Super habitable planet

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Artist's impression of a super-habitable planet. The red color is supposed to indicate vegetation.

A super-habitable planet is a suspected type of exoplanet that could be better suited for the formation and evolution of living things than Earth . The concept was first proposed by René Heller and John Armstrong, who wanted to show the public that the habitable zone (HZ) is just one of the prerequisites for a planet with life. Heller and Armstrong state that it is not clear why the earth should have the best physiochemical parameters for living things, since planets could have better conditions for the formation and evolution of living things than the earth, although they are not Earth-like. Although they still believe that water is essential for life, they suspect that the earth is not ideal for biodiversity ; In other words, they define a superhabitable world as a rocky planet or moon on which there would be more diverse flora and fauna , which would prove that its environment is more receptive to life.

Heller and Armstrong also point out that not only rocky planets within the habitable zone can be habitable, as tidal warming can make terrestrial and icy worlds habitable outside the HZ, such as in Europe's inner ocean. The authors express that for the identification of habitable and super-habitable planets a plan is needed that is more biocentric than geocentric or anthropocentric . Heller and Armstrong have proposed setting up a profile of exoplanets that includes, among other features, their spectral class , mass, and position within their planetary system. Judging by the authors' statements, such super-habitable worlds would likely be larger, older, and warmer than Earth as they orbit a Class K star in the main sequence . According to current science, there is no celestial body confirmed to be habitable outside of our solar system .

properties

Heller and Armstrong explain that a number of properties are necessary to classify an exoplanet or exomoon as super-habitable; the size and mass that would be optimal for plate tectonics are approximately 1.3 Earth radii and 2 Earth masses . It would also have higher gravity , which would increase the retention of gases during the formation of the planet. This makes it likely that they will have a denser atmosphere containing higher levels of oxygen and greenhouse gases , which would bring the average temperature to the 25 ° C optimal conditions for plants. A denser atmosphere would also affect the relief of the sea floor, making it more regular, and shrinking lake basins , which would improve the diversity of marine life in shallow waters.

Another factor to consider is the spectral class of the star. Class K stars are not as massive as the Sun and are stable longer (15 to 30 billion years, compared to 10 billion for the Sun, a class G star), which would give living things more time to evolve. A superhabitable world would have to stay in the center of the habitable zone of its star system for a long time .

Surface, size and composition

Size comparison of some exoplanets (surfaces: fantasy representation), including Kepler-62e (second from the left with a radius of 1.6 earth radii), with the earth (right)

An exoplanet with a larger volume, more complex terrain, or a higher surface fraction of liquid water could be more suitable for life than Earth. Since the volume of a planet is often related to its mass, it can be assumed that a higher mass means a stronger gravity, which results in a denser atmosphere.

Some studies indicate that there is a natural limit of 1.6 earth radii, below which almost all planets are earth-like , i.e. consist primarily of stone, iron and water. As a rule, it is likely that objects below 6 M ⊕ have an earth-like composition. Above this limit, the planet's density decreases as its volume increases, causing it to become an ocean planet first, and eventually a gas planet . In addition, the high mass of superplanets can mean that they have no plate tectonics . Therefore, it is to be expected that any exoplanet with an Earth-like density and a radius below 1.6 Earth radii is suitable for life. However, other studies indicate that ocean planets represent a transition phase between Earth-like planets and so-called mini-Neptunes , especially when they orbit a red dwarf. Although ocean planets could be habitable, because of the average depth of water and lack of land mass, they would not fall under the term "super-habitable" established by Heller and Armstrong. From a geological point of view, the optimal mass of planet is around 2 Earth masses, so it needs to have a radius that maintains Earth's density between 1.2 and 1.3 Earth radii.

The mean depth of the oceans affect the habitability of the planet. Since shallow waters receive more light and heat and are therefore more comfortable for most aquatic life, it is conceivable that exoplanets with a lower ocean depth would be more habitable. Massive exoplanets would have regular gravity on their surface, which could mean shallower and more hospitable sea basins.

geology

With the presence of large amounts of water on the planet, plate tectonics can maintain high levels of carbon dioxide (CO 2 ) in the atmosphere. This process is common in geologically active planets with a high rotational speed. The more massive a planet is, the longer it will generate internal heat, which is an important factor in plate tectonics. However, excess heat due to higher pressure and the viscosity of the earth's mantle , which prevents the lithosphere from shifting , can also slow down plate tectonics. Research indicates that plate tectonics peaks in activity in objects between 1 and 5 Earth radii mass, with an optimal mass of around 2 Earth masses.

