A superhabitable planet is a theoretical type of extrasolar planet or extrasolar moon which has better conditions for life than the planet Earth. The concept was introduced in 2014 by René Heller and John Armstrong. They have both criticized the language used in the search for habitable planets so they propose clarifications because a circumstellar habitable zone (HZ) is not enough to define a planet's habitability. They argue that it is not clear why Earth offers the most suitable conditions for life, because "planets could be non-Earth-like, yet offer more suitable conditions for the emergence and evolution of life than Earth did or does". While still assuming that life requires water, they hypothesize that Earth may not represent the optimal conditions for biodiversity; in other words, a superhabitable world would be defined as a terrestrial planet or moon that could support more diverse flora and fauna than on Earth, as it would empirically show that its environment is more hospitable to life.
Heller and Armstrong also point out that not all rocky planets in the habitable zone may be habitable, and that tidal heating can render terrestrial or icy worlds habitable beyond the stellar HZ, such as in Europa's internal ocean. The authors propose that in order to identify a habitable (or superhabitable) planet, a characterization concept is required that is biocentric, rather than geo or anthropocentric. Heller and Armstrong proposed to establish a profile for exoplanets according to stellar type, mass and location in their planetary system, among other features. According to Heller and Armstrong, these superhabitable worlds would likely be larger, warmer, and older than Earth, and be orbiting around K-type main-sequence stars.
Characteristics[]
Heller and Armstrong have proposed a series of basic characteristics required to classify an exoplanet or exomoon as "superhabitable". For the size, it is required to be about 2 Earth masses, and 1.3 Earth radii. This will provide an optimal size for plate tectonics. In addition, it would have a greater gravitational attraction that would increase retention of gases during the planet's formation. Therefore it would be likely that they have a denser atmosphere that will offer greater concentration of oxygen and greenhouse gases, which in turn raise the average temperature to optimum levels for plant life, to about 25 °C (77 °F). A denser atmosphere might also influence the surface relief, making it more regular and decreasing the size of the ocean basins, which will improve the diversity of marine life in shallow water.
Other factors include the type of star the planet orbits. K-type stars are optimal since they are less massive than the Sun, and are stable on the main sequence for a very long time (20 to 70 billion years, compared to 10 billion for the Sun), which gives more time for the emergence of life and evolution. A superhabitable world would also require to be located near the center of the habitable zone of its star system for a long time.