One might think that typical earthly characteristics have endowed our world with its blue treasure. But now a study shows that liquid water could also occur on planets with a primordial hydrogen-helium atmosphere in the long term, according to a study. According to the model simulations, under certain circumstances it can produce a greenhouse effect that allows for a mild climate for billions of years. The study thus suggests that the previous search for potentially habitable worlds in space is too narrow, say the scientists.
On our planet, it has become the elixir of life: liquid water is the prerequisite for the existence of all terrestrial organisms and it is assumed that their history of development began in the oceans. This importance therefore also characterizes the research field of astrobiology: In the search for extraterrestrial life, the question of the potential for liquid water on distant celestial bodies plays an important role. That is why astronomers are particularly interested in exoplanets that resemble our planet. “One of the reasons why water on Earth can be liquid is the atmosphere,” says co-author Ravit Helled from the University of Zurich. “With its natural greenhouse effect, it captures just the right amount of heat to create the necessary conditions for oceans, rivers and rain,” explains the scientist.
The greenhouse effect is primarily based on gases such as carbon dioxide or methane, as is generally known from the human-caused increase in these substances in the atmosphere. However, many exoplanets could have other atmospheric compositions and even the Earth did not have the gas envelope it has today in its early days. “When our planet formed from cosmic gas and dust, it accumulated an atmosphere composed mainly of hydrogen and helium – a so-called primordial atmosphere,” explains Helled. However, in the course of its evolution, the earth lost this original atmosphere and developed a new one that allowed an optimal greenhouse effect for the existence of liquid water on the surface.
Targeting exotic greenhouse conditions
As part of their study, Helled and her colleagues have now investigated the extent to which more massive rocky planets – so-called super-Earths – could have accumulated particularly extensive primordial atmospheres, which were preserved and ensured mild conditions. “Such massive primordial atmospheres could also cause a greenhouse effect – similar to today’s earth’s atmosphere. We therefore wanted to find out whether these atmospheres can create the necessary conditions for liquid water,” says Helled. Another important factor was also in focus: the possible existence time of such conditions, which appears to be important for the development potential of life.
In order to pursue their research questions, the researchers simulated the development of many planets in different configurations and combinations of features over billions of years. In addition to the properties of the planetary atmospheres, they also took into account the intensity of the radiation they receive from their stars. Another factor was the internal heat that planets can give off to the outside. Because while this geothermal energy plays only a minor role in the conditions on the earth’s surface, it could make a significant contribution to warming on planets with massive primordial atmospheres, the scientists explain.
More potential in the search for life in space
As they report, their simulations revealed that in many of the simulated combinations of conditions, the original atmosphere was lost due to the intense radiation from nearby stars. In some cases, however, the hydrogen-helium atmosphere was maintained for a long time and, thanks to its comparatively high density, was able to ensure a favorable greenhouse effect: not too much, so that the water does not evaporate, and not too little, so that it does not freeze completely. “Under certain circumstances, conditions can arise that allow liquid water to exist,” summarizes first author Marit Mol Lous from the University of Bern. As for the internal heat factor, it also became apparent: “In simulated cases, where enough geothermal heat reaches the surface, even intense radiation from a star is not necessary for surface conditions to exist that allow liquid water to exist says Mol Lous.
“Perhaps most importantly, our results show that in some cases the favorable conditions can last for very long periods of time – up to tens of billions of years,” the researcher points out. Her colleague Christoph Mordasini from the University of Bern continues: “Since the presence of liquid water is a likely prerequisite for life and life on Earth may have taken many millions of years to develop, this could be the horizon for the Significantly expand the search for extraterrestrial life forms. According to our results, it could even develop on so-called free-floating planets that do not orbit a star,” says Mordasini.
However, the researchers emphasize that caution is still required when interpreting the study results. Because it remains unclear how often the theoretically appropriate atmospheres and constellations of exoplanets in space actually occur, and also how probable the emergence of life is. “That’s a question for astrobiology. However, our work has shown that our Earthcentric vision of a livable planet may be too narrow,” concludes Mordasini.
Source: University of Bern, specialist article: Nature Astronomy, doi: 10.1038/s41550-022-01699-8