Life may unfold in various forms throughout the universe, constrained by
universal physical laws and consistent principles of organic evolution,
but propelled to great variety in detail by local conditions and the
specifics of planetary history. What is known of the chemical and
physical conditions of any planetary environment and its history enables
us to make educated and plausible speculations about the nature and
history of life on that world.
Within our Solar System, there is an enormous diversity of planetary
environments. On Earth, life evolved on a geologically complex,
water-rich world, which today has an oxidizing atmosphere, although this
was not always the case. On Mars, the surface is bitterly cold and dry,
and the atmosphere very thin. Whether or not life ever existed on the
Red Planet is a matter for speculation, but we do know that early in its
history, Mars was a warmer, wetter world. Today Venus is a planet with
an incredibly hot surface and a dense choking atmosphere, and it seems
unlikely, although not impossible, that life could ever evolve here. On
the gas giant planets, such as Jupiter and Saturn, it is possible that
life might exist in the dense atmospheres of these cloud covered worlds,
and might even have evolved on some of their exotic moons such as the
sulphur-rich, volcanic world Io, Icy Europa with its possible
sub-surface ocean, or Titan with its lakes of liquid petroleum gas on
the surface.
Discussions of the great variety of life forms that could evolve in
these diverse environments have become particularly relevant in recent
years with the discovery of around 300 exoplanets in orbit around other
stars and the possibilities for the existence of life in these planetary
systems.
