  Future NASA concept missions that are currently under study, like Habitable
Exoplanet Imaging Mission (HabEx) & Large Ultra-Violet Optical Infra Red
(LUVOIR) Surveyor, would discover a large diversity of exoplanets. We propose
here a classification scheme that distinguishes exoplanets into different
categories based on their size and incident stellar flux, for the purpose of
providing the expected number of exoplanets observed (yield) with direct
imaging missions. The boundaries of this classification can be computed using
the known chemical behavior of gases and condensates at different pressures and
temperatures in a planetary atmosphere. In this study, we initially focus on
condensation curves for sphalerite ZnS, H2O, CO2 and CH4. The order in which
these species condense in a planetary atmosphere define the boundaries between
different classes of planets. Broadly, the planets are divided into rocky (0.5
- 1.0RE), super-Earths (1.0- 1.75RE), sub-Neptunes (1.75-3.5RE), sub-Jovians
(3.5 - 6.0RE) and Jovians (6-14.3RE) based on their planet sizes, and 'hot',
'warm' and 'cold' based on the incident stellar flux. We then calculate planet
occurrence rates within these boundaries for different kinds of exoplanets,
\eta_{planet}, using the community co-ordinated results of NASA's Exoplanet
Program Analysis Group's Science Analysis Group-13 (SAG-13). These occurrence
rate estimates are in turn used to estimate the expected exoplanet yields for
direct imaging missions of different telescope diameter.
