Planets With Large Oceans are Probably Common in the Milky Way

Within our Solar Systems, there are several moons where astronomers believe life could be found. This includes Ceres, Callisto, Europa, Ganymede, Enceladus, Titan, and maybe Dione, Mimas, Triton, and the dwarf planet Pluto. These “ocean worlds” are believed to have abundant liquid water in their interiors, as well as organic molecules and tidal heating – the basic ingredients for life.

Which raises the all-important question: are similar moons to be found in other star systems? This is the question NASA planetary scientist Dr. Lynnae C. Quick and her team from NASA’s Goddard Space Flight Center sought to address. In a recent study, Quick and her colleagues examined a sample of exoplanet systems and found that ocean worlds are likely to be very common in our galaxy.

Their study, “Forecasting Rates of Volcanic Activity on Terrestrial Exoplanets and Implications for Cryovolcanic Activity on Extrasolar Ocean Worlds“, recently appeared in the journal Publications of the Astronomical Society of the Pacific. Joining Dr. Quick were researchers Aki Roberge of NASA Goddard, Amy Barr Mlinar of the Planetary Science Institute (PSI), and Matthew M. Hedman – a physicist from the University of Idaho.

For the sake of their study, Dr. Quick and her team considered whether other systems in the galaxy might also have planets with interior ocean that are geologically active. This would result in plume activity that future exoplanet-hunting telescopes might be able to detect. As Dr. Quick explained in a recent NASA press release:

“Plumes of water erupt from Europa and Enceladus, so we can tell that these bodies have subsurface oceans beneath their ice shells, and they have energy that drives the plumes, which are two requirements for life as we know it. So if we’re thinking about these places as being possibly habitable, maybe bigger versions of them in other planetary systems are habitable too.”

While current telescopes are not sophisticated enough to detect plumes on exoplanets, Dr. Quick and her team began conducting a mathematical analysis in 2017 to see how likely extrasolar ocean worlds are. To do this, they selected 53 exoplanets that are similar in size to Earth, though some were up to eight times more massive. They then sought to determine how much energy each could be generating and releasing in the form of heat.

Using the Solar System’s ocean worlds as a template, the sources of this heat could come down to one of two possibilities. First, there’s the slow decay of radioactive materials in a planet’s crust and mantle (aka. radiogenic heat). Second, there’s tidal force, where the gravitational pull of another object causes the interior of the planet to flex and stretch, thereby generating heat that needs an escape route.

In the case of terrestrial (rocky) planets like Earth, this heat is released through the crust and mantle in the form of volcanism and plate tectonics. In the case of moons like Europa, Enceladus, Triton, et al. it results in cryovolcanism (where water breaks through the icy surface to form plumes), or through the migration of the icy crust. Either way, knowing how much heat is discharged lets scientists know if the body could be habitable.

For instance, too much volcanic activity can turn the surface of a planet into a molten wasteland while volcanic gases can create a toxic plume of an atmosphere. Too little activity, meanwhile, can lead to a thin atmosphere without enough greenhouse gases, resulting in a cold, barren surface. The same goes for cryovolcanism, where too much can heat an interior ocean that is too hot to support life, and too little will lead to the ocean freezing.

Ultimately, their analysis confirmed that more than a quarter of the 53 exoplanets they sampled (26%, which works out to 14 planets) were likely to be ocean worlds and that the majority of these would be capable of releasing more energy than either Europa or Enceladus. In addition, the team took a look at the TRAPPIST-1 system, which has seven rocky exoplanets that were confirmed by astronomers in 2017.

Multiple studies have been conducted on these planets that indicate that they could be water, which this study supports. According to the calculations of Dr. Quick and her team, TRAPPIST-1 e, f, g and h could all be ocean worlds, which means that 4 among the 14 ocean worlds the scientists identified in this study could be found in just one system.

Given that very few exoplanets have been studied directly (i.e. the Direct Imaging method), the analysis employed in this study is subject to a lot of uncertainties and some assumptions. Still, these and other studies that place constraints on planetary habitability will come in handy when next-generation instruments like the James Webb Space Telescope (JWST) and the Nancy Grace Roman Space Telescope take to space.

As Aki Roberge, a NASA Goddard astrophysicist who collaborated with Quick on this analysis, said:

“Future missions to look for signs of life beyond the solar system are focused on planets like ours that have a global biosphere that’s so abundant it’s changing the chemistry of the whole atmosphere. But in the solar system, icy moons with oceans, which are far from the heat of the Sun, still have shown that they have the features we think are required for life.”

And then there are missions like NASA’s Europa Clipper (which scheduled to launch sometime in the 2020s) which will explore the surface and subsurface of Europa to learn more about its interior. There’s also NASA’s Dragonfly mission, which will travel to Titan in the 2030s to explore the moon’s atmosphere and surface to learn more about its rich prebiotic conditions and organic chemistry.

“Forthcoming missions will give us a chance to see whether ocean moons in our solar system could support life,” says Quick, who is a science team member on both of these missions. “If we find chemical signatures of life, we can try to look for similar signs at interstellar distances.”

Between analysis that can narrow the search for potentially habitable exoplanets, and missions to potentially habitable bodies in our Solar System, scientists are much more likely to be able to find life beyond Earth and our Solar System. And that life, according to these most recent findings, could be quite plentiful – be it extra-terrestrial or extrasolar!

Further Reading: NASA, Astronomical Society of the Pacific