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.[n. 1] 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 (20 to 70 billion years, compared to 10 billion for the Sun, a G-class star), giving 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.
Surface, size and composition
An exoplanet with a larger volume than that of Earth, or with a more complex terrain, or with a larger surface covered with liquid water, could be more hospitable for life than Earth. Since the volume of a planet tends to be directly related to its mass, the more massive it is, the greater its gravitational pull, which can result in a denser atmosphere.
Some studies indicate that there is a natural limit, set at 1.6R⊕, below which nearly all planets are terrestrial, composed primarily of rock-iron-water mixtures. Generally, objects with a mass below 6 M⊕ are very likely to be of similar composition as Earth. Above this limit, the density of the planets decreases with increasing size, the planet will become a "water world" and finally a gas giant. In addition, most super-Earths' high mass may cause them to lack plate tectonics. Thus, it is expected that any exoplanet similar to Earth's density and with a radius under 1.6 R⊕ may be suitable for life. However, other studies indicate that water worlds represent a transitional stage between mini-Neptunes and the terrestrial planets, especially if they belong to red dwarfs or K dwarfs. Although water planets may be habitable, the average depth of the water and the absence of land area would not make them superhabitable as defined by Heller and Armstrong. From a geological perspective, the optimal mass of a planet is about 2 M⊕, so it must have a radius that keeps the density of the Earth among 1.2 and 1.3R⊕.
The average depth of the oceans also affects the habitability of a planet. The shallow areas of the sea, given the amount of light and heat they receive, usually are more comfortable for aquatic species, so it is likely that exoplanets with a lower average depth are more suitable for life. More massive exoplanets would tend to have a regular surface gravity, which can mean shallower—and more hospitable—ocean basins.
Plate tectonics, in combination with the presence of large bodies of water on a planet, is able to maintain high levels of carbon dioxide (CO
2) in its atmosphere. This process appears to be common in geologically active terrestrial planets with a significant rotation speed. The more massive a planetary body, the longer time it will generate internal heat, which is a major contributing factor to plate tectonics. However, excessive mass can also slow plate tectonics because of increased pressure and viscosity of the mantle, which hinders the sliding of the lithosphere. Research suggests that plate tectonics peaks in activity in bodies with a mass between 1 and 5M⊕, with an optimum mass of approximately 2M⊕.
If the geological activity is not strong enough to generate a sufficient amount of greenhouse gases to increase global temperatures above the freezing point of water, the planet could experience a permanent ice age, unless the process is offset by an intense internal heat source such as tidal heating or stellar irradiation.
Another feature favorable to life is a planet's potential to develop a strong magnetosphere to protect its surface and atmosphere from cosmic radiation and stellar winds, especially around red dwarf stars. Less massive bodies and those with a slow rotation, or those that are tidally locked, have a weak or no magnetic field, which over time can result in the loss of a significant portion of its atmosphere, especially hydrogen, by hydrodynamic escape.
Temperature and climate
The optimum temperature for Earth-like life in general is unknown, although it appears that on Earth organism diversity has been greater in warmer periods. It is therefore possible that exoplanets with slightly higher average temperatures than that of Earth are more suitable for life. The thermoregulatory effect of large oceans on exoplanets located in a habitable zone may maintain a moderate temperature range. In this case, deserts would be more limited in area and would likely support habitat-rich coastal environments.
However, studies suggest that Earth already lies near to the inner edge of the habitable zone of the Solar System, and that may harm its long-term livability as the luminosities of main-sequence stars steadily increase over time, pushing the habitable zone outwards. Therefore, superhabitable exoplanets must be warmer than Earth, yet orbit further out than Earth does and closer to the center of the system's habitable zone. This would be possible with a thicker atmosphere or with a higher concentration of greenhouse gases.
The star's type largely determines the conditions present in a system. The most massive stars O, B, and A have a very short life cycle, quickly leaving the main sequence. In addition, O-type stars produce a photoevaporation effect that prevents the accretion of planets around the star.
On the opposite side, the less massive M-and K-types are by far the most common and long-lived stars of the universe, but their potential for supporting life is still under study. Their low luminosity reduces the size of the habitable zone, which are exposed to ultraviolet radiation outbreaks that occur frequently, especially during their first billion year of existence. When a planet's orbit is too short, it can cause tidal locking of the planet, where it always presents the same hemisphere to the star, known as day hemisphere. Even if the existence of life were possible in a system of this type, it is unlikely that any exoplanet belonging to a red dwarf star would be considered superhabitable.
Dismissing both ends, systems with a K-type stars offer the best habitable zones for life. K-type stars allow the formation of planets around them, have a long life expectancy, and provide a stable habitable zone free of the effects of excessive proximity to its star. Furthermore, the radiation produced by a K-type star is high enough to allow complex life without the need for an atmospheric ozone layer. They are also the most stable and their habitable zone does not move very much during its lifetime, so a terrestrial analog located near a K-type star may be habitable for almost all of the main sequence.
