3 Trillion Suns!

January 7th, 2009 | Posted by paul in Uncategorized


That’s the new upper estimate of the number of stars in our galaxy, according to the report in Science News..  Scientist Mark Reid and a team of astronomers at of the Harvard-Smithsonian Center for Astrophysics, used the Very Long Baseline Array to look at star-forming regions across the galaxy. What they observed is that these stellar nurseries are moving in an elliptical orbit, rather than a circular one, and moving more than 100,000 miles per hour faster than previously believed. This extra velocity can only be explained by much larger overall galactic mass, which Reid estimates is at least 50% larger than previous estimates of galactic size.  These estimates pegged the Milky Way’s stellar population at around 400 billion stars, but now that figure has been revised upward to between 1.8 and 3 trillion!

It’s here that I like to wax mathematical with my own (rough) estimate of Drakes Equation.

Lets assume 3 trillion suns is the more accurate figure.  Based on the growing evidence of an abundance of habitable planets, as evidence by the ever widening range of exoplanets being discovered, it’s not too much of a stretch to assume that there is at least one rocky, Earth-like planet for every 10 G-type stars (ones like our sun)  By earth-like I mean that it is rocky and not gaseous, and it’s size is somewhere between 4000 and 16000 miles in diameter (the Earth is 8000 miles).  However scientist are beginning to believe that both K and F type stars are also long lived enough that planets at an appropriate distance could also be habitable.  In this case habitable is defined as having the right conditions for liquid water.  Also, scientists now believeit is possible for habitable planets to form around binary star systems as well.  Finally there is what is called a habitable zone around the galaxy, which is in my opinion too conservative, and is not supported by the majority of scientists.  If we put this all together, G-type, plus a smaller number of K and M type stars gives us about 8% of the total stars capable of supporting habitable planets.  If we cut out most (but not all) of the multiple star systems, and eliminate all the stars closer than 20,000 light-years to the galactic core (just to be conservative), we’re left with about 4% of the stars in our galaxy that our friendly to life-supporting planets should they exist.  And if 1 out of 10 of those stars actually have an earth-like planet, with 3 trillion total stars, leaves us with about 12 billion Earth-like planets in our galaxy capable of supporting liquid water and in turn carbon-based life as we know it. And I believe this to be a conservative estimate, but we’d really know until the Kepler data comes in.

At this point Drake’s Equation become very murky, as our understanding of the formation and likelihood of life forming, given the right conditions, is almost unknown.  What we do know is that many of the major chemical precursors for life have already been identified in space, including water, alcohol, ammonia, carbon monoxide, carbon dioxide, and acetic acid, which gives vinegar its piquant flavor.

Either way what these numbers tell us, is that there could be upwards of 12 billion stars with habitable planets around them.  That or these planets could be terraformed to support life by a life-bearing space civilization.  The ideas of billions of living worlds in our very own galaxy is a prospect worthy of investigation, which is why the search for earth-like planets has become the most important objective of space science both at NASA and abroad.

To get a scifi perspective, Star Trek talks of thousands of inhabited worlds, Star Wars of millions, and our very own Milky Way could be home to billions.


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