Drake Gets a Bump

January 31, 2006

The Drake Equation is a grand guess. It’s a set of probabilities which added together give a rough idea of the number of communicating civilizations in the galaxy. It is not meant to provide a real answer, but to bound the problem. Here’s one of the ways it’s written:

#Civ’s = N_m * f_p * n_e * f_l * f_i * f_c * f_L

That is, the number of communicating civilizations in the Milky Way is a factor of: how many stars there are in the Milky Way (N_m) times the fraction of stars which have planets around them (f_p), times the number of those planets in a system capable of sustaining life (n_e), times the fraction of those planets where life does evolve (f_l), times the fraction of those where intelligent life evolves (f_i), times the fraction of those who choose to communicate (f_c), times the fraction of those that are doing so now (f_L).

When Drake first formed the equation most of the factors were wild guesses. Most still are today.

However, over time we’ve begun to learn some of the constraints of the constraints. N_m is pretty much a given. We know the Milky Way galaxy has between 100 and 400 billions stars in it. The rest of the factors, however, decrease in certainty. f_p was once unknown, but we’ve begun to observe planets around other stars (not so in Drake’s time).

When Carl Sagan explained the DE on his documentary “Cosmos” he assigned a guess of 1 in 10 for f_p*. It turns out he was pretty close if not even conservative. Unfortunately many of those extrasolar planets have very eccentric orbits precluding Earth like planets. That’s a hit to the DE and it needs to be incorporated into the DE in either the f_p or n_e factor.

The remaining four factors (life, intelligent life, communicating, and doing so now) are still highly speculative. But the good news (if you’re looking for life outside of our system) is that the DE just got a small bump. The number of stars capable of holding planets has just been revised upward. Here’s the article I read which prompted this blog entry:

http://www.spaceflightnow.com/news/n0601/30singlestars/

Here are some sites about the Drake Equation:

http://en.wikipedia.org/wiki/Drake_equation

http://www.activemind.com/Mysterious/Topics/SETI/drake_equation.html

http://www.pbs.org/wgbh/nova/origins/drake.html

PS: * It's been some time since I've seen Cosmos, so I'd forgotten what number Sagan used. I just found a clip of his Drake Equation segment from Cosmos on YouTube. In fact Sagan gives f_p a value of 1 in 4 which may still prove correct, but is a liberal guess with today's knowledge. Here's the link

http://www.youtube.com/watch?v=0Ztl8CG3Sys