Low-mass planets with a primordial atmosphere of hydrogen and helium can have the temperatures and pressures that allow water into the liquid phase. The presence of liquid water is beneficial to life, so these planets may host exotic habitats for billions of years. Credit: © (CC BY-NC-SA 4.0) – Thibaut Roger – Universität Bern – Universität Zurich.
Life on Earth started in the oceans. The potential for liquid water is therefore an important ingredient in the search for life on other planets. To find it, scientists have traditionally searched for planets similar to our own. Still, prolonged liquid water doesn’t necessarily have to happen 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 of Competence in Research (NCCR) PlanetS, report in a study published in the journal natural astronomythat favorable conditions can even exist for billions of years on planets that hardly resemble our home planet.
primeval greenhouses
“One of the reasons water on Earth can be liquid is the atmosphere,” explains co-author Ravit Helled, a professor of theoretical astrophysics at the University of Zurich and member of the NCCR PlanetS. “With its natural greenhouse effectit traps just the right amount of heat to create the right conditions for oceans, rivers and rain,” said the researcher.
However, the Earth’s atmosphere was very different in ancient history. “When the planet was first formed from cosmic gas and dust, it accumulated an atmosphere composed mainly of hydrogen and helium — a so-called primordial atmosphere,” Helled explains. However, in the course of its development, the Earth lost this primordial atmosphere.
Duration of liquid water conditions for planets on a wide range of semi-major axes (1 au to 100 au) and shell masses (10−1.8 up to 10−6 m† Planets receive solar radiation based on the brightness evolution of a sun-like star. a†cNuclear masses of 1.5 (a), 3 (b) and 8 m †c† The total evolution duration is 8 Gyr. The color of a dot indicates how long there were continuous surface pressures and temperatures through which liquid water, lqw† These range from 10 Myr (purple) to over 5 Gyr (yellow). Gray crosses correspond to cases where no liquid water lasts longer than 10 Myr. In these simulations, atmospheric loss is not taken into account. dResults for planets with a core mass of 3 mbut including the restriction that the surface temperature must remain between 270 and 400 K. Each panel contains an ‘unbound’ box where the distance is set to 106au and solar radiation has become negligible. Credit: natural astronomy (2022). DOI: 10.1038/s41550-022-01699-8
Other, more massive planets can accumulate much larger primordial atmospheres, which in some cases can keep them indefinitely. “Such massive primordial atmospheres can also create a greenhouse effect, much like Earth’s current atmosphere. So we wanted to know if these atmospheres could help create the necessary conditions for liquid water,” Helled says.
Liquid water for billions of years
To do this, the team thoroughly modeled countless planets and simulated their evolution over billions of years. They were responsible not only for the properties of the planets’ atmospheres, but also for the intensity of the radiation from their respective stars and for the internal heat of the planets that radiated outward. While on Earth this geothermal heat plays only a minor role in surface conditions, it can make a greater contribution on planets with massive primordial atmospheres.
“What we found is that in many cases primordial atmospheres have been lost to intense radiation from stars, especially on planets that are close to their stars. But in the cases where the atmosphere persists, the right conditions for liquid water can arise.” reports Marit Mol Lous, Ph.D. 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, radiation from a star like the sun is not even needed, so there are conditions on the surface that allow the existence of liquid water.”
“Perhaps most importantly, our results show that these conditions can persist for very long periods of time — up to tens of billions of years,” emphasizes the researcher, who is also a member of the NCCR PlanetS.
Planets with a primordial hydrogen-helium atmosphere exhibit a wide range of conditions that allow liquid water. Credit: © (CC BY-NC-SA 4.0) – Thibaut Roger – Universität Bern – Universität Zurich.
Broadening horizons for the search for extraterrestrial life
“To many, this may come as a surprise. Astronomers usually expect liquid water to exist in regions around stars that receive just the right amount of radiation: not too much, so that the water doesn’t evaporate, and not too little, so it doesn’t all freeze.” , explains co-author Christoph Mordasini, professor of theoretical astrophysics at the University of Bern and member of the NCCR PlanetS.
“Since the availability of liquid water is a likely prerequisite for life, and life likely took many millions of years to emerge on Earth, this could significantly expand the horizons for the search for extraterrestrial life forms. Based on our results, it could even can appear on so-called free-floating planets, which do not orbit a star,” says Mordasini.
Duration of liquid water on unbound planets. The planets are simulated with different core masses and shell masses. Grid dot colors indicate how long there were continuous surface pressures and temperatures that allowed liquid water, τlqw. The longest simulated duration was 84 billion years for a 10 m core and a 0.01m envelope. If planets with primordial atmospheres can harbor liquid water, the duration on untethered planets could be much longer, because the internal heat source can evolve more slowly than the host star. Contour lines indicate the beginning of liquid water for planets withlqw> 100 Myr. Credit:natural astronomy (2022). DOI: 10.1038/s41550-022-01699-8
Still, the researcher remains cautious: “While our results are exciting, they should be taken with a grain of salt. For such planets to have liquid water for a long time, they must have the right amount of atmosphere. We don’t know how common that is.” .”
“And even under the right circumstances, it’s unclear how likely it is for life to arise in such an exotic potential habitat. That’s a question for astrobiologists. Yet our work has shown that our Earth-centered idea of a life- friendly planet might be too narrow,” concludes Mordasini.
Marit Mol Lous, Potential long-term habitable conditions on planets with primordial H–He atmospheres,natural astronomy (2022). DOI: 10.1038/s41550-022-01699-8† www.nature.com/articles/s41550-022-01699-8
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