July 11th 2023.
The chances of finding life on other planets have been given a boost as new research suggests that many more exoplanets in the Milky Way may have liquid water than previously thought. Water is essential for life to occur, and so the focus of scientists searching for it has been on planets where it could feasibly form in a liquid state.
Up until now, this has been in the circumstellar habitable zone, the region around stars that is warm enough to maintain liquid water, and more specifically, the CHZ around M-dwarf stars. These common stars, which make up around 70% of the stars in the Milky Way, are cooler than our Sun and are where most known exoplanets orbit.
However, according to lead author Lujendra Ojha, the chances of finding life beyond the CHZ have now increased significantly due to two main mechanisms of liquifying water.
"As Earthlings, we are lucky at the moment because we have just the right amount of greenhouse gases in our atmosphere to make liquid water stable on the surface," said Dr Ojha. "However, if Earth were to lose its greenhouse gases, the average global surface temperature would be approximately -18 degrees Celsius, and most surface liquid water would completely freeze. A few billion years ago, this actually happened on our planet and surface liquid water completely froze."
Dr Ojha and his team highlighted two main ways in which liquid water can be maintained outside of the CHZ. Firstly, heat generated from radioactivity deep in the Earth can warm water enough to keep it liquid, a phenomenon that can be seen in places like Antarctica and the Canadian Arctic. Secondly, the gravitational effects of nearby planets and moons can churn the water on other bodies, an effect similar to the Moon's on tides, but much stronger.
"There is even some evidence to suggest that this might be even happening currently in the south pole of Mars," said Dr Ojha.
The findings suggest a 100-fold increase in the chances of finding water – and possibly life – in our galaxy. According to Dr Ojha, "We modelled the feasibility of generating and sustaining liquid water on exoplanets orbiting M-dwarfs by only considering the heat generated by the planet. We found that when one considers the possibility of liquid water generated by radioactivity, it is likely that a high percentage of these exoplanets can have sufficient heat to sustain liquid water – many more than we had thought."
"Before we started to consider this sub-surface water, it was estimated that around one rocky planet every 100 stars would have liquid water. The new model shows that if the conditions are right, this could approach one planet per star. So we are a hundred times more likely to find liquid water than we thought."
The implications of these findings are huge. With around 100 billion stars in the Milky Way Galaxy, the odds are looking good for the possibility of life existing elsewhere in the universe. Both Nasa and the European Space Agency have missions bound for Jupiter’s icy moons, Clipper and Juice respectively, and these will arrive in 2030 to further explore the possibility of life in our solar system.
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