Artist's conception of Kepler-22b, a possible ocean world. Image credit: NASA/Ames/JPL-Caltech

Aug 7, 2015 Ocean Planets in the Habitable Zone: Probably Not Habitable

Sen—With nearly two thousand exoplanets confirmed, we're learning that some very common types of exoplanets are absent from our own Solar System. Ocean planets, covered in global oceans potentially 100 kilometers deep, are less foreign to our Solar System than hot Jupiters or super-Earths. Jupiter's moons Europa and Callisto very likely have global oceans beneath their icy surfaces. If Europa or Callisto had migrated in towards the Sun their surfaces could have melted, resulting in something very close to the ocean planets which are still mostly unknown. So far we have confirmed only two planets with measured densities that suggest a large water content—Kepler-22b and GJ 1214b.

While the masses of ocean planets might be similar to Earth, a larger ratio of water to rock would make the planets larger than Earth. They might be completely covered by a global ocean with over 100 times our total ocean volume. Simulations of planet formation suggest that ocean worlds could be relatively common. They would form in the outer parts of a solar system where water and other ices are common and then later migrate inwards where the ice would melt into liquid water.

An ocean planet, since it needs liquid water on the surface, must fall within the "classic" habitable zone (HZ). The exact location and size of the HZ depends mainly on the amount of energy radiated by the star and the long-term composition of a planet's atmosphere. The atmospheric composition determines how much of the stellar energy is reflected or absorbed by the planet and how much insulation (greenhouse effect) the atmosphere provides for the surface.

Carbon dioxide is one of the most important greenhouse gases we observe in our Solar System, second only to water vapor, so many models of planetary atmospheres tend to focus on carbon dioxide as the main insulator. Despite formally being in the HZ, too much insulation will make a planet surface too hot (like Venus), while too little insulation will make the planet surface too cold (like Mars). Neither of those surfaces is particularly hospitable for life as we know it.

But the "classic" HZ may not apply for ocean planets. There is one important assumption that goes into the HZ models used by astronomers everywhere: That the level of carbon dioxide in the atmosphere of the planet is kept constant through the carbonate-silicate cycle. The carbonate-silicate cycle on Earth works to regulate the amount of atmospheric carbon dioxide through the interaction of our atmosphere, our oceans, and our plate tectonics.

The cycle requires that plate tectonics are active on the exoplanet, that is, that the planetary crust is broken up into pieces that shift around on top of a molten mantle. This is definitely not a guarantee everywhere; of the rocky planets in our Solar System, only Earth has active plate tectonics. The carbonate-silicate cycle also requires that the atmosphere be in contact with the rocky surface and that the rocky surface be in contact with the ocean floor. This is where ocean planets throw a wrench into the standard models.

Ocean planets most likely still have a rocky core and crust, but the pressure at the bottom of the deep ocean would cause a layer of high pressure ice to form. This ice layer would separate the planetary crust from the liquid water, thus breaking the all-important carbonate-silicate cycle. While some people postulate that so-called "ice tectonics" could still facilitate the transport of carbon dioxide between ocean and crust through the ice layer, this phenomenon is relatively unstudied and so cannot be counted on to regulate planetary climate.

However, all is not lost even without the carbonate-silicate cycle. A more sluggish, but still effective, ocean carbon cycle exchanges carbon dioxide directly between the atmosphere and the ocean. This process also operates on Earth and likely would on ocean planets, as well. In this case, the amount of carbon dioxide in the atmosphere is determined by the average surface temperature of the planet and the ocean volume. In an equilibrium state, where the amount of atmospheric carbon dioxide remains steady, carbon dioxide gets dissolved into ocean water at the same rate it is being evaporated into the atmosphere.

The ocean carbon cycle is not ideal, but it is not a lost cause either. Daniel Kitzmann of the University of Bern, Switzerland and his co-authors performed a detailed study of the conditions under which an ocean planet would remain habitable around a Sun-like star for six billion years. Since the atmospheres and conditions on ocean planets are completely unknown, they made a few, not unreasonable assumptions: That there is no transport of carbon dioxide through the high pressure ice layer at the ocean floor, that the total amount of carbon dioxide on the planet remains constant, that the planet has an Earth-like mass, and that the global ocean is the same temperature everywhere. These assumptions let them calculate the most optimistic cases for an ocean world around a Sun-like star.

Kitzmann and co-authors first calculate the amount of carbon dioxide dissolved into the liquid ocean at changing temperatures, also taking into account the change in ocean volume with temperature. It was this first step in the analysis which showed that the ocean carbon cycle would have a positive feedback on the planet, rendering most ocean planets inhospitable in a short time.

They then worked toward finding the conditions that would let the ocean planet remain habitable for six billion years, which is most of the star's life. Here they also take into account that a star's energy output increases over its lifetime. They tested a wide range of possible carbon dioxide levels, from values below Earth's carbon dioxide content to values higher than Venus's. They found that the long-term habitability of an ocean planet is highly restricted by the total carbon dioxide content of the planet, and that carbon dioxide levels near Venus's is more preferred than Earth's. A slight nudge away from the perfect carbon dioxide level leads to inhospitable planets.

Balancing the ocean carbon cycle is much more difficult than the carbonate-silicate cycle because the ocean carbon cycle has a positive feedback on itself. If there is a slight change in one direction, the cycles exacerbates the change and eventually results in a runaway effect. The ocean volume is determined by surface temperature: When the temperature is hotter more of the water is evaporated into the atmosphere, leaving a smaller ocean volume. A hotter temperature also means the water is less efficient at dissolving and storing the carbon dioxide. Both of those effects contribute to the positive feedback.

Let's say, for example, the temperature is slightly too hot. A hotter temperature leads to a smaller ocean volume, which leads to less carbon dioxide being absorbed into and stored in the ocean. Since the ocean stores less carbon dioxide, there must be more carbon dioxide in the atmosphere, which produces a greenhouse effect that raises the surface temperature even more. This perpetuates the positive feedback until the ocean planet is in a runaway greenhouse effect, loses all of its water, and is inhospitable. An ocean planet can just as easily spiral into a runaway snowball effect if the surface temperature falls too low.

Even at the perfect level of carbon dioxide there is only a small range of orbital distances from the star where an ocean planet will retain liquid water on the surface for six billion years. This region is much smaller than the "classic" HZ defined for planets with the benefit of a carbonate-silicate cycle. While Venus levels of carbon dioxide are preferred, they are only effective if the ocean planet orbits at twice the orbital distance of Venus (almost as far as Mars). If Earth had to rely only on the ocean carbon cycle with our present level of atmospheric carbon dioxide we would soon evolve to a runaway greenhouse effect and no longer be habitable. Venus would have lost that race even sooner.

The moral of this story is that "habitable zone" does not automatically translate to "hospitable for life as we know it." To understand the habitability of planets unlike our own, we need to have a better understanding of how atmospheric chemistry works without the benefit of the carbonate-silicate cycle. There is much more to the story of what makes a planet habitable than just the surface temperature, and the "classic" HZ may not always apply (hint:Kepler-452b). In the future we might study Europa to learn more about ice tectonics, and future telescopes like the James Webb Space Telescope might teach us more about the atmospheres of ocean planets.



The shaded green region shows the combinations of carbon dioxide content and orbital distance that leads to long-term habitable ocean planets. Blue lines show the orbital distance corresponding to the freezing point of water at a particular carbon dioxide content, and red lines show the same for the boiling point. Limits are shown for past, current, and future amounts of Solar energy, and Venus's position is indicated for reference. Image credit: Kitzmann et al (2015).