What conditions allowed the emergence of life?

Habitable Zone: A Fragile Balance Between Heat and Cold

Liquid water, an indispensable condition for life

Our Galaxy contains about 200 billion stars and statistics tell us it would contain 2,000 billion planets. That seems enough to say that life has certainly found many places among the habitable zones of these billions of stars to develop. However, so many favorable conditions must be met that this strongly restricts the possibilities.

The first indispensable condition is the presence of water in liquid state. Water is abundant in the material Universe, which is composed of 74% hydrogen, 24% helium, 1% oxygen, and all other elements combined represent only 1% of ordinary matter.

The biochemical role of liquid water

Scientists believe that liquid water is vital because of its role in biochemical reactions. It is even considered an indispensable element for a viable ecosystem because it greatly promotes the transport of materials necessary for biochemical activity.

A planet must therefore have liquid water and keep it long enough, billions of years, to have a chance of preserving life. No one knows how life appears; the transition from the inanimate to the animate is still a mystery, but life as observed on our planet is based on the chemistry of carbon in solution in liquid water.

A carbon chemistry in aqueous solution

If life exists elsewhere, it must be based on the chemistry of carbon. Life did not take the path of silicon, which is much more abundant on Earth than carbon.

Specialists have tried to imagine life in a chemistry other than that of carbon, and all say it is much more complicated because carbon (C) is the essential component of organic compounds. It is the basis of a multitude of compounds and associates very well with other atoms, in particular with hydrogen, oxygen, nitrogen, phosphorus, and sulfur.

The difficult preservation of water on the surface

Since in the Universe the water molecule (H²O) is present everywhere, all planets must have some. The difficulty will be to keep water on the surface in liquid state for billions of years, and for that the habitable planet must gather a very large number of conditions favorable to its stability.

The planet must therefore remain in orbit in a habitable zone.

What is the size of the habitable zone?

Numerical models show that if we move Earth away from the Sun by 12%, it will then receive only 79% of the Sun's heat. At that location the climate runs away, and Earth becomes covered very quickly with ice in a few tens of years. Yet Earth has always been habitable despite the evolution of the Sun's heat.

In the beginning the Sun was 25% less bright than today, and yet there has always been liquid water on Earth's surface with on average a warmer climate than today. This is the Faint Young Sun paradox.

The Faint Young Sun paradox

This old enigma from 1972 was raised by astronomers Carl Sagan (1934-1996) and George Mullen. It happened when evidence of liquid water and bacterial life was found in geological layers from the beginning of Earth's formation. Sagan and Mullen suggested at the time that a greenhouse effect 3 times greater than today must have existed thanks to ammonium and methane.

Thanks to this primordial greenhouse effect, Earth was able to retain the weak heat emitted by the young Sun. As the Sun shone more and more, the carbonate-silicate cycle took over to stabilize Earth's climate.

The limits of the habitable zone and the loss of water

If the planet is capable of stabilizing its climate, the upper limit of the habitable zone recedes to 1.6 AU. The inner limit of the habitable zone was calculated by computer models at 5% (0.95 AU). At that location, closer to the Sun, the climate system runs away. The temperature increases, the greenhouse effect increases, and the oceans evaporate. The bonds of water molecules are broken in the atmosphere, and hydrogen, being very light, disappears into space. Little by little the planet loses all its water.

It is therefore between 0.95 and 1.65 AU that Earth can keep liquid water on its surface. It will need to remain in this zone for a very long time to allow the evolution of life. It is thanks to the carbonate-silicate cycle and plate tectonics that Earth was able to keep its atmosphere.

Carbonate-Silicate Cycle and Plate Tectonics

The carbonate-silicate cycle, Earth's thermostat

The Carbonate-Silicate Cycle plays a crucial role in regulating Earth's climate over long periods. This cycle begins when atmospheric CO2 dissolves in rainwater, forming carbonic acid (H2CO3). The products of this weathering are then transported to the oceans, where marine organisms use them to build calcium carbonate shells (CaCO3). When they die, these organisms accumulate on the ocean floor. Plate tectonics then recycles the CO2 in subduction zones, releasing it in gaseous form into the atmosphere via volcanic eruptions. Without this volcanic release, atmospheric CO2 would be exhausted in about 400,000 years. The cycle continues when volcanic CO2 is again dissolved by rainwater, which transports it to the ocean depths. This process is essential to the functioning of Earth's "climate machine."