If geological activity is not strong enough to generate greenhouse gases that would bring global temperatures above freezing, the planet could experience a permanent ice age without an intense internal heat source .

Magnetosphere

Another positive quality for life is the planet's potential to create a magnetosphere that protects the surface and atmosphere from cosmic rays and solar winds , especially around red dwarfs . Less massive celestial bodies, those with a slow or bound rotation, have either a weak or no magnetic field, which over time can lead to the loss of the atmosphere.

The climate of a warmer and wetter rocky planet could be similar to the tropical regions of the world. In the picture, a mangrove tree in Cambodia .

Temperature and climate

Although the optimal temperature for terrestrial life is unknown, there is evidence that the diversity of living things was higher during warmer periods. So it is possible that exoplanets with slightly higher average temperatures would be more suitable for living things. The regulating effect of large oceans on the temperature of planets within the habitable zone could fall within a moderate range. In this case, deserts would likely be smaller and promote coastal habitats.

However, studies indicate that the earth is already on the inner edge of the solar system's habitable zone , which could damage its quality of life in the long term as the luminosity of main sequence stars increases over time, shifting their habitable zone outwards. For this reason, superhabitable planets should be warmer than Earth and have a closer orbit to the center of their habitable zone. This would either be possible with a thicker atmosphere or more greenhouse gases .

star

An example of a system for predicting the habitable zone location around types of stars based on their stellar luminosity

Conditions in the star system depend largely on the spectral class of the star. The most massive stars of the classes O, B and A have a very short life cycle and quickly leave the main sequence . In addition, O and B stars produce a photoevaporation effect that prevents planets from accretion around a star.

On the other hand, the less massive M and K class stars are the most common and long-lived stars in the universe, but their potential for life is still being studied. Their low luminosity reduces the size of the habitable zone exposed to outbreaks of ultraviolet radiation, especially within the first billion years of their existence. If a planet's orbit is too short, it can cause the planet to bind to a rotation with the star always facing the same hemisphere. Even if life were possible in such a system, it is unlikely that an exoplanet orbiting a red dwarf would be classified as super-habitable.

Taking both ends into account, class K stars provide the best habitable zones for living things. Class K stars enable the formation of planets, have a long life expectancy and offer a habitable zone free from the effects of being too close to the star. Also, the radiation produced by a class K star is high enough to allow complex life without an ozone layer . They are also the most stable stars and their habitable zones change only minimally during their lifetime, which would mean that an Earth-like planet orbiting such a star would be habitable for almost as long as the time of the star in the main sequence.

Orbit and rotation

Artist's impression of Kepler-186f . Some superhabitable planets could be very similar to Earth.

Researchers do not yet agree on the optimal rotational speed of exoplanets, but it shouldn't be too high or too low. In the latter case, problems that have been observed with Venus would arise . Venus needs 243 days to rotate around its own axis, which is why it cannot build up an earth-like magnetic field.

Ideally, the orbit of a super-habitable world should be at the center of its habitable zone.

the atmosphere

There are no strong arguments that the Earth's atmosphere is optimally composed to accommodate life. On Earth, the oxygen (O 2 ) content rose to 35% in the period when coal was first formed. This period coincided with the periods of greatest biodiversity . Based on the assumption that a significant amount of oxygen in the atmosphere is necessary for the development of complex life, it seems that the amount of oxygen relative to the total atmosphere limits the maximum size of the planet for optimal superhabitability and sufficient biodiversity.

In addition, the density of the atmosphere should be higher in more massive planets, which supports the hypothesis that super- habitable conditions could also exist on a super-earth .

Age

In a biological context, planets older than Earth could have greater biodiversity as native species had longer time to develop , adapt, and stabilize their environment for their descendants.

