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Planetans - Oceanic Planets

April  9, 2016

In 2004 the team around the astronomers Alain Léger and Franck Selsis proposed a new class of planets, which they called ocean planets (Léger et al. 2004). Such planets have about twice the Earth's radius, but have a lower density than rock planets. Up to 50 percent of their mass may be water. Such bizarre worlds are not to be confused with Earth-like planets that have a vast ocean as deep as the Earth's ocean. The Léger model, however, would differ very drastically from the Earth, as the water layer would be several thousand kilometers thick. Even the liquid part of it could be 100 and more kilometers deep. In order to explore and understand the conditions on such a world, one has to resort to methods of high-pressure physics. So far, only two submarines have dived to depths of 11 kilometers and these two expeditions were technical masterpieces of their time. Would it be possible one day to explore a 100 kilometer deep ocean at all?

Schematic comparison of the Earth, a rocky planet of 6 Earth masses and an ocean planet of 6 Earth masses. Data according to Léger et al. (2004). The most striking feature of the ocean planet is its outer shell of water or ice.

And there is another important difference as compared to Earth. An ocean planet in this sense would have no land surface. The silicatic mantle of the planet would be buried deep underneath the ice. There would be no terrain above sea level. The entire planet would be completely covered by a single, world-wide ocean.

In order to avoid confusion with oceanic planets in the earthly sense, Leonid Ksanfomality from the Russian Academy of Sciences later proposed the name "planetan", a composition of "planet" and "ocean" (Ksanfomality 2014) .

Presumed formation
Léger et al. assumed that such water planets would be fundamentally different from rocky Earth-like planets. They proposed a formation in the outer regions of a protoplanet disk, about 5-10 astronomical units from a Sun-like star, where the planet would accumulate a solid core consisting of rocks and a high proportion of water ice. In addition, the planet would contained carbon dioxide, ammonia, hydrogen and helium. The total mass of the planet should not exceed 8 Earth masses, so that the hydrogen and helium content relative to the other gases remains small. The planets Uranus and Neptune correspond to such an "ice giant" in their inner composition, but are surrounded by a sheath of hydrogen and helium, containing 1 to 4 Earth masses, and the proportion of hydrogen and helium would be lower in a future water planet with a mass limit at 8 Earth masses.

By interactions with the protoplanet disk and other objects of the solar system, the ice planet would migrate towards a lower orbit, possibly even into the habitable zone. During this migration into increasingly warmer zones, it would lose most of its original hydrogen and helium content until only water, carbon dioxide and ammonia would make up the majority of the volatile compounds. Part of the ice layer would melt, a global ocean would form, and the atmosphere would have a high proportion of water vapor.

It is very important for the understanding of a water planet in this case that the separation of water and the mineral constituents already takes place during the hot phase of planetary formation. Very early, a nucleus is formed, while water and the other compounds are initially present in gaseous form. Condensation occurs later. As a result, the ice mantle would be completely above the mineral core.

An Earth-like planet, on the other hand, would accumulate its much smaller water supply only much later in its origin. An often-read idea is that Earth is said to have received its water mainly from comets and crystal water in meteorites. On Earth, the liquid ocean is thus in contact with the solid crust, and continents and volcanic islands protrude beyond the water surface. On a planetan however, there would not be such contact zone.

The phase diagram of water as a key to the nature of planetans
To understand this, we must look at the behavior of water at high pressures and temperatures. At sea level and so-called standard pressure (1 atmosphere or 100 kiloPascal (kPa)) pure water at 0 ° C is changing into ice and at 100 ° C into the vapor phase. However, these transitions can be postponed. For example, if one increases e.g. pressure,  water will still remain liquid beyond 100 ° C. This effect can be exploited, among other things, in steam cookers where, thanks to the sealed lid, the water can still remain liquid some degrees above 100 ° C and thus help with a faster cooking. However, the existence of the liquid phase is limited on the temperature axis: at 374 ° C, the highest temperature is reached at which liquid water can still exist. However, 220 sea level pressures (or 22 MPa) is required to maintain water in this phase. Only some degrees warmer and water will change into what is known as a "supercritical fluid."  In this state the density is still that of liquid water, but as a result of the high temperature, the water molecules are already so free from one another that they move like gas molecules. The point of this transition is called the critical point.

