Toward Solving the Drake Equation

The Drake equation is an attempt to quantify the number of detectable alien civilizations (N) in the Milky Way galaxy.  Here's the equation:

R* = Star formation rate in the galaxy
f_p   = Fraction of stars with planets
n_e   = Average number of planets per star that can support life
f_l    = Fraction of suitable planets that develop life
f_i    = Fraction of above that develop intelligent life
f_c   = Fraction of civilization that emit detectable signals (eg. radio waves)
L    = Length of time over which above civilizations emit the signals.

The equation is useful in that it demonstrates how utterly ignorant we are about this topic.  Depending upon the various assumptions made about each of the above variables, the galaxy is either teeming with life, or we're the only ones here.

Well, astronomy has made some progress on the first few variables.  In recent years, all-sky infrared surveys have mapped the star-forming regions in the galaxy, and they claim star formation rates of ~2-3 solar masses per year.  Basic info here.  Great, so that's one variable with a reasonable estimate.  Only 6 more to go, but these are much harder to determine.

In an exciting new development, astronomers have announced that they have a reasonable estimate of the second variable, the fraction of stars with planets.  The article, published in the January 12 issue of the Nature, estimates that stars in the Milky Way average at least 1 planet per stars.  Well how many stars are there in the Milky Way?  Answer: A few hundred billion!!  So if each one has at least one planet, that's a LOT of planets.  This is great, but how did they determine this number?

Well, they monitored stars for a phenomenon called gravitational microlensing (or just microlensing to the cognoscenti).  Microlensing occurs when a massive object passes in front of a star.  Because the gravity from the object can bend the light (as predicted by Einstein), it can act as a lens, effectively magnifying the light seen from the background star.  This technique had been used famously by the MACHO Project in the 1990s to determine if much of the "dark" matter in the galaxy was simply planets and brown dwarfs (it isn't).

The published result is from the PLANET collaboration, which has been monitoring high magnification events from large area microlensing surveys such as OGLE.  As shown in the illustration above, every once in a while, during a microlensing event from a star, an additional lensing will occur (over a shorter duration) by a planet companion of the lensing star.  The great thing about microlensing searches for planets is that they're more sensitive to planets at larger orbital radii, whereas transit and radial velocity techniques are efficient at finding planets that are very close to the star.

Though the numbers are quite small, the authors suggest that smaller planets similar to Earth are more common than Jupiter mass planets.

So, we now have at least a decent estimate for two of the seven variables in Drake's equation, and we're investigating a third.  The astronomers are doing their part.  I'd like to hear from the biologists and anthropologists what their best guesses are for the other four.