Here are some interesting facts about our sun--facts that have a direct bearing on such things as the Drake Equation and the Rare Earth hypothesis:
- The Milky Way is 130,000 light years across, but only 5,000 light years thick (it's shaped like a pancake)
- There are 3 trillion (or 300,000 million) stars in the Milky Way
- Our sun is roughly half-way from the centre of the core of the galaxy and half-way through its lifespan at 4.5 billion years old
- it takes our sun 250 million years to orbit the centre of the galaxy
- Every second, our sun converts about 5 million tons of mass into energy; Earth intercepts only a billionth of this energy
- Stars that are larger than our sun burn out at a much quicker rate; a star with twice the mass as ours will burn out in only 1 billion years (it's likely that any orbiting planet could not ignite and sustain complex life given that short time-frame); further, these larger stars emit high levels of life-threatening ultraviolet radiation
- stars that are smaller than our sun can last well over 10 billion years and comprise as much as 95% of all stars in the Milky Way, leaving only 150 billion stars either the same or larger than our own (the so-called G class of yellow suns)
- M red dwarf stars are the most common of the small stars (they represent 80% of all stars in the galaxy); they burn their fuel more slowly and are quite dim--in fact, when you look into the sky at night, you cannot see any of these stars
- small stars have a significantly smaller habitable zone for any potential life-bearing planets; moreover, the habitable zone would have to be quite close to the sun, causing gravitational effects that would likely prevent the formation of life (including orbital "lock" so that only one side is facing the sun at any given time); finally, smaller stars are less stable and produce flares with great frequency
- our sun is situated in the galactic habitable zone, which boasts two major characteristics: i) a region fairly devoid of interstellar matter, and ii) a region rich in metallicity (the outer stars lack critical heavier elements which assist in the creation of life)
- 60% of yellow G class stars like ours exist in binary pairs or triplets; multiple star systems tend to sweep-up nearby material making it difficult for planets to form, and any planet that does form will be in a highly eccentric orbit--not good for life; thus, out of all the galaxy's stars, no more than 90 billion stars are of the solitary yellow G class
- it's appearing more and more that our solar system's composition is unique; most stars have so-called "Hot Jupiters" orbiting around them--gas giants that orbit very closely to the star; again, it's highly unlikely that a life-bearing planet could exist in such a system; and to make it even more difficult for life-bearing planets to exist, most outer solar system gas giants tend to be in eccentric orbits (unlike Jupiter which is in a near-circular orbit), which again would cause great instability to the solar system
- statistics that I'd like to know: of the 150 billion solitary G class stars, how many are in the galactic habitable zone (my guess is 10%, or 15 billion stars), and how many of them have Jupiters in a circular and outer orbit (my guess is 5%, bringing our total down to 750 million potentially habitable solar systems at our current time); how many of these 750 million solar systems have an Earth-like planet in its habitable zone (my guess, another 5%, bringing us down to 37.5 million)? How many of these 37.5 million planets developed life? Let's say 0.5%, which bring us to 18.75 million. How many of these developed complex life? Let's say 0.05%, bring us down to 937,500. How many of these developed industrial age intelligent societies (my guess, 0.05, bringing the total down to 46,880)?
So using this rather armchair approach to the Drake Equation we get a figure of N=46,880 or so. Of course, this doesn't speak to how long life has been able to get going in the Galaxy, so there may have been many other civilizations throughout its history--but again, I don't think this window has been open for very long, possibly only within the last 750 million to 1,000 million years. Also, my estimates may be quite liberal. My figure of 0.05 for industrial age societies may be significantly exaggerated.
BTW, many of these statistics were taken from William C. Burger's excellent book, "Perfect Planet, Clever Species."