In the search for habitable exoplanets that can harbor life like Earth, liquid water has played a key role in signaling the possibility of life. While scientists have searched for bodies of water similar to those found on Earth, a study suggests that liquid water could exist on the surface of exoplanets for billions of years under other conditions, too. Researchers from the University of Bern, the University of Zurich and the National Center of Competence in Research (NCCR) Planets have explained why the search for habitable exoplanets may require a different approach than the current one.
According to Ravit Helled, professor of theoretical astrophysics at the University of Zurich and co-author of the study, “One of the reasons water on Earth can be liquid is the atmosphere.” He said Earth’s natural greenhouse effect traps the right amount of heat to develop favorable conditions for oceans, rivers and rain.
When the Earth was formed, the atmosphere was mainly made up of hydrogen and helium, which is called a primordial atmosphere. While Earth lost this atmosphere over time, some larger planets may retain it indefinitely.
“Such massive primordial atmospheres can also cause 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,” said Helled.
In their study, published in Nature Astronomy, the researchers modeled numerous planets and simulated their evolution over billions of years. They observed not only the properties of the atmosphere, but also the radiation intensity of the stars of the planets and their internal heat that radiated outwards.
“What we found is that in many cases primordial atmospheres were lost due to intense radiation from stars, especially on planets close to their stars. But in the cases where the atmosphere remains, the right conditions for liquid water can arise,” says Marit Mol Lous, PhD student and lead author of the study.
The researcher emphasized that their findings suggest that conditions on the planets can last for up to tens of billions of years. However, another co-author, Christoph Mordasini, said that even if the planets have the right conditions, “it’s unclear how likely life is to develop in such an exotic potential habitat.”