Are our ideas of the habitable zones around a star too Earth-centric? Of course. We’ve only found one example of life in the universe so far, so we all draw conclusions. But there are alternatives, as researchers from the University of Bern and the University of Zurich have just reported in a study published in the journal Nature Astronomy. According to the study, favorable conditions could even prevail for billions of years on planets that barely resemble our home planet.

“One of the reasons water can be liquid on Earth is its atmosphere,” explained study co-author Ravit Helled, professor of theoretical astrophysics at the University of Zurich and a member of the NCCR PlanetS. “With its natural greenhouse effect, it traps just the right amount of heat to create the right conditions for oceans, rivers and rain.” In Earth’s early days, however, the atmosphere looked very different. “When the planet first formed from cosmic gas and dust, it accumulated an atmosphere composed mainly of hydrogen and helium – a so-called primordial atmosphere,” Helled said. As it evolved, however, Earth lost this original atmosphere.

Other, more massive planets, however, can accumulate much larger urate atmospheres, which they can hold indefinitely in some cases. “Such massive urate atmospheres can also produce a greenhouse effect – much like Earth’s atmosphere today. So we wanted to find out if these atmospheres can help create the conditions necessary for liquid water,” Helled says. 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 planets’ atmospheres, but also the radiation intensity of the respective stars, as well as the internal heat of the planets radiating outward. While this geothermal heat plays only a minor role in surface conditions on Earth, it can make a larger contribution on planets with massive urate atmospheres.

“We have found that in many cases the urate atmospheres are lost to intense radiation from the stars, especially on planets that are close to their star. But in the cases where the atmospheres were preserved, the right conditions for liquid water can occur,” reports Marit Mol Lous, a doctoral student and lead author of the study. In cases where enough of the Earth’s heat reaches the surface, radiation from a star like the Sun would not be needed at all for conditions to exist at the surface that would allow liquid water to exist for the very long time needed for life to form.

“Since the presence of liquid water is a likely prerequisite for life, and life probably took many millions of years to evolve on Earth, this could greatly expand the horizon for the search for extraterrestrial life forms. According to our results, it could even arise on so-called free-floating planets that do not orbit a star,” says Christoph Mordasini, professor of theoretical astrophysics at the University of Bern. Nevertheless, the researcher remains cautious: “Our results, while exciting, 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 conditions, it’s unclear how likely life is to evolve in such an exotic potential habitat. That’s a question for astrobiologists.”