Superhabitable world
A superhabitable world is a hypothetical type of planet or moon that is better suited than Earth for the emergence and evolution of life. The concept was introduced in a 2014 paper by René Heller and John Armstrong, in which they criticized the language used in the search for habitable exoplanets and proposed clarifications. The authors argued that knowing whether a world is located within the star's habitable zone is insufficient to determine its habitability, and that the prevailing model of characterization was geocentric or anthropocentric in nature. Instead, they proposed a biocentric model that prioritized characteristics affecting the abundance of life and biodiversity on a world's surface.
If a world possesses more diverse flora and fauna than there are on Earth, then it would empirically show that its natural environment is more hospitable to life. To identify such a world, one should consider its geological processes, formation age, atmospheric composition, ocean coverage, and the type of star that it orbits. In other words, a superhabitable world would likely be larger, warmer, and older than Earth, with an evenly-distributed ocean, and orbiting a K-type main-sequence star. In 2020, astronomers, building on Heller and Armstrong's hypothesis, identified 24 potentially superhabitable exoplanets based on measured characteristics that fit these criteria.
Stellar characteristics
A star's characteristics is a key consideration for planetary habitability. The types of stars generally considered to be potential hosts for habitable worlds include F, G, K, and M-type main-sequence stars. The most massive starsO, B, and A-type, respectivelyhave average lifespans on the main sequence that are considered too short for complex life to develop, ranging from a few hundred million years for A-type stars to only a few million years for O-type stars. Thus, F-type stars are described as the "hot limit" for stars that can potentially support life, as their lifespan of 2 to 4 billion years would be sufficient for habitability. However, F-type stars emit large amounts of ultraviolet radiation, and without the presence of a protective ozone layer, could disrupt nucleic acid-based life on a planet's surface.On the opposite end, the less massive red dwarfs, which generally includes M-type stars, are by far the most common and long-lived stars in the universe, but ongoing research points to serious challenges to their ability to support life. Due to the low luminosity of red dwarfs, the circumstellar habitable zone is in very close proximity to the star, which causes any planet to become tidally locked. The primary concern for researchers, however, is the star's propensity for frequent outbreaks of high-energy radiation, especially early in its life, that could strip away a planet's atmosphere. At the same time, red dwarfs do not emit enough quiescent UV radiation to support biological processes like photosynthesis.
Dismissing both ends, astronomers are led to conclude that G and K-type stars—yellow and orange dwarfs, respectively—provide the best life-supporting characteristics. However, a limiting factor to the habitability of yellow dwarfs is their higher emissions of ionizing radiation and shorter lifespans compared to cooler orange dwarfs. Therefore, researchers conclude that orange dwarfs offer the best conditions for a superhabitable world.
Also nicknamed "Goldilocks stars", orange dwarfs emit low enough levels of ultraviolet radiation to eliminate the need for a protective ozone layer, but just enough to contribute to necessary biological processes. Additionally, the long average lifespan of an orange dwarf provides a more stable habitable zone throughout the star's lifetime, providing more time for life to develop.
Planetary characteristics
Age
It is necessary for the age of any superhabitable world to be greater than Earth's age. This necessity is based on the belief that as a planet or moon ages, it experiences increasing levels of biodiversity, since native species have had more time to evolve, adapt, and stabilize the environmental conditions suitable for life. However, the eventual exhaustion of a world's internally generated heat means that there is also an upper limit to the age of any habitable world; internal cooling would lead to changes to the average global temperature and atmospheric composition. Therefore, the optimal age range for a superhabitable world would be roughly 58 billion years.Orbital characteristics
During the main sequence phase, a star burns hydrogen in its core, producing energy through nuclear fusion. Over time, as the hydrogen fuel is consumed, the star's core contracts and heats up, leading to an increase in the rate of fusion. This causes the star to gradually become more luminous, and as its luminosity increases, the amount of energy it emits grows, pushing the habitable zone outward. Studies suggest that Earth's orbit lies near the inner edge of the Solar System's HZ, which could harm its long-term livability as it nears the end of its HZ lifetime.Ideally, the orbit of a superhabitable world should be further out and closer to the center of the HZ relative to Earth's orbit, but knowing whether a world is in this region is insufficient on its own to determine habitability. Not all rocky planets in the HZ may be habitable, while tidal heating can render planets or moons habitable beyond this region. For example, Jupiter's moon Europa is well beyond the outer limits of the Solar System's HZ, yet as a result of its orbital interactions with the other Galilean moons, it is believed to have a subsurface ocean of liquid water beneath its icy surface.
