June 21, 2019 : Worlds Like Venus, The Earth, Mars, Io Or Titan Fuel The Analyses Of Planetologists Upon The Concept Of Habitable Zone

The discovery or the identification of a multitude of exoplanets which are planets evolving beyond the Solar System fuels our studies on the potential habitability of several worlds orbiting other stars than the Sun. Planetologists have developed the concept of Habitable Zone in order to identify the exoplanets which may host oceans, lakes, seas or rivers of liquid water. The Habitable Zone represents an area around the star where the environmental temperature is high enough or low enough to allow the potential presence of water in its liquid form on the surface of the planetary body, the planet, the moon or the Dwarf Planet. In the Solar System, Venus, the Earth and Mars are apparently in the Habitable Zone. However, a multitude of factors must be taken into account for the determination of the likelihood of oceans or seas of liquid water on any world. The atmosphere of Venus is remarkably dense and engenders a major greenhouse effect which prevents the development of oceans or seas of liquid water on its surface. The atmosphere of Mars is too thin to allow the presence of pockets, seas or oceans of liquid water on its surface. Moreover, the environmental temperature of Mars is generally too low to allow the presence of liquid water on its surface.

In the Solar System, we have identified many worlds which may contain subsurface oceans of liquid water. One can mention Europa and Ganymede around Jupiter, Enceladus and Titan around Saturn or the Dwarf Planet Pluto beyond Neptune. Tidal forces can play a major role in the development of a subsurface ocean, volcanoes, cryovolcanoes or geysers around Gas Giants. For instance, geysers developing in