Q:  What's so special about the region between 1 and 10 microns?


         There are two reasons why this is a particularly fruitful 
    region of the spectrum for exoplanet observations.

         1) the Earth's atmosphere has some good observing windows
                in this region, so we can see celestial sources clearly
                with ground-based telescopes
  

         2) the peak wavelength of a blackbody spectrum of temperature T
                can be computed as

                                    2.9 x 10^(-3) m*K       2,900 micron*K
               peak wavelength  =  -------------------  =  ----------------
                                           T                     T


               So for typical exoplanet temperatures of T = 100 - 2000 K,
               the peak of thermal radiation will lie in the range
      
                         peak  ~   1 - 30   microns

               The long wavelength end of this range is very hard to 
               observe from the ground .