Did We Found Superhabitable Planet ?

Did We Found Superhabitable Planet ?
A size comparison and artist's impression
of Kepler-442b (1.34 R⊕) to the Earth (right).
A Superhabitable planet is a hypothetical type of exoplanet or exomoon that may be better suited than Earth for the emergence and evolution of life. The concept was introduced in 2014 by René Heller and John Armstrong, who have 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. Heller and Armstrong state that it is not clear why Earth should offer the most suitable physicochemical parameters to living organisms, 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 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.

Despite the scarcity of information available, the hypotheses presented a superhabitable planets can be summarized as a preliminary profile, even if there is no scientific consensus.
  • Mass: approximately 2M⊕.
  • Radius: to maintain a similar Earth density, its radius should be between 1.2 and 1.3R⊕.
  • Oceans: percentage of surface area covered by oceans should be Earth-like but more distributed, without large continuous land masses. The oceans should be shallow; the light then will penetrate easier through the water and will reach the fauna and flora, stimulating an abundance of life down in the ocean.
  • Distance: shorter distance from the centre of the habitable zone of the system than Earth.
  • Temperature: average surface temperature of about 25 °C (77 °F).
  • Star and age: belonging to an intermediate K-type star with an older age than the Sun (4.5 billion years) but younger than 7 billion years.
  • Atmosphere: somewhat denser than Earth's and with a higher concentration of oxygen. That will make life larger and more abundant.

There is no confirmed exoplanet that meets all these requirements. After updating the database of exoplanets on 23 July 2015, the one that comes closest is Kepler-442b, belonging to an orange dwarf star, with a radius of 1.34R⊕ and a mass of 2.34M⊕, but with an estimated surface temperature of −2.65 °C (27.23 °F).

"The Earth just scrapes the inner edge of the Solar System's habitable zone, the area in which temperatures allow Earth-like planets to have liquid surface water. So from this perspective, Earth is only marginally habitable. That led us to ask: could there be more hospitable environments for life on terrestrial planets?"

- René Heller

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