However, for many years old star systems had a low metallicity and thus little planet formation, which is why old planets might have been rare in the beginning, but the number of metallic objects in the universe has increased since its inception. The first extrasolar discoveries, mostly gas planets near their stars, so-called " Hot Jupiter ", indicated that planets would be rare in systems with a low metallicity, which aroused the suspicion of a time limit for the appearance of the first planets. In 2012, the observations of the Kepler telescope enabled experts to find out that this ratio is much more restrictive in systems with hot Jupiters and that planets could form to some extent in such low-metallicity stars. It is now believed that the first objects with the mass of the earth were formed between the first 7 and 12 billion years. Given the greater stability and life expectancy of late main sequence stars of spectral class K compared to the Sun (class G), it is possible that superhabitable planets of class K stars that have an orbit within their habitable zone will give living things a longer, more stable, and better environment could offer than the earth.

Summary

Artist's impression of a size comparison of a super-habitable exoplanet (1.34 earth radii) to the earth (right).

Despite the scarcity of information available, the hypothesis presented above can be summarized in a preliminary profile even if there is no scientific agreement.

  • Mass: about 2 earth masses.
  • Radius: To maintain an Earth-like density, the radius should be between 1.2 and 1.3 Earth radii.
  • Oceans: The surface area covered by oceans should be the same, but more distributed.
  • Distance: A shorter distance from the center of your habitable zone than Earth.
  • Temperature: average surface temperature of approximately 25 ° C.
  • Star and Age: orbiting a medium-class K star older than the Sun (4.5 billion years) but younger than 7 billion years.
  • Atmosphere: slightly denser than that of the earth with a higher proportion of oxygen.

There is no confirmed exoplanet that meets all of these conditions. According to the latest state of the database for exoplanets from July 23, 2015, Kepler-442b probably comes closest to the criteria mentioned. It orbits an orange dwarf star, has a radius of 1.34 earth radii and a mass of 2.34 earth masses, but a surface temperature of −2.65 ° C.

Appearance

"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? "

“The earth only scratches the inner edge of the habitable zone of the solar system, the area where temperatures allow earth-like planets to have liquid water on the surface. This led us to ask: could there be an even more favorable environment for life on rocky planets? "

- René Heller.

The appearance of a super-habitable planet should generally be Earth-like. The main differences would be derived from its mass in accordance with the profile. Its denser atmosphere would prevent inland ice from forming because of the smaller temperature differences between different regions of the planet . It would also have a higher concentration of clouds and frequent rainfall.

The vegetation would probably be different due to increased air density, temperature, radiation output and increased precipitation. Because K-class stars emit different light, plants could take on other colors than green. The vegetation would cover more regions than on Earth, which would make them visible from space. In general, the climate on a super-habitable planet would be more uniform (e.g. uniformly humid) or more stable than on Earth, which also has less habitable areas such as glaciers or deserts. If there was enough oxygen in the atmosphere, the planet could be accessible even to people without spacesuits . But you would have to adapt to the high force of gravity, through increased muscle strength or bone density, etc.

frequency

Heller and Armstrong speculate that the number of super-habitable planets may be much higher than those of the more Earth-like ones : Less massive stars in the main sequence are much more common than larger and brighter stars, so there are more orange dwarfs than solar counterparts. It is believed that roughly 9% of all stars in the Milky Way are K class.

Another point that supports the predominance of super-habitable planets is the fact that most of the conditions of a super-habitable world can only be met by a higher mass. A celestial body with a mass of 2 to 3 Earth masses would have plate tectonics that last longer and a larger surface area than Earth. Likewise, it is likely that due to the effect of gravity on the planet's crust, its oceans are shallower, its gravitational field is stronger, and it has a denser atmosphere.

In contrast, planets of one earth mass have a higher number of conditions. For example, some planets could experience shorter plate tectonics, ending up with a lower air density than Earth, increasing the likelihood of a global ice age, perhaps even a permanent snowball earth . Another negative effect of a low density of the atmosphere can manifest itself in thermoacoustic waves, which lead to large differences in global climate and can increase the likelihood of natural disasters. In addition, having a weaker magnetosphere can result in loss of the atmosphere, which would turn the planet into a desert planet like Mars . All of these examples could prevent the evolution of life on the planet's surface. In any case, the multitude of scenarios that would make a planet with the mass of Earth uninhabitable are less likely to be found on a planet that fulfills the basics of a super-habitable world, which would make them more common.

literature

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