Asuperhabitable world is a hypothetical type of planet or moon that is better suited than Earth for theemergence and evolution oflife. The concept was introduced in a 2014 paper by René Heller and John Armstrong, in which they criticized the language used in the search forhabitable exoplanets and proposed clarifications.^[2] The authors argued that knowing whether a world is located within the star'shabitable zone is insufficient to determine its habitability, and that the prevailing model of characterization was geocentric oranthropocentric in nature. Instead, they proposed abiocentric model that prioritized characteristics affecting the abundance of life andbiodiversity on a world's surface.^[2]

If a world possesses more diverse flora and fauna than there are on Earth, then it would empirically show that itsnatural environment is more hospitable to life.^[3] 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, andolder than Earth, with an evenly-distributed ocean, and orbiting aK-type main-sequence star.^[4] In 2020, astronomers, building on Heller and Armstrong's hypothesis, identified 24 potentially superhabitableexoplanets based on measured characteristics that fit these criteria.^[5]

A star's characteristics is a key consideration forplanetary habitability.^[6] The types of stars generally considered to be potential hosts for habitable worlds include F, G, K, and M-type main-sequence stars.^[7] The most massive stars—O,B, andA-type, respectively—have average lifespans on themain sequence that are considered too short for complex life to develop,^[8] ranging from a few hundred million years for A-type stars to only a few million years for O-type stars.^[9] 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.^[10] However, F-type stars emit large amounts ofultraviolet radiation, and without the presence of a protective ozone layer, could disruptnucleic acid-based life on a planet's surface.^[10]

On the opposite end, the less massivered dwarfs, which generally includes M-type stars, are by far the most common and long-lived stars in the universe,^[11] but ongoing research points to serious challenges totheir ability to support life. Due to the lowluminosity of red dwarfs, thecircumstellar habitable zone (HZ)^[a] is in very close proximity to the star, which causes any planet to becometidally locked.^[14] The primary concern for researchers, however, is the star's propensity for frequent outbreaks ofhigh-energy radiation, especially early in its life, that couldstrip away a planet's atmosphere.^[15] At the same time, red dwarfs do not emit enough quiescent UV radiation (i.e., UV radiation emitted duringinactive periods) to support biological processes like photosynthesis.^[3]

Dismissing both ends, astronomers are led to conclude that G andK-type stars—yellow and orange dwarfs, respectively—provide the best life-supporting characteristics. However, a limiting factor to thehabitability of yellow dwarfs is their higher emissions ofionizing radiation and shorter lifespans compared to cooler orange dwarfs.^[16] Therefore, researchers conclude that orange dwarfs offer the best conditions for a superhabitable world.^[3][16]

Also nicknamed "Goldilocks stars", orange dwarfs emit low enough levels of ultraviolet radiation to eliminate the need for a protectiveozone layer, but just enough to contribute to necessary biological processes.^[17][3] Additionally, the long average lifespan of an orange dwarf (18 to 34 billion years, compared to 10 billion for the Sun) provides a more stable habitable zone throughout the star's lifetime, providing more time for life to develop.^[18][19][17]

It is necessary for the age of any superhabitable world to be greater thanEarth's age (~4.5 billion years).^[19] 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.^[19] 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.^[20] Therefore, the optimal age range for a superhabitable world would be roughly 5–8 billion years.^[20]

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 (HZ) outward.^[23][24] Studies suggest that Earth's orbit lies near the inner edge of theSolar System's HZ,^[14] 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,^[25][26] but knowing whether a world is in this region is insufficient on its own to determine habitability.^[3] Not allrocky planets in the HZ may be habitable, whiletidal heating can render planets or moons habitable beyond this region. For example, Jupiter's moonEuropa is well beyond the outer limits of the Solar System's HZ, yet as a result of its orbital interactions with the otherGalilean moons, it is believed to have asubsurface ocean ofliquid water beneath its icy surface.^[27]

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 (or both) may be superhabitable, and that scientists should be keen to look for biosignatures on exoplanets with these orbital characteristics.^[28]

Assuming that a greater surface area would provide greater biodiversity, the size of a superhabitable world should generally be greater than 1R_🜨, with the condition that its mass is not arbitrarily large.^[29] Studies of the mass-radius relationship indicate that there is a transition point between rocky planets and gaseous planets (i.e.,mini-Neptunes) that occurs around 2M_🜨 or 1.7R_🜨.^[30][31] Another study argues that there is a natural radius limit, set at 1.6R_🜨, below which nearly all planets areterrestrial, composed primarily of rock-iron-water mixtures.^[32]

