Earth Sky | A livable water world doesn't have to be like Earth
Earthly life requires water. So scientists contemplating life outside our solar system have traditionally thought of watery exoplanets, similar to our own aquatic world. An exoplanet that is similar to Earth – in terms of atmosphere, distance to stars, mass, etc. – suggests a possible habitable world. But scientists said in late June 2022 that long-term liquid water would not have occurs under conditions similar to Earth. Exoplanets can be a little bigger and may not even be located near a star!
The new study comes from researchers at the University of Bern, the University of Zürich and PlanetS’s National Center for Competence in Research (NCCR). This suggests that several different exoplanets from Earth may be able to retain liquid water for billions of years. This study focuses on the super-Earth world, with a thick primordial atmosphere of hydrogen and helium. Earth also had a hydrogen-helium atmosphere shortly after it formed, billions of years ago.
The researchers published their peer-reviewed findings in the journal Natural Astronomy on 27 June 2022.
Liveable water world
Since the search for exoplanets began decades ago, astronomers have focused on finding rocky planets like Earth. A world that could have lakes and oceans. As logic seemed to dictate, it would be the world most likely to support life. But now, new research is showing that even a planet so different from Earth in many ways could still have oceans… and possibly life.
On Earth, water remains liquid in part due to the greenhouse effect of the atmosphere. Although there are still areas of ice, such as at the poles, lakes and oceans remain largely liquid. This has allowed life to thrive on our planet. Study co-author Ravit Helled at the University of Zürich, said:
One of the reasons why water is liquid on Earth is its atmosphere. With its natural greenhouse effect, it traps the right amount of heat to create the right conditions for oceans, rivers and rain.
Ancient atmosphere and liquid water
But several billion years ago, when Earth first formed, it was a very different world. The atmosphere was very different from what it is today, as Helled notes:
When the planet first formed from cosmic gas and dust, it accumulated an atmosphere composed mostly of hydrogen and helium, which is called the primordial atmosphere.
On Earth, that atmosphere was gradually transformed by life into an atmosphere composed mainly of nitrogen and oxygen. However, on some other planets, especially the larger and more massive, the primordial atmosphere could persist for billions of years. However, as it happens, the atmosphere can also create a greenhouse effect. Can some of the planets support liquid water?
hell said:
Such a massive primordial atmosphere could also have caused a greenhouse effect, much like Earth’s atmosphere today. We therefore wanted to find out if this atmosphere could help create the necessary conditions for liquid water.
How did a non-Earth-like planet become a habitable water world
The researchers ran simulations of many different possible planetary environments. The computer model takes into account a variety of factors, including the composition of the atmosphere, the amount of radiation from the host star and the planet’s internal heat. For a planet with a massive primordial atmosphere, they found something interesting. Some of the planets can indeed have liquid water on their surfaces. The paper’s lead author, Marit Mol Lous, explains:
What we found was that in most cases the primordial atmosphere was lost due to intense radiation from stars, especially on planets close to their stars. But in cases where the atmosphere persists, the right conditions for liquid water can occur. In cases where sufficient geothermal heat reaches the surface, radiation from a star such as the sun is not even necessary so the conditions prevailing at the surface allow the presence 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.
Implications for habitability and life
If there really is liquid water in such a world for billions of years, that seems promising for the prospects for life. Our own planet is teeming with millions of different types of life, which began to evolve in just the last four to five billion years. What could possibly happen on a planet with liquid water longer than that? According to study co-author Christoph Mordasini:
Since the availability of liquid water is likely a prerequisite for life, and life may take millions of years to appear on Earth, this can greatly broaden the horizons for the search for alien life forms. Based on our results, it could even appear on so-called free-floating planets, which do not orbit around stars.
However, there are still many questions to be answered. Mordasini continued:
While our results are interesting, they should be considered 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 it is. And even under the right conditions, it’s unclear how likely life would be to emerge in such a potentially exotic habitat. That is a question for astrobiologists. However, with our work, we show that our Earth-centered idea of a life-friendly planet may be too narrow.
Bottom line: A new study from researchers in Europe shows how a potentially habitable water world doesn’t have to be like Earth. The results could help broaden the search for alien life.
Source: Potential long-term habitable conditions on planets with ancient H–He atmospheres
Through the University of Bern
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