Experimentally, this transition is very impressive. When water is heated in a high-pressure vessel, two phases can be observed below the critical point: boiling water at the bottom and steam above. When the critical point is exceeded, however, the two phases disappear immediately. The supercritical fluid fills the whole boiler, the liquid and gaseous phases no longer exist.

The existence of liquid water is also limited in the other direction of the temperature and pressure scale. At standard pressure, liquid water can still exist in a temperature range of 0-100 ° C. When the pressure drops - for example in a vacuum bell - the water will still freeze at 0 ° C, but will boil at less than 100 ° C. This range becomes narrower as the pressure decreases. At 0 ° C and 6 Pa pressure, finally, the point is reached from which on liquid water will be observed for the last time. At this triple point, ice, liquid water and water vapor can exist side by side. At even lower pressures, ice will no longer melt, but will form water vapour directly. This process is known as sublimation.

When  pressure and temperature are now plotted on the two axes of a graph - the temperature on the X axis and the pressure on the Y axis - the existence of the three phases of  water can be clearly distinguished from each other. The right half of the diagram shows the pressures and temperatures at which water is vapor, whereas the left side is occupied by the ice phase, and in the upper right half of the diagram liquid water will be found.

For the consideration of water planets, however, we are interested in the changes which occur when liquid water is exposed to ever higher pressure but the temperature remains constant. For a start, let us consider the conditions we encounter in the world's oceans.

In the deeper zones of the sea, the temperature of the water is about 4 ° C at most places. This is the temperature at which liquid water has the highest density under standard pressure. Water at 4 ° C will accumulate near the sea floor without any further disturbances (in the world's seas the great currents form a considerable disturbance, as they constantly transport cold water from the edge of the polar regions into the deep sea). Cooling the water further and freezing it into ice, the density of the water decreases at the same time, making an ice cube lighter than the same volume of liquid water. For this reason ice floats on top of the water surface - a very remarkable property of water, vital for the biology of life in oceans, rivers and lakes!

With each meter of sea depth, the pressure increases by about 10 kPa or 1 standard atmosphere. The water at a certain depth carries the common load of all overlying layers of water. Liquid water is difficult to compress because the individual water molecules are already present in a very dense and space-saving arrangement. This changes only at very great depths. 10 km below sea level, the density of the water is about 5% higher than at the same temperature at sea level. Even at the deepest parts of the sea, water is still liquid.

However, with increasing pressure, the arrangement of the water molecules will change. If there were even deeper parts of the ocean, one could observe something strange at a depth of about 63 kilometers. At such depth the pressure of the water column above would press the 4 ° C cold water into another form of ice. But this ice, called ice VI, differs from the ice we know from the surface. Ice VI exists only under high pressure and is denser than liquid water, therefore remains at the seabed! Ice VI would melt at 82 ° C.

Let us stay at the 4 ° C line. At this temperature, ice VI can exist up to a pressure of 2 GPa, corresponding to a sea depth of 200 kilometers. Even higher pressures will cause a new change in the ice crystals, forming ice VII. This kind of ice exhibits an unexpected property: Ice VII is more resistant to heating and will remain in crystalline form even at temperatures above 700 ° C.

At pressures of about 62 GPa or 620 kilometers deep, we would encounter ice X. At least theoretically, even more highly compressed ice forms are possible, but are not tested so well. The triple point of ice X, ice VII and supercritical water is 727 ° C and 42 GPa. In the 2,000-kilometer or even deeper ice layer of a planetan, such ice forms would extend to the rocky bottom. Half of such an ocean planet may be composed of water, but the greatest part of the water would be in such exotic ice forms!

GJ1241b - Model for a hot ocean planet
One of the best candidates to date for an ocean planet is GJ1241b. This planet orbits a cool red dwarf once every 1.6 days. With approximately 2.7 Earth radii and 6.6 Earth masses, the planet's density would be only around 1.9 g / cm3. This would be compatible with a planet consisting of an iron-nickel core, a silicatic shell, and about 50% of its mass in water. The atmosphere was estimated as a few percent of the total mass. It consists of water vapor, carbon dioxide and small amounts of hydrogen and helium. Marcy (2009) indicates a temperature of 190 ° C for the water surface. This ocean would be liquid in the upper layers.