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 generateinternal heat—a major contributing factor to plate tectonics.^[29] Too much mass, however, can slow plate tectonics by increasing the pressure of the mantle.^[29] It is believed that plate tectonics peak in bodies between 1 and 5M_🜨, and from this perspective, a planet can be considered superhabitable up to around 2M_🜨.^[33] Assuming this planet has a density similar to Earth's, its radius should be between 1.2 and 1.3R_🜨.^[33][29]

An important geological process isplate tectonics, which appears to be common in terrestrial planets with a significant rotation speed and aninternal heat source.^[34] If large bodies of water are present on a planet, plate tectonics can maintain high levels ofcarbon dioxide (CO
₂) in its atmosphere and increase theglobal surface temperature through thegreenhouse effect.^[35] However, if tectonic activity is not significant enough to increase temperatures above thefreezing point of water, the planet could experience apermanent ice age, unless the process is offset by another energy source liketidal heating orstellar irradiation.^[36] 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 arunaway greenhouse effect by trapping heat and preventing adequate cooling.

The presence of amagnetic field is important for the long-term survivability of life on the surface of a planet or moon.^[22] A sufficiently strong magnetic field effectively shields a world's surface and atmosphere againstionizing radiation emanating from theinterstellar medium and its host star.^[22][37] A planet can generate an intrinsic magnetic field through adynamo that involves an internal heat source, an electrically conductive fluid likemolten iron, and a significantrotation speed, while a moon could be extrinsically protected by its host planet's magnetic field.^[22] Less massive bodies and those that aretidally 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 byhydrodynamic escape and become adesert planet.^[29] If a planet's rotation is too slow, such as with Venus, then it cannot generate an Earth-likemagnetic field. A more massive planet could overcome this problem by hosting multiplemoons, which, through their combined gravitational effects, can boost the planet's magnetic field.^[38]

The appearance of a superhabitable world should be similar to the conditions found in thetropical climates of Earth.^[39] Due to the denser atmosphere and less temperature variation across its surface, such a world would lack any majorice sheets and have a higher concentration of clouds, whileplant life would potentially cover more of the planet's surface and be visible from space.^[39]

When considering the differences in the peak wavelength of visible light forK-type stars and the lower stellar flux of the planet, surface vegetation may exhibit colors different than the typical green color found on Earth.^[40][41] Instead, vegetation on these worlds could have a red, orange, or even purple appearance.^[42]

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.^[43] In addition, the greater surface gravity of a superhabitable world could reduce the average ocean depth and create shallowocean basins, providing the optimal environment formarine life to thrive.^[44][45][46] 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.^[43][47]

In general, theclimate of a superhabitable planet would be warm, moist, and homogeneous, allowing life to extend across the surface without presenting large population differences.^[48][49] These characteristics are in contrast to those found on Earth, which has more variable and inhospitable regions that include frigidtundra and drydeserts.^[50] Deserts on superhabitable planets would be more limited in area and would likely support habitat-rich coastal environments.^[51]

The optimum surface temperature for Earth-like life is unknown, although it appears that on Earth, organism diversity has been greater in warmer periods.^[52] It is therefore possible that exoplanets with slightly higher average temperatures than that of Earth are more suitable for life.^[53] The denser atmosphere of a superhabitable planet would naturally provide a greater average temperature and less variability of the global climate.^[54][46] Ideally, the temperature should reach the optimal levels for plant life, which is 25 °C (77 °F). In addition, a large distributed ocean would have the ability to regulate a planet's surface temperature similar to Earth'socean currents, and could allow it to maintain a moderate temperature within the habitable zone.^[55][51]

There are no solid arguments to explain if Earth's atmosphere has the optimal composition,^[56] but relative atmospheric oxygen levels is required to meet the high-energy demands of complex life (O
₂).^[57] Therefore, it is hypothesized that oxygen abundance in the atmosphere is essential for complex life on other worlds.^[56][57]

In September 2020,Dirk Schulze-Makuch and colleagues identified 24 contenders for superhabitable planets out of more than 4000 confirmed exoplanets and exoplanet candidates.^[5] The criteria included measurable factors like type of star, and the planet's age, mass, radius, and surface temperature. The authors also considered more hypothetical factors like the presence of abundant water, a large moon, and a geological recycling mechanism like plate tectonics.^[20]

Kepler-1126b (KOI-2162.01) andKepler-69c (KOI-172.02) are the only objects in the list that have been confirmed as exoplanets.^[58] However, earlier research on Kepler-69c suggests that because its orbit lies near the inner edge of the HZ, its atmosphere could likely be in a runaway greenhouse state, which could heavily impact its prospects for habitability.^[59] The full list can be found below.^[60]

• "Habitable Worlds Catalog".
• "NASA Exoplanet Archive".

• Hypothetical planet types
• Astrobiology

• Articles with short description
• Short description is different from Wikidata
• Use dmy dates from October 2019