Superhabitability describes a perfect storm of life-friendly factors. In their recent paper, astrobiologists René Heller and John Armstrong describe no fewer than 18 of them.

First and foremost, superhabitability arises on terrestrial planets with masses two to three times that of Earth. Planets that size have a number of things working for them, including:

• Long tectonic activity periods to induce carbon-silicate cycles that are active on long timescales
• Enhanced magnetic shielding to protect against cosmic and stellar high-energy radiation
• Large surface areas, allowing for enhanced biodiversity — but not so large that life can’t properly emerge, evolve, and diversify on account of excess gravity
• Smooth surfaces allowing for more shallow seas, allowing for increased habitability (life explodes in warm, shallow waters)
• Optimal land-to-ocean-fraction and distribution; supercontinents are relatively bad (like Earth’s Gondwana, which featured vast interior deserts); diversified continents also provide atmospheric moderating effects
• Atmospheres thicker than that of Earth, allowing for slightly warmer temperatures — but not too high

In addition, superhabitable planets or moons should be biologically diverse, because higher biodiversity makes planets more habitable in the long term.

Cosmologically speaking, superhabitable planets should orbit stars that are slightly smaller than our Sun, specifically K dwarf suns. Ideally, these stars should be old and have the capacity to burn for a long, long time.

It would also help if the planet was in a solar system with other habitable planets, thus increasing the chances of panspermia.

Superhabitable worlds should also experience early planetary bombardment from celestial objects, like asteroids and comets. This provides essential organic molecules and volatiles. Typically, these active periods taper off over time, resulting in eras of prolonged stability and a dramatically decreased chance of mass extinction events.

Fascinatingly, the scientists argue that stable, nearly-circular orbits are not a requirement.

Not surprisingly, these planets should ideally reside within a solar system’s habitable zone, and feature a spin that’s conducive to life. On this last point, the authors go against conventional wisdom:

It is uncertain whether any given spin rate is desirable for life, as long as it helps keep the surface uniformly habitable, while radical changes in such a spin rate might be detrimental. Did the existence of the Moon encourage life to evolve by changing the diurnal and tidal cycles, or was this an impediment to evolution? Could moderate changes of a world’s obliquity or rotation rate even force life to adapt to a broader range of environmental conditions, thereby triggering more diverse evolution? Ultimately, is it possible that a terrestrial planet without a massive moon, or a planet more subject to changes in spin, could be superhabitable?

Given all these factors, the scientists say that the Earth is only “marginally” habitable.

More: http://online.liebertpub.com/doi/full/10.1089/ast.2013.1088