Water Worlds and Ocean Planets
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Larger and jumbo images.
While more than two-thirds
of Earth's surface is covered
with liquid water, some
planets may lack dry land
altogether (more on image
from Rosetta and APOD).
On April 18, 2013, astronomers working on NASA's Kepler Mission announced their discovery of two planetary systems that appear to host three super-Earth-size planets (Kepler 62e, 62f, and 69c) in habitable-zone orbits, where the surface temperature of the planet may be suitable for liquid water. Although the Kepler-62 system has five detected planets, the Kepler-69 system has only two thus far. Kepler 62 is a K-2 dwarf star that is smaller, cooler, and only a fifth as bright as our Sun, Sol, while Kepler 69 is a Sun-like G-type dwarf with perhaps 93 percent of Sol's mass and 80 percent of its brightness. Kepler-62f is only 40 percent larger in diameter than Earth, making it the extra-Solar planet closest to Earth's size detected in the habitable zone of another star and is likely to have a rocky composition. Similarly, Kepler-62e orbits on the inner edge of its star's habitable zone and is roughly 60 percent larger than Earth. Modelling suggest that both 62e and 62f may be "water worlds" with global oceans. Kepler-69c may be around 70 percent larger than Earth, and its orbit of 242 days around its Sun-like star resembles that of Venus in the Solar System (NASA news release; and Kepler news release).
ARC, JPL, CalTech, NASA
Larger and jumbo illustrations
(from left: Kepler-69c, 62f, 62e, and Earth).
The Kepler Mission has
found three more super-Earths in
in habitable-zone orbits (more).
On May 8, 2012, astronomers working with NASA's Spitzer Space Telescope revealed that they had directly detected infrared light from the innermost planet "e" around 55 Cancri A. They were able to determine that the tidally-locked planet is relatively dark and that its star-facing side is heated to more than 2,000 Kelvin (3,140 degrees F or 1,730 degrees C). The new observations indicate that the planet is composed of some 20 percent in light elements, which supports the hypothesis that the planet is a "water world," where a large rocky core is surrounded by a layer of very hot water under tremendous presssure in a "supercritical" state (where it is both liquid and gas) under a layer of steam -- similar to GJ 1214 b (NASA science news and video).
Larger and jumbo illustrations in visual
and infrared light (bottom).
New observations of 55 Cnc Ae's infrared
light (which the planet radiates relatively
brightly compared with its parent star)
support the hypothesis that this super-Earth
also is a water world (more).
A Range of Water Worlds
Although some 70 percent of the Earth's surface is covered by water, some astronomers have been using the terms "water world" or "ocean planet" for planets that have an even higher proportion of water (solid or liquid) relative to the composition of the entire planet, so that they have a low density. An important concern has been distinguishing between large planets with thick hydrogen-helium gas envelopes from worlds with similar densities due to a high water content (Adams et al, 2008). In addition, an ocean world's suitability for habitation by Earth-type life is limited if the planet is completely covered by liquid water at the surface, even more restricted if a pressurized, solid "ice" layer is located between the global ocean and the heavier elements and minerals of the lower rocky mantle.
Rocky ("terrestrial") planets like the Earth with
a silicate mantle around a metallic core should
exude water from their cooling mantle (a "magma
ocean" that solidifies from the bottom up) and
even more by outgassing steam which falls back
to the surface to form seas and oceans of water,
as the planet cools within the first 150 million
years after formation (more).
Unlike the case for carbon, the outer Solar System can only have provided as much as 10 percent of the surface water found on Earth given the lower proportion of deuterium found in Earth's water. Rocky ("terrestrial") planets like the Earth are thought to develop within the relatively hotter, inner region of a circumstellar dusk disk around their host star. These planets form with a hot silicate mantle around a metallic core. Mathematical modelling indicates that planets with one to three percent water content should exude water onto its surface as its mantle cools (where this planetary "magma ocean" solidifies from the bottom up). With a water content of just 0.01 percent, on the other hand, a rocky planet similar to the Earth in elemental composition should outgas enough steam into the atmosphere to fall out as rain to form seas, if not oceans, of water as the planet cools within the first tens of million years after formation. Under this model of ocean formation, rocky planets with 0.5 to five Earth-masses are likely to form oceans with the first 150 million years after formation. In the Solar System, however, only the Earth was large and lucky enough to have sufficient mass to gravitationally hold on to its vast oceans of water within the star's warm "habitable zone" for over four billion years and foster the development of its Earth-type life (Linda T. Elkins-Tanton, 2010; and Steve Nerlich, Universe Today, November 20, 2010 -- also in Astrobiology, March 4, 2011).
