Astronomers are meticulously scrutinizing distant exoplanets that share similarities in diameter, distance from their parent stars, presence of a solid surface, water content, atmospheric composition, and other factors. Several dozen such exoplanets have been identified, with Proxima Centauri b, located near our closest star, being one of them. Scientists are earnestly preparing to explore the possibility of life in this particular system.
However, it is essential to recognize that planets like Earth, despite being a known example of a habitable environment, may not necessarily represent the optimal conditions for the emergence and development of life. Anthropocentrism and geocentrism often skew our perspectives, limiting our ability to consider a broader range of possibilities.
Many experts believe that numerous planets may provide more suitable conditions for fostering a thriving biosphere, referred to as “superhabitable” worlds. Currently, over 20 candidate planets for this designation have been identified.
The concept of superhabitable exoplanets was first introduced in a 2014 article by René Heller and John Armstrong, published in the journal Astrobiology. Their calculations outlined the characteristics of an ideal world capable of supporting a more diverse biosphere, including a richer variety of flora and fauna than Earth.
When evaluating potential superhabitable planets, it is crucial to begin by examining their respective stars.
The most massive and largest luminaries can be discarded immediately. They are too violent, unpredictable, and do not live long. In general, the violent streams of particles emanating from such stars prevent the formation of planets. But even if this happens, and even if life manages to appear on such a planet, its life will be short. In less than a billion years, the planet and everything on it will be destroyed by a supernova explosion.
At the opposite end are the smallest and faintest stars, primarily red dwarfs. There’s not much good here either. In order for the weak light to sufficiently warm the planet, it would have to be very close to its star. At this distance, its ultraviolet radiation becomes extremely dangerous, and any outbreak of activity threatens to completely sterilize the surface.
Our Sun belongs to the middle class, stars of spectral class G, yellow dwarfs – moderately hot, not too bright, and with fairly predictable behavior. All this is wonderful for life, but the best option is class K orange dwarfs. They are 1.5-2 times smaller than our star and several times fainter. But this is precisely what makes their character much calmer. The activity of orange dwarfs is noticeably lower; they use “internal fuel” more economically to maintain thermonuclear reactions.
As a result, the lifespan of such stars is much longer. The stable lifespan of the Sun is estimated at about 10 billion years, most of which have already passed. But orange dwarfs can persist for 20 or 50 billion years – at least in theory.
This period is longer than the time that has passed since the beginning of the Universe, and not a single example of the destabilization of such a star with the transition to a red giant is still unknown. So an orange dwarf can give life billions more years for quiet evolution.
For the biosphere to be rich and diverse, it needs more space. The larger the planet, the longer it will retain the “internal heat” of its depths, and with it the currents that arise in the liquid core and create a global magnetic field that protects the surface from streams of deadly particles continuously arriving from space.
This same thermal energy provides mixing of the mantle, and hence geological activity, including plate tectonics. Such processes fill the atmosphere with carbon dioxide, which serves as the main source of carbon for the biosphere on Earth.
On the other hand, calculations show that if the planet is too large and massive, then its gravity will be too strong. It will squeeze the lithosphere so tightly that no plate tectonics will be possible.
In addition, all the moisture may be squeezed out to the surface, surrounding the planet with an endless ocean. The useful substances will be too diluted, and the conditions will be too homogeneous for the emergence of life, and even more so for the development of its most complex forms.
Calculations show that optimal geological conditions for superhabitability are provided by planets no more than five times heavier than ours, optimally about two Earth masses, with a radius of approximately 1.2-1.3 Earth’s.
Such dimensions will make it possible to maintain a strong magnetosphere for a very long time, as well as high geological activity, which brings valuable minerals to the surface and fills the atmosphere with carbon dioxide. Among other benefits, the extra weight will allow the planet to retain a denser atmosphere, which means a warmer, more stable climate.
Exoplanets of this size are called super-Earths, and they are quite widespread. By some estimates, almost a third of the Milky Way’s planets are of this type, and the closest (GJ 699 b) is located just six light years from the Sun, near Barnard’s star.
Unfortunately, it cannot be called superhabitable: the star is too cold, and the temperature on GJ 699 b remains much below -100 °C.
In order for a planet to consistently maintain a comfortable temperature, it should not be too close, but not too far from its star (as we have already seen, life generally does not like extremes). This area is called the habitable zone.
The Earth is located within this region of the Sun, but again, it is far from ideal. Our planet’s orbit passes close to its inner boundary, and over time this could become a problem.
The fact is that stars radiate more and more over time, and over time it will become too hot on Earth. So a true super-habitable planet would be closer to the middle of this zone of its sun.
At the same time, it is desirable that the climate there be warmer. On Earth, life has mastered almost all ecological niches, reaching the very poles. But it is clear that icy wastelands are not the best place to thrive, and ecosystems living in the humid and warm tropics reach maximum numbers and diversity.
The current average global surface temperature of the Earth is about 15 °C, but for super habitability it is better to be slightly above 20 °C. It is quite possible to maintain such a temperature even at a greater distance from the star.
This can be achieved by a thicker and denser atmosphere, with an increased concentration of carbon dioxide: it can warm the planet due to the greenhouse effect. Ideally, there should be more oxygen in the air.
It is difficult to imagine that any other gas is widespread enough and could serve as a more convenient oxidizing agent for maintaining energy processes in living organisms.
It is not without reason that it was in the late Carboniferous period of the Earth, when the concentration of this gas reached 35 percent, that the biosphere was even more diverse than it is today.
The rather vast oceans allow the climate to be further moderated. A reasonable combination of seas and continents is useful for evolution, and in this too the Earth is almost – and only almost – ideal.
Calculations carried out in 2022 showed that water and land on the surface of our planet are presented in very good proportions. The feedback chains that arise between them make it possible to maintain both plate tectonics and the global cycle of minerals. On the other hand, smaller oceans with longer coastlines would give life much more opportunities to develop.
Where is the best?
At the moment, the existence of not a single exoplanet has been confirmed that would fully satisfy all these characteristics. Located 1,200 light-years away, Kepler-442b is an orange dwarf system with a mass of 2.36 Earths – but orbits too far from its star and has an average temperature of only 4°C.
Planet KOI-4878.01 is slightly warmer than Earth, but its dimensions are most likely the same as ours.
The ideal place has not been found – but this does not mean that somewhere in the vastness of the Galaxy this best of worlds is not waiting to be discovered.