A large uncertainty factor is the composition and mass fraction of the atmosphere. A massive atmosphere around an otherwise rocky body would also lower the medium density of the planet and give the false image of a water world. Bean et al. (2006), however, were able to isolate transmission spectra of the outer atmosphere of GJ1241b. The spectra revealed a structureless cloud cover, which would be compatible with a water vapor atmosphere surrounding the entire planet. The hydrogen content can then only be small, since hydrogen would produce distinct clouds and nebulae in the outer atmospheric layers. But nothing like this was seen.

Hypothetical view of the surface of the water planet GJ1214b. Almost no sunlight could penetrate the dense cloud layers of the steam atmosphere. The only source of light on the surface would be the flashes of gigantic thunderstorms.

OGLE-2005-BLG-390Lb - a frozen water world?
The planet OGLE-2005-BLG-390Lb, discovered via gravitational lensing, offers a completely different example. Its center star is an M-class star with an estimated age of 9-10 billion years. The planet has a mean mass of 5.5 Earth masses and orbits its star at a distance of 2.6 astronomical units. The surface temperature is between -238 and -226 ° C. In the case of a high water content, the ocean would now be completely frozen. About 5 billion years ago, however, the heat released by the decay of radioactive elements would have been sufficient to allow at least an ocean of liquid water under an upper ice cover. In the past, this planet would have been a greater counterpart of Jupiter's moon Europa. (Ehrenreich et al, 2006).

Climatic instability on water planets
On Earth, the carbonate-silicate cycle is a very important factor for climate stabilization. In this cycle carbon dioxide is extracted from the atmosphere in the form of carbonates. At the same time, silicates near the surface are weathered away. However, without a silicate surface above sea level, this cycle does not exist. Instead, only the solubility of atmospheric carbon dioxide in seawater governs overall climate, but in a regulation which is very unfavorable for living creatures. If the temperature of the water rises, it can keep less carbon dioxide dissolved and gives off the excess to the atmosphere. This enhances the greenhouse effect, leading to further rise in temperature, as a result of which more carbon dioxide is emitted. However, if the planet cools, seawater can absorb more carbon dioxide, the greenhouse effect of the atmospheric carbon dioxide will be weakened and temperatures drop. A planetan would be a more unstable environment than a terrestrial planet, with the global ocean vulnarable to either complete evaporation in a runaway greenhouse effect or complete glaciation in the case of a weakend greenhouse effect. The habitability of such planets is not completely excluded, but it would be subject to more severe limitations. (Kitzmann et al. 2015).

In summary, the class of  deep ocean planets or planetans represents a very bizarre category of objects, which still require a lot of research.

Léger, Selsis, Sotin, Guillot, Despois, Mawet, Ollivier, Labèque, Valette, Brachet, Chazelas, Lammer (2004): A new family of planets? „Ocean-Planets". Icarus 169, pp. 499-504
Ehrenreich, Lecavelier des Etangs, Beaulieu, Grasset (2006): On the possible properties of small and cold extrasolar planets: Is OGLE 2005-BLG-390Lb entirely frozen? The Astrophysical Journal 651, pp. 535-543
Bean, Miller-Ricci Kempton, Homeier (2010): A ground-based transmission spectrum of the super-Earth exoplanet GJ1214b. Nature 468, pp. 669-672
Marcy (2009): Water world larger than Earth. Nature 462, pp. 853-854
Ksanfomality (2014): Planetans – Oceanic Planets. Solar System Research, pp. 79-89
Kitzmann, Alibert, Godolt, Grenfell, Heng, Patzer, Rauer, Stracke, von Paris (2015): The unstable CO2 feedback cycle on ocean planets.

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Manche Planeten, deren Radius dem größerer erdähnlicher Planeten entspricht, zeigen eine ungewöhnlich geringe Dichte. Es...

Posted by Exoplaneten on Samstag, 9. April 2016
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