Last week we posted a piece on how delicately balanced the solar system must be in order for biologically advanced life to exist on earth. The balance needed to sustain such life, however, involves far more than just the solar system. It turns out that the star which supports the life-bearing planet must also meet a number of requirements.
Daniel Bakken at Evolution News and Views mentions a few of those conditions. He notes among other things that in order to support life a star must be (I'm paraphrasing him here):
1. A single star, not a binary star: Many stars in our galaxy are binaries.
2. Must be just the right size and temperature: The stellar classification system used by astronomers ranks stars in surface temperature, and assigns letters to them, in the sequence O, B, A, F, G, K and M, O being the hottest, largest stars and M being the lowest in temperature, and smallest in mass.
It's likely that only F and G dwarf stars are homes for potentially habitable planets as they are stable for billions of years, yet are large enough so their habitable zones are outside the tidal locking limit (the region where the star's gravity "locks" the planets rotation so that only one side of the planet ever faces the star). The Sun is a G-type dwarf star. F-type dwarf stars are slightly larger. Stars between O and A are large and evolve too quickly, providing only a short-lived habitable zone (the zone around the star where temperatures are moderate enough to allow for liquid water and life to exist).
The K and M dwarf stars are smaller red dwarfs. A serious problem with red dwarf stars in the K and M classes is their energetic flares and coronal ejection events. Potentially habitable planets need to orbit closer to these stars to be in their habitable zones. Yet the exposure to their stellar winds and more frequent and energetic flares becomes a serious issue for habitability. Because of these stars' smaller mass, ejections get released with more violence. Any planet's atmosphere would be subject to this ionizing radiation and likely expose any surface life to much more damaging radiation.
3. Emit just the right amount of UV radiation: Life needs enough UV radiation to allow chemical reactions, but not so much as to destroy complex carbon molecules like DNA. Just this flux requirement alone requires the host star to have a minimum stellar mass of 0.6 solar masses (a solar mass is equal to the mass of our sun), and a maximum mass of 1.9 solar masses.
In other words, our sun is not just an average star. In many respects it's unusual. Not only does it satisfy the above conditions, but it's also located in just the right place in our galaxy. It's between spiral arms where it's less likely to collide with debris, and also far enough from the center of the galaxy and not too high above nor too far below the plane of the galactic disk so that harmful radiation from the center is blocked by the intervening matter.
When we factor together all the conditions that must be met by the various parts of the solar system in order for intelligent life to exist anywhere within it the probability that any other star/planet system in the universe would duplicate these traits is dramatically diminished. It may be that, contrary to popular belief, earth is the only place in this enormous universe in which advanced life exists or even could exist. That's a stunning possibility.