According to Heller and Armstrong, knowing whether or not a planet is in its host star’s habitable zone (HZ) is insufficient to determine its habitability: it is not clear why Earth should offer the most suitable physicochemical parameters to living organisms, as “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 planetary habitability conditions for maximum biodiversity; in other words, they define a superhabitable world as a terrestrial planet or moon that could support more diverse flora and fauna than there are 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 a habitable zone (HZ) 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 these authors, such superhabitable worlds would likely be larger, warmer, and older than Earth, and orbiting K-type main-sequence stars.
Heller and Armstrong proposed that a series of basic characteristics are required to classify an exoplanet or exomoon as superhabitable; for size, it is required to be about 2 Earth masses, and 1.3 Earth radii 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. It is therefore 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 may also influence the surface relief, making it more regular and decreasing the size of the ocean basins, which would improve diversity of marine life in shallow waters.
Other factors to consider are the type of star in the system. K-type stars are less massive than the Sun, and are stable on the main sequence for a very long time (18 to 34 billion years, compared to 10 billion for the Sun, a G-class star), giving more time for the emergence of life and evolution.
In addition, K-type stars emit less ultraviolet radiation (which can damage DNA and thus hamper the emergence of nucleic acid based life) than G-type stars like the Sun.
Heller and Armstrong speculate that the number of superhabitable planets around Kepler 442-like stars can far exceed that of Earth analogs: less massive stars in the main sequence are more abundant than the larger and brighter stars, so there are more orange (K) dwarfs than solar analogues. It is estimated that about 9% of stars in the Milky Way are K-type stars.
In September 2020, astronomers identified 24 superhabitable planet contenders, from among more than 4000 confirmed exoplanets at present, based on astrophysical parameters, as well as the natural history of known life forms on the Earth.
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