An indispensable plate tectonics

In summary, to keep water in liquid state, a living planet with intense geological activity is needed. However, the phenomenon of plate tectonics is absent on the other planets of the solar system. It seems that if the planet is too small like Mars, it cannot have plate tectonics, but if the planet is larger (super-Earth), convection is less efficient, and there will be only one large plate. Yet Venus, which is the same size as Earth, does not benefit from plate tectonics!

N.B.:
The circumstellar habitable zone, also called the ecosphere, is a theoretical region in the shape of a circular tube around a star where the surface temperature of orbiting planets allows the presence of liquid water. Although this zone could potentially harbor life, the conditions necessary for it to develop are so numerous that the probabilities remain very low.

Astronomical Conditions for a Planet to Keep its Liquid Water

The stability of planetary orbits

Another essential condition for maintaining water in liquid state concerns the astronomical characteristics of the star around which a planet orbits. These characteristics are limited by the fact that 60% of stars are binary systems, which is not conducive to the emergence of life, because the orbits of planets in these systems are often irregular and chaotic. For a planet to be habitable, its orbital eccentricity must be low, close to 0 (circular orbit). Earth's eccentricity, which is 0.01, is ideal. In contrast, observations of exoplanets reveal an average eccentricity of 0.29, which is considerable. A high eccentricity makes the planetary orbit unstable, exposing the planet to gravitational perturbations from other planets (work by Jacques Lascar), which also affects temperature stability. Among the observed solar systems, few offer nearly circular orbits like those of our solar system.

The mass of the star, a decisive criterion

Another crucial condition for preserving liquid water is the mass of the star. Stars have masses varying from about 1/100th to 100 times that of the Sun. Stars more massive than the Sun, between 1.2 and 1.5 solar masses, emit too much ultraviolet radiation, which is not favorable to the emergence of life. Moreover, their lifespan is too short to allow the development of life.

Stars smaller than the Sun emit a significant amount of X-rays and particles harmful to life. About 75% of stars have a mass of about 0.5 solar mass. These stars emit little light, which brings the habitable zone closer to the star. Because of this proximity, potentially habitable planets synchronize their rotation with their star by tidal effect, resulting in one frozen face and another burning. Although a "temperate region" may exist at the boundary, this situation is not ideal for maintaining water in liquid state. Moreover, at this distance, these planets have no magnetic field, because synchronization prevents differential rotation of the planetary core, exposing them further to solar radiation.

In summary, stars of 0.9 to 1.2 solar mass are the most conducive to the emergence of life. Computer simulations are not surprising, because they model the ideal conditions corresponding to our solar system. However, they help researchers better understand the complex conditions necessary for the emergence of life.

N.B.:
Eccentricity defines the shape of an elliptical orbit; it varies between 0 and 1. 0 for circular orbits. A high eccentricity decreases the smallest axis (perihelion) and increases the largest axis (aphelion), but does not modify the major axis.

The States of Water or Phase Transitions

The phase diagram of pure water

Water presents itself in a single form only for a given pressure-temperature pair. The phase diagram below summarizes these equilibria and highlights two remarkable points, the triple point and the critical point.

The Phases of Water

Triple point and phase transitions

Pure water presents itself in a single phase (solid, liquid, or gaseous) for a specific pressure and temperature. However, at the triple point, the three phases coexist simultaneously at a precise temperature and pressure. A pressure-temperature pair can correspond to a phase transition, that is, a change of state, such as melting (solid to liquid), solidification (liquid to solid), sublimation (solid to gas), condensation (gas to solid), vaporization (liquid to gas), or liquefaction (gas to liquid). Beyond the critical point, water enters a fluid phase, where it is both gaseous and liquid, at a pressure of 218 atmospheres and a temperature of 374°C.