According to a 2023 paper by Jonathan Jernigan and colleagues, marine biological activity increases on planets with increasing obliquity and eccentricity. The authors suggest that planets with a high obliquity or eccentricity may be superhabitable, and that scientists should be keen to look for biosignatures on exoplanets with these orbital characteristics.
Mass and size
Assuming that a greater surface area would provide greater biodiversity, the size of a superhabitable world should generally be greater than 1 R?, with the condition that its mass is not arbitrarily large. Studies of the mass-radius relationship indicate that there is a transition point between rocky planets and gaseous planets that occurs around 2 M? or 1.7 R?. Another study argues that there is a natural radius limit, set at 1.6 R?, below which nearly all planets are terrestrial, composed primarily of rock-iron-water mixtures.Heller and Armstrong argue that the optimal mass and radius of a superhabitable world can be determined by geological activity; the more massive a planetary body, the longer time it will continuously generate internal heata major contributing factor to plate tectonics. Too much mass, however, can slow plate tectonics by increasing the pressure of the mantle. It is believed that plate tectonics peak in bodies between 1 and 5 M?, and from this perspective, a planet can be considered superhabitable up to around 2 M?. Assuming this planet has a density similar to Earth's, its radius should be between 1.2 and 1.3 R?.
Geology
An important geological process is plate tectonics, which appears to be common in terrestrial planets with a significant rotation speed and an internal heat source. If large bodies of water are present on a planet, plate tectonics can maintain high levels of carbon dioxide in its atmosphere and increase the global surface temperature through the greenhouse effect. However, if tectonic activity is not significant enough to increase temperatures above the freezing point of water, the planet could experience a permanent ice age, unless the process is offset by another energy source like tidal heating or stellar irradiation. On the other hand, if the effects of any of these processes are too strong, the amount of greenhouse gases in the atmosphere could cause a runaway greenhouse effect by trapping heat and preventing adequate cooling.The presence of a magnetic field is important for the long-term survivability of life on the surface of a planet or moon. A sufficiently strong magnetic field effectively shields a world's surface and atmosphere against ionizing radiation emanating from the interstellar medium and its host star. A planet can generate an intrinsic magnetic field through a dynamo that involves an internal heat source, an electrically conductive fluid like molten iron, and a significant rotation speed, while a moon could be extrinsically protected by its host planet's magnetic field. Less massive bodies and those that are tidally locked are likely to have a weak to non-existent magnetic field, which over time can result in the loss of a significant portion of its atmosphere by hydrodynamic escape and become a desert planet. If a planet's rotation is too slow, such as with Venus, then it cannot generate an Earth-like magnetic field. A more massive planet could overcome this problem by hosting multiple moons, which, through their combined gravitational effects, can boost the planet's magnetic field.
Surface features
The appearance of a superhabitable world should be similar to the conditions found in the tropical climates of Earth. Due to the denser atmosphere and less temperature variation across its surface, such a world would lack any major ice sheets and have a higher concentration of clouds, while plant life would potentially cover more of the planet's surface and be visible from space.When considering the differences in the peak wavelength of visible light for K-type stars and the lower stellar flux of the planet, surface vegetation may exhibit colors different than the typical green color found on Earth. Instead, vegetation on these worlds could have a red, orange, or even purple appearance.
An ocean that covers a large portion of a world's surface with fractionate continents and archipelagos could provide a stable environment across its surface. In addition, the greater surface gravity of a superhabitable world could reduce the average ocean depth and create shallow ocean basins, providing the optimal environment for marine life to thrive. For example, marine ecosystems found in the shallow areas of Earth's oceans and seas, given the amount of light and heat they receive, are observed to have greater biodiversity and are generally seen as being more comfortable for aquatic species. This has led researchers to speculate that shallow water environments on exoplanets should be similarly suitable for life.