  Exoplanetary science has reached a historic moment. The James Webb Space
Telescope will be capable of probing the atmospheres of rocky planets, and
perhaps even search for biologically produced gases. However this is contingent
on identifying suitable targets before the end of the mission. A race
therefore, is on, to find transiting planets with the most favorable
properties, in time for the launch. Here, we describe a realistic opportunity
to discover extremely favorable targets - rocky planets transiting nearby brown
dwarfs - using the Spitzer Space Telescope as a survey instrument. Harnessing
the continuous time coverage and the exquisite precision of Spitzer in a 5,400
hour campaign monitoring nearby brown dwarfs, we will detect a handful of
planetary systems with planets as small as Mars. The survey we envision is a
logical extension of the immense progress that has been realized in the field
of exoplanets and a natural outcome of the exploration of the solar
neighborhood to map where the nearest habitable rocky planets are located (as
advocated by the 2010 Decadal Survey). Our program represents an essential step
towards the atmospheric characterization of terrestrial planets and carries the
compelling promise of studying the concept of habitability beyond Earth-like
conditions. In addition, our photometric monitoring will provide invaluable
observations of a large sample of nearby brown dwarfs situated close to the M/L
transition. This is why, we also advocate an immediate public release of the
survey data, to guarantee rapid progress on the planet search and provide a
treasure trove of data for brown dwarf science.
