Ocean worlds around red dwarfs … common?
We all want to know if there’s life elsewhere in the universe. So, naturally, we’d also like to know how common Earth-like worlds with liquid water are. On July 11, 2023, Lujendra Ojha of Rutgers University announced that ocean worlds around red dwarf (aka M-dwarf) stars may be 100 times more common than scientists previously believed. Ojha presented his new paper at the Goldschmidt Conference in Lyon, France. The study focused on planets orbiting red dwarf stars, the most common stars in our galaxy. And the findings could even apply to planets outside of the habitable zones of their stars.
The researchers first published their peer-reviewed results in Nature Communications on December 6, 2022. You can also read the presentation abstract of the new paper on the Goldschmidt Conference website.
Ocean worlds may be common
The study showed that despite the harsh conditions around red dwarf stars, liquid water can still exist on rocky exoplanets. It might exist in locations that scientists hadn’t been thought as much about, such as underground. Ojha said:
We know that the presence of liquid water is essential for life. Our work shows that this water can be found in places we had not much considered. This significantly increases the chances of finding environments where life could, in theory, develop.
Overall, Ojha said that the chances of a rocky exoplanet orbiting a red dwarf star having liquid water is much greater than scientists first thought. About 100 times greater, in fact. As Ojha noted:
Before we started to consider this subsurface water, estimates were that about 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 100 times more likely to find liquid water than we thought. There are around 100 billion stars in the Milky Way galaxy. That represents really good odds for the origin of life elsewhere in the universe.
Some of these planets may be eyeball planets. Those are hypothetical planets where most of the planet is frozen on the surface except for a patch of liquid water on the side of the planet facing its sun. The Goldschmidt Conference abstract states:
Another notable, common feature of these planets is tidal locking, possibly leading to an eyeball-like climate state, where most of the planet is frozen, with the exception of the substellar point [subsolar point], where liquid water may exist. In such cold, icy, rocky planets, basal melting may provide an alternative means of forming liquid water in a subsurface environment shielded from high-energy radiation. Basal melting is responsible for the formation of subglacial liquid water lakes in various areas of Earth.
Exo-Earths similar to icy moons
Red dwarf stars are known for being very volatile. They emit massive amounts of solar radiation that can be deadly for any life that may exist on an orbiting planet. That radiation can even strip a planet of its atmosphere if it is too close to its star. Yet even those planets, or ones that are frozen on the surface, might still be able to hold onto liquid water … on the inside. This would make them similar to the icy moons in the outer solar system, like Europa and Enceladus. Frozen on the outside, but with oceans beneath the icy surface. Ojha said:
Some of the moons you find in the solar system (for example, Europa or Enceladus) have substantial underground liquid water, even though their surfaces are completely frozen. This is because their interior is continually churned by the gravitational effects of the large planets they orbit, such as Saturn and Jupiter. This is similar to the effect of our moon on tides, but much stronger. This makes the moons of Jupiter and Saturn prime candidates for finding life in our solar system, and many future missions have been planned to explore these bodies.
Interior heat could sustain subsurface oceans
We modeled 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.
The paper again references basal melting, saying:
On such cold, icy exo-Earths, basal melting of regional/ global ice sheets by geothermal heat provides an alternative means of forming liquid water … We show that even with a modest, moon-like geothermal heat flow, subglacial oceans of liquid water can form at the base of and within the ice sheets on exo-Earths.
Earth as an analog for ocean worlds
Even though the Earth is substantially different from these smaller icy moons, it also serves as an analog for what could happen to exo-Earths. As Ojha explained:
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. However, if Earth were to lose its greenhouse gases, the average global surface temperature would be approximately -18 degrees Celsius (-0.4 F). Most surface liquid water would completely freeze. A few billion years ago, this actually happened on our planet, and surface liquid water completely froze.
However, this doesn’t mean that water was completely solid everywhere. For example, heat from radioactivity deep in the Earth can warm water enough to keep it liquid. Even today, we see this happening in places like Antarctica and the Canadian Arctic. There, despite the frigid temperature, large underground lakes of liquid water exist, sustained by the heat generated from radioactivity. There is even some evidence to suggest that this might be happening currently in the south pole of Mars.
A separate study from scientists in Japan last year also made the case for plentiful ocean worlds around red dwarf stars. The scientists estimated that about 10% of rocky planets in the habitable zone of red dwarfs could have oceans.
And yet another study from 2022, from researchers at the University of Chicago, also supports the existence of such worlds.
Bottom line: A new study says that the chances of ocean worlds – rocky planets with liquid water – orbiting red dwarf stars is much greater than previously thought.