Liquid water on rocky planets may be 100 times more likely

It’s easy to think of Earth as a water world, with its vast oceans and beautiful lakes, but compared to many worlds, Earth is particularly wet. Even the icy moons of Jupiter and Saturn have much more liquid water than Earth. Earth is unusual not because it has liquid water, but because it has liquid water in the warm habitable zone of the Sun. And as a new study in Nature Communications shows that Earth may be even more unusual than we thought.

Water is one of the most common molecules in the universe. Hydrogen is the most abundant element in space, and oxygen is readily produced as part of the stellar CNO fusion cycle. So we would expect water-rich planets to be abundant in star systems. But this does not mean that liquid water will be abundant. In our solar system, two types of worlds have liquid water. Earth and gas giant moons.

Like other warm terrestrial planets such as Venus and Mars, Earth had liquid water in its youth. Mars was too small to hold its water. Much of it evaporated into space, while some froze in its surface crust. Venus was large enough to hold water, but its extreme heat evaporated much of it in its thick atmosphere. We’re still not entirely sure how Earth managed to hold its oceans, but it was probably a combination of a strong magnetic field and an extra dose of water from asteroids and comets during the heavy bombardment period.

The icy moons of Jupiter and Saturn are another story. They were far enough from the Sun to hold water from their formation. They quickly formed a thick layer of ice to prevent water from evaporating into space. But these moons are small worlds and would freeze very quickly were it not for the tidal forces exerted by their gas giant.

Because planets with cold gas are likely to have icy moons, the general consensus is that we are much more likely to find life on a Europa-like world than on an Earth-like one. But this new study differs. He argued that liquid water is much more likely to be found on super-Earths.

Super-Earths span a mass range from several Earth masses to the mass of Neptune. For the most part, they are likely to be gaseous worlds with dense atmospheres. At the smaller end, they are likely to be more Earth-like. Based on the exoplanets we’ve discovered so far, super-Earths are the most common. And most of them are likely to be outside their star’s habitable zone in the cold regions of the star system. So they are probably rich in water. But they’re also unlikely to be found orbiting a gas giant, so it’s generally assumed that their ice sheet will be mostly frozen over time.

The reason is related to the different freezing and melting points of ice. The kind of ice we have on Earth melts at about 0 °C. But this is only true at around Earth’s atmospheric pressure. At higher pressures, there are several varieties of ice with different melting points. Although it’s a bit complicated, generally at higher pressures ice can have a much higher melting point. So even if the super-Earth is geologically active, it may not be warm enough to melt the ice.

This new study shows that super-Earths don’t have to be hot enough to create a deep ocean. Through geothermal and nuclear heating, it can melt a thin layer of water on its surface, and thanks to cracks and various water phase transitions, water can creep up to the layer just below the frozen surface. This process would be sufficient to create a rich oceanic layer of liquid water. Because the heat of the super-Earth lasts for billions of years, it can support a liquid ocean long enough for life to develop.

Based on what we know about exoplanets, super-Earth oceans may be 100 times more common than those on Earth-like worlds or icy moons. And that means life has even more possible homes than we thought.

Reference: Ojha, Lujendra et al. “Liquid water on cold exo-Earths by basal melting of ice sheets.” Nature Communications 13.1 (2022): 7521.