f_planets -- this is the fraction of star with planets. Recall from Lecture 14 that extrasolar planets are now known to exist around other stars:

http://exoplanets.org/

There are over 300 known extrasolar planets out of (more or less) a couple thousand stars that have been searched. Thus, about 10% of stars have planets.

There is a pretty big uncertainty about this number. Remember that the radial velocity technique is most sensitive to big planets close to their stars. So how many small, far out planets are we missing?

Also, the Hubble Space Telescope has been used to identify a 13 billion year old planet (from a time when the metallicity of the galaxy and Universe was thought to be quite low). Do we really understand everything we'd like to about how, when, and where planets form?

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