  Study of exoplanets is one of the main goals of present research in planetary
sciences and astrobiology. Analysis of huge planetary data from space missions
such as CoRoT and Kepler is directed ultimately at finding a planet similar to
Earth -- the Earth's twin, and answering the question of potential
exo-habitability. The Earth Similarity Index (ESI) is a first step in this
quest, ranging from 1 (Earth) to 0 (totally dissimilar to Earth). It was
defined for the four physical parameters of a planet: radius, density, escape
velocity and surface temperature. The ESI is further sub-divided into interior
ESI (geometrical mean of radius and density) and surface ESI (geometrical mean
of escape velocity and surface temperature). The challenge here is to determine
which exoplanet parameter(s) is important in finding this similarity; how
exactly the individual parameters entering the interior ESI and surface ESI are
contributing to the global ESI. Since the surface temperature entering surface
ESI is a non-observable quantity, it is difficult to determine its value. Using
the known data for the Solar System objects, we established the calibration
relation between surface and equilibrium temperatures to devise an effective
way to estimate the value of the surface temperature of exoplanets for further
analysis with our graphic methodology.
  A new approach, called Mars Similarity Index (MSI), is introduced to identify
planets that may be habitable to the extreme forms of life. MSI is defined in
the range between 1 (present Mars) and 0 (dissimilar to present Mars) and uses
the same physical parameters as ESI. We are interested in Mars-like planets to
search for planets that may host the extreme life forms, such as the ones
living in extreme environments on Earth; for example, methane on Mars may be a
product of the methane-specific extremophile life form metabolism.