Orbit and rotation
Experts have not reached a consensus about what is the optimal rotation speed for an exoplanet, but it should not be too fast nor too slow. The latter case can cause some problems similar to those observed in Venus, which completes one rotation every 243 Earth days and as a result, cannot generate an Earth-like magnetic field.
There are no solid arguments to explain if Earth's atmosphere has the optimal composition to host life. On Earth, during the period when coal was first formed, atmospheric oxygen (O
2) levels were up to 35%, and coincided with the periods of greatest biodiversity. So, assuming that the presence of a significant amount of oxygen in the atmosphere is essential for exoplanets to develop complex life forms, the percentage of oxygen relative to the total atmosphere appears to limit the maximum size of the planet for optimum superhabitability and ample biodiversity[clarification needed].
In a biological context, older planets than Earth may have greater biodiversity, since native species have had more time to evolve, adapt and stabilize the environmental conditions to sustain a suitable environment for life that can benefit their descendants.
However, for many years it was thought that since older star systems have lower metallicity, they should display low planet formation, and thus such old planets may have been scant in the beginning, but the number of metallic items in the universe must have grown steadily since its inception. The first exoplanetary discoveries, mostly gas giants orbiting very close to their stars, known as Hot Jupiters, suggest that planets were rare in systems with low metallicity, which invited suspicion of a time limit on the appearance of the first objects landmass. Later, in 2012, the Kepler telescope's observations allowed experts to find out that this relationship is much more restrictive in systems with Hot Jupiters, and that terrestrial planets could form in stars of much lower metallicity, to some extent. It is now thought that the first Earth-mass objects should appear sometime between 7 and 12 billion years. Given the greater stability of the orange dwarfs (K-type) compared to the Sun (G-type) and longer life expectancy, it is possible that superhabitable exoplanets belonging to K-type stars, orbiting within its habitable zone, could provide a longer, steadier, and better environment for life than Earth.
Despite the scarcity of information available, the hypotheses presented above on 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 center 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 appearance of a superhabitable planet should be, in general, very similar to Earth. The main differences, in compliance with the profile seen previously, would be derived from its mass. Its denser atmosphere probably prevent the formation of ice sheets as a result of lower thermal difference between different regions of the planet. Also, it has a higher concentration of clouds, and abundant rainfall.
Probably the vegetation is very different due to the increased air density, precipitation, temperature, and stellar flux. For the type of light emitted from the K-type stars, plants may take other colors than green. The vegetation would cover more regions than vegetation here on Earth, making this visible from space.
In general, the climate of a superhabitable planet would be warmer, moist, homogeneous and have stable land, allowing life to extend across the surface without presenting large population differences, in contrast to Earth that has inhospitable areas such as glaciers, deserts and tropical regions. If the atmosphere contains enough molecular oxygen, the conditions of these planets may be bearable to humans even without the protection of a space suit, provided that the atmosphere does not contain excessive toxic gases, but would require some adaptation to the increased gravity, such as an increase in muscles and in bone density, etc.
Heller and Armstrong speculate that the number of superhabitable planets 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 dwarfs than solar analogues. It is estimated that about 9% of stars in the Milky Way are K-type stars.
Another point favoring the predominance of superhabitable planets in regard to Earth analogs is that, unlike the latter, most of the requirements of a superhabitable world can occur spontaneously and jointly simply by having a higher mass. A planetary body close to 2 or 3M⊕ should have longer-lasting plate tectonics and also will have a larger surface area in comparison to Earth. Similarly, it is likely that its oceans are shallower by the effect of gravity on the planet's crust, its gravitational field more intense and, a denser atmosphere.
By contrast, Earth-mass planets may have a wider range of conditions. For example, some may sustain active tectonics for a shorter time period and will therefore end up with lower air density than Earth, increasing the probability of developing global ice coverage, or even a permanent Snowball Earth scenario. Another negative effect of lower atmospheric density can be manifested in the form of thermal oscillations, which can lead to high variability in the global climate and increase the chance for catastrophic events. In addition, by having a weaker magnetosphere, such planets may lose their atmospheric hydrogen by hydrodynamic escape easier and become a desert planet. Any of these examples could prevent the emergence of life on a planet's surface. In any case, the multitude of scenarios that can turn an Earth-mass planet located in the habitable zone of a solar analogue into an inhospitable place are less likely on a planet that meets the basic features of a superhabitable world, so that the latter should be more common.
- The habitable zone (HZ) is a region present around each star where a terrestrial planet or moon that has an atmospheric pressure and a suitable combination of gases, could maintain liquid water on its surface. However, planets in the HZ may not be habitable, as tidal heating during the planet's orbit can be an additional heat source that causes a planet to enter a runaway greenhouse state.
- The initials "HZD" or "Habitable Zone Distance" mark the position of a planet about the center of the habitable zone of the system (value 0). A negative HZD value means that the orbit of a planet is smaller near its star —the center of the habitable zone— while a positive value means a wider orbit around its star. The values 1 and −1 mark the boundary of the habitable zone. A superhabitable planet should have a HZD of 0 (the optimal location within the habitable zone).
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