.

While it’s impossible in this solar system for the habitable zone to be inside the sun’s rotation lock radius, this is certainly the case for cooler stars. It’s unfortunate that the rotation lock radius doesn’t shift inward with lower mass nearly as much as the habitable zone does. Therefore, only the more massive of red stars have much likelihood of hosting life-bearing worlds in addition such stars narrow habitable zones.


For the most part, blue star’s advantages and disadvantages are the opposite of red stars.

Advantages: Less tendency to suddenly brighten or flare, Wide habitable zones, Habitable zone well outside the star’s rotation lock radius. However, this is all that can be said about massive blue star’s ability to host life.

Disadvantages: By far the biggest ones are their incredibly short lifetimes (sometimes only a few million years!) and their enormous radiation output. The latter means that even if any Earth-sized or larger rocky-metallic planets do form within the habitable zone and keep suitable orbits, the star will severely irradiate the planet’s surface and, perhaps even, strip the planet of its atmosphere – unless the planet is lucky enough to have an unusually strong magnetic field. Even with such a field, massive blue stars will certainly explode into a supernova far too quickly to allow formation of anything more than primitive microbial life.


Stars neither small red ones nor large blue ones offer some mix of advantages and disadvantages, all of them in a less extreme form than the stars we examined. Obviously, there is an optimal mass of a star if it is to have a reasonable probability of hosting a life-bearing world. The star must have a habitable zone outside the star’s rotation lock radius, it must not emit radiation intense enough to sterilize planets within the habitable zone, it must exist long enough to permit complex life and even more complex ecosystems to form, it must have a stable brightness regime, it must flare only a small amount if at all.

Obviously our Sun qualifies as such a star. However, other stars undoubtedly qualify as having high potential for life as well. The question is “What range of stellar masses is most optimal for a lifebearing world?” The sun does seem optimal for us. However, our star is the only one known to harbor a life-bearing world. That means the optimal mass of a life-bearing star could be either larger or smaller than ours (I’m inclined to lean toward “somewhat smaller”). The next posts will delve further into this most profound of all questions in astronomy and biology.

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