In our search for exoplanets, we found more than three dozen potentially habitable worlds. It is estimated that there are between 8 and 20 billion potentially inhabited Earth-like worlds in our galaxy alone. But there is a big difference between potentially habitable and truly habitable planets, and scientists are starting to narrow down their definitions.
The classic definition of a potentially habitable world is a rocky planet that can support liquid water on its surface. This means that it has a fairly dense atmosphere, and its distance from the star is in the so-called habitable zone.
In many ways, this simple definition is too broad and too narrow. Liquid water is considered a defining factor because it is central to life on Earth. There may be other life forms that do not require fluids. Saturn's moon Titan has a sea of liquid methane, and some researchers have studied how methane may play a role in alien cells, similar to the role of water in life on Earth. Even if we restrict life to be like Earth, being outside the habitable zone of an exoplanet does not mean that there can be no liquid water on its surface. Jupiter is far beyond this zone, but liquid water is also under the icy surface of its two moons.
Habitable zones of two planetary systems: Kepler-452 and Kepler-186 compared to the habitable zone of the solar system
At the same time, staying in the habitable zone is not enough. There are three rocky planets in the Sun's habitable zone, but Venus is a toxic greenhouse world, and while Mars may have primitive life, it is hardly Earth's rich Eden. Both Venus and Mars had liquid water on the surface in their youth. Life may have received and was upon them, but could not live long enough.
Even if we just consider the habitable zone of the star, it says nothing about the stability of the star itself. Most potentially habitable worlds revolve around small red dwarf stars, as they make up nearly 75% of all stars. Red dwarfs are much colder than our Sun, so their habitable zones are very close to a star. But red dwarfs often go through periods when they emit huge stellar flares and X-ray flares. This is likely to deprive potentially habitable planets of their atmospheres.
All this gives astronomers a lot of food for thought, but for biologists it all comes down to lipids. Lipids are made up of fatty acids and they form the building blocks of cell membranes. For cells to function, cell membranes must be permeable, and permeability depends on the composition and pressure of the atmosphere. Recently, a Scientific Reports article examined this relationship and its impact on habitability.
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So far, it is believed that Europa's ocean can extend 80 km in depth, and the ice shell - 20 km.
The article discusses the limits of the lipid solubility tolerance of carbon dioxide and nitrogen. Based on data from anesthesia and diving, he sets a limit of 0.1 bar for carbon dioxide and 2 bar for nitrogen. This limit narrows the habitable zone for stars, especially in the colder range. For our solar system, this takes Mars and Venus out of the zone. The decrease is most significant for red dwarf stars. While red dwarfs are common, they may be far less likely to harbor life.
It should be emphasized that this is still based on what we know about life on Earth. But this is an important study. The number of known exoplanets is growing exponentially, and such research will help us narrow down candidates for where life could exist.
Author: Dmitry Lyalin