NASA (Clockwise from top left: Io, Europa, Callisto, and Ganymede -- larger image)
Many of the large moons of gas giants like Jupiter and Saturn formed with a high
water content, whose crust would liquify at the surface if such gas giants and their
moons were to migrate sufficiently inward into the habitable zones of their host
stars. Except for Io, the other three large Galilean moons all have a layer of ice
and possibly liquid water surrounding their rocky cores. Ganymede, like Europa,
has a very thin atmosphere of oxygen, while Callisto appears to have its oxygen
locked up in ice and rocks as its atmosphere is composed of mostly of carbon
While water worlds can be "super-Earths," such worlds can also include smaller planets and even moons of massive that formed out of abundant ices -- like water (H2O). ammonia (NH3), and carbon dioxide (CO2) -- outwards of the snow (or ice or frost) line but later migrated inwards into their host star's habitable zone (Marc J. Kuchner, 2003; Alain Léger et al, 2004; Ben Mathiesen, PhysOrg.com, February 2, 2007; and short discussion and illustration at the Institut d'Astrophysique Spatiale). At the lower bound of planetary mass, a water world may only be limited by the gravitational pull needed to hold on to its atmosphere and surface water. Ice-rich planets that have migrated inward into orbit too close to their host stars may develop thick steamy atmospheres but still retain their volatiles for billions of years, even if their atmospheres undergo slow hydrodynamic escape (Kennedy et al, 2008; and Marc J. Kuchner, 2003).
Super-Earths can form with sufficient ices to become water worlds, but such planets would have to be constrained to around 10 Earth-masses to avoid forming a thick, hydrogen-helium atmosphere. Assuming an iron-rich planet with an internal structure like Earth, modelling results for the first discovered super-Earth (GJ 876 d) suggest the existence of a minimum threshold in planetary diameter above which a super-Earth "most certainly" has a high water content (where thick layers of water and pressurized ice surround a rocky mantle and core), which would be around 24,000 kilometers (or nearly 15,000 miles) in the particular case of GJ 876 d (Valencia et al, 2007). Given the same mass, water worlds are around 40 to 50 percent larger than rocky planets (Fortney et al, 2007).
Water worlds may be most likely type of super-Earth to be potentially habitable for photosynthesis-based Earth-type life (von Bloh et al, 2009 and 2008). In the case of Gliese 581 d, modelling simulations indicated that a planet with up to 8.4 Earth-masses has a sufficient amount of volatiles that it builds up a much denser atmosphere than an Earth-size planet, which prevents the atmosphere it from freezing out due to tidal locking in an inner orbit around a dim red dwarf. One 2008 study was optimistic that simple microbial life could have developed on Gl 581 d, although "adverse environmental conditions" might prove too much for complex life (von Bloh et al, 2008). As photodissociation of water can produce oxygen molecules (O2) as in Earth-like planets, however, atmospheric accumulation of oxygen without the oxidation of rocks through soil weathering and oxidation of volcanic gases indicate that the detection of O2 would not be a reliable signature of Earth-type life (Alain Léger et al, 2004).
By comparison, GJ 1214 b orbits its similarly dim red dwarf star at an even closer distance than Gl 581 d and so the planet is thought to be quite a bit hotter. This planet's discoverers were able to calculate its radius as well as its mass, which they determined are "consistent with a composition of primarily water enshrouded by a hydrogen–helium envelope that is only 0.05 percent of the mass of the planet" and is steamy hot and has been losing mass to space over the lifetime of the planet (Charbonneau et al, 2009). Despite a smaller mass than Gl 581 d, GJ 1214 b is also likely to have a very hot but dense steam atmosphere around a rocky core (with possibly a hot ocean of water and ice layer under crushing pressure in between). Model simulations for planets in tidally locked, synchronous orbits (where the same side of the planet is warmed by the host stars, such as dim red dwarfs) indicate that heat transfer by winds can efficiently reduce the temperature differential between the perpetual day and night sides of such planets (Merlis and Schneider, 2010).
Potential Water Worlds
0.0 G2 V Earth 1.0 1.0 1.0 0.95-1.37
365.24 1.0 0.017 59 15
20.4 M2.5 V Gl 581 d 7.1-13.8 =>1.5 0.22 0.11-0.28 66.9 0.183 ~0 cold? cold?
40.9 G8 V 55 Cnc Ae ~8.6 2.05 +/- 0.15 0.016 0.57-1.13 0.74 0.002 0.57 >3,100 >1,700
42.1 M4.5 V GJ 1214 b 5.57-7.53 2.53-2.74 0.014 ~0.1-0.2 1.6 0.0044 <0.27 >540 >280
~1,200 K2 V Kepler-62e >1 1.61 0.427 0.4-1.0? 122.4 0.335 0.10 86 30
~1,200 K2 V Kepler-62f >1 1.41 0.718 0.4-1.0? 267.3 0.732 0.15 -19 -28
? G? KOI 877.01 6-40 2.6 <0.1 ? 6.0 0.016 ? hot hot
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