Long-range liquid water also on non-Earth-like planets?

(Bern) (PPS) Liquid water is an important prerequisite for the development of life on a planet. As researchers from the University of Bern, the University of Zurich and the National Center for Efficiency in Research (NCCR) PlanetS report in a new study, contrary to what was previously assumed, water can also be found on planets that are vastly different from Earth for billions. Years. This challenges our previous notions of potentially habitable planets.

Life on land began in the oceans. Therefore, in the search for life on other planets, the possibility of liquid water is an important requirement. To find it, researchers have traditionally searched for planets similar to ours. However, long-term liquid water does not necessarily have to occur under conditions similar to those on Earth. Researchers from the University of Bern and the University of Zurich, who are members of the National Center for Efficiency at Research Planets, reported in a study published in Nature Astronomy that billions of years of favorable conditions can exist even on planets that are barely the same. our planet.

old greenhouses

“One of the reasons water on Earth is a liquid is the atmosphere,” explains study co-author Ravit Helled, professor of theoretical astrophysics at the University of Zurich and a member of NCCR PlanetS. “With its natural greenhouse effect, it captures the right amount of heat to create the necessary conditions for oceans, rivers, and rain,” says the researcher.

However, Earth’s atmosphere was very different in its early days. “When the planet first formed from cosmic gas and dust, it accumulated an atmosphere composed primarily of hydrogen and helium – the so-called primordial atmosphere,” explains Held. But during its evolution, the Earth lost this original atmosphere.

Other, more massive planets could gather much larger primordial atmospheres, which in some cases could sustain them. “Such massive primordial atmospheres can also cause a global warming effect – similar to Earth’s atmosphere today. So we wanted to see if these atmospheres could create the conditions for liquid water,” says Held.

Liquid water for billions of years

To do this, the team modeled countless planets and simulated their evolution over billions of years. They took into account not only the properties of the atmospheres of the planets, but also the intensity of radiation from their stars and the internal heat emitted by the planets outward. While this geothermal temperature plays a minor role in Earth’s surface conditions, it can contribute significantly to planets with massive primordial atmospheres.

“We have found that in many cases the original atmosphere has been lost due to intense radiation from stars – especially on planets close to their star. However, in cases where the atmosphere has been preserved, conditions can arise in which liquid water can exist,” According to Marit Mole Luce, a doctoral student and lead author of the study. According to the researcher from the University of Bern and the University of Zurich, “In cases where sufficient geothermal heat reaches the surface, the intense radiation from a star like the Sun is not necessary even for the surface conditions that support the presence of liquid allowing water ».

“Perhaps most importantly, our results show that these conditions can persist for very long periods – up to tens of billions of years,” stresses the researcher, who is also a member of NCCR Planets.

Expanding the search for extraterrestrial life

“This may come as a surprise to many. Astronomy typically expects liquid water to be in the regions around stars that receive the right amount of radiation: not so much that the water doesn’t evaporate, and not so little that it doesn’t freeze completely,” explains study co-author Christophe Mordasini. Professor of Theoretical Astrophysics at the University of Bern and a member of NCCR PlanetS.

“Since the presence of liquid water is a potential requirement for life and life on Earth is likely to take several million years to evolve, this could significantly expand the prospects for the search for extraterrestrial life. According to our results, it can evolve even on so-called planets. Free-floating that does not revolve around a star,” says Mordasini.

However, the researcher remains cautious: “Our results are exciting, but they must be treated with caution. Because in order to have liquid water for such planets in the long term, they must have the appropriate atmosphere. We do not know how often this happens.” And even under the right conditions, no one knows how likely it is for life to develop in such a potentially exotic habitat. “This is an astrobiology question. However, through our work we have shown that our Earth-centric concept of a habitable planet may be too narrow,” concludes Mordasini.

Information about the publication:
Marit Moll Los, Ravitt Heald and Christophe Mordassini: Long-term habitable conditions on planets with H-He primordial atmospheres, Natural Astronomy, June 2022 DOI: 10.1038/s41550-022-01699-8

nature.com/articles/s41550-022-01699-8