Life on Earth arose in the oceans. Therefore, in the search for life on other bodies, in the solar system and other stellar systems, the presence of liquid water may be an important factor. In recent decades we have realized that liquid water can survive on very different stars . From the salty slurries that fill the subsurface of Mars , to the oceans hundreds of kilometers deep that we think occupy the interiors of many moons in the solar system , we’ve found that our little piece of the Milky Way alone could contain 25 times more liquid water than we thought . We also know that planets like Uranus and Neptune contain large amounts of this compound, although we don’t know in what state it appears there.
A recent study by a Swiss team has studied the persistence of liquid water on planets with very different conditions , calling into question the very restrictive ideas we had about the habitable zone of a star. This zone would be that region around a star in which liquid water could occur on the surface of a planet. In the solar system it would be between a little beyond the orbit of Venus and a little before that of Mars. However, why limit ourselves to the surface ? Why not also consider in that habitable zone the oceans that are under kilometers of ice? Or the possible oceans that could harbor larger worlds, like Neptune or even Jupiter, deep in their atmosphere?
The main difference between Earth and Mars and Venus, other than their distance from the Sun, is their atmosphere . This is key in allowing the presence of liquid water on the surface of our planet. Venus’s atmosphere is too thick and too efficient at trapping the sun’s heat, driving the planet to temperatures above 400 degrees Celsius. The atmosphere of Mars, on the other hand, is too thin and does not retain heat or exert enough pressure for there to be water on the surface of the red planet.
When it formed, Earth’s atmosphere was very different. Mainly, it contained more hydrogen and helium , since these are the main components of matter in the universe and the most abundant elements in the solar system. The Sun and Jupiter are composed of more than 99% hydrogen and helium , and the atmospheres of Saturn, Uranus, and Neptune contain mostly these elements. However, the terrestrial gravity was not enough to retain them indefinitely and they were lost over time. The planets that are capable of retaining these atmospheres will also be capable of generating a certain greenhouse effect , which allows them to maintain a high temperature in their interior, creating, perhaps, the necessary conditions for liquid water to last.
To study it, the team modeled the atmospheres of such planets , following their evolution over billions of years. They not only considered different compositions and sizes, but also the amount of energy they receive from their star . They found that these primordial atmospheres of hydrogen and helium not only disappear due to the lack of gravity, but also due to the star’s radiation , especially when the planet in question orbits close to the star .
But in those cases in which the atmosphere persists, the conditions may exist for liquid water to exist. They even saw that in worlds where the internal energy is large enough , the energy received from the star may not be necessary to guarantee that liquid water, as in fact occurs in the moons and smaller bodies in which we believe there would be oceans of the liquid element. They also found that these conditions could last for billions of years , a necessary condition to ensure that life can not only arise, but also evolve and spread.
This team proposes that, given the right conditions, this liquid water could even exist and persist on wandering planets, planets that don’t orbit any stars . Either because they formed independently, or because they were expelled from their corresponding solar system. These results therefore considerably expand the possibilities of finding liquid water on exoplanets and in our own solar system, and thereby also expand the possibilities of finding some form of extraterrestrial life on any of these bodies. However, the team recommends taking the results with a grain of salt. Although this is possible, because there are configurations that allow it, they do not know how likely it is that these conditions will appear on a natural planet.
Furthermore, the existence of liquid water is not the only requirement for the presence of life. An energy source and the basic chemical materials are also necessary. It could even be said that not even water is necessary for the development of life , but this would be to venture into fields of astrobiology that have been little studied and developed.
Marit Mol Lous et al, 2022, Potential long-term habitable conditions on planets with primordial H–He atmospheres, Nature Astronomy, DOI: 10.1038/s41550-022-01699-8