Re: paper on transits by extrasolar planets in OGLE data

[Stupendous Man <richmond@a188-l009.rit.edu>]

  Chris asks:

> Here's a question:  If _every_ star had a Jupiter sized planet
> and the orbital planes were random with respect to our line of
> sight what fraction of the stars _could_ show a transit?

  Order-of-magnitude calculation: assume star of 1 solar radius,
planet of 0.1 solar radius (like Jupiter), planet's orbital radius
3 million km (equivalent to orbital period of 4 days, typical of
the "hot Jupiters").  

     (scribble, scribble, scribble)

  Hmm.  I end up with a probability of around 7 percent that
such a planet will have the right orbital orientation to transit
its star, as seen from the Earth.  

  You can find a proper calculation in papers by Borucki and collaborators;
for example, I see that in "The Vulcan Photometer: A Dedicated 
Photometer for Extrasolar Planet Searches", PASP, 113, 782 (2001)

     http://adsabs.harvard.edu/cgi-bin/nph-bib_query?
           bibcode=2001PASP..113..439B&db_key=AST&high=3c321cbf8318006

a probability of about 10 percent is derived with similar assumptions.

  So, _if_ all stars have such planets, and _if_ the OGLE observational
selection effects were small (which is almost certainly wrong), 
_then_ one might have expected OGLE to find 5000 transiting systems
out of 52,000 stars.

  Both of the "_if_" condition above aren't justified, of course.
There will be papers to come in which people use the OGLE results
to try to estimate the fraction of stars with such planets -- just
wait and see.  

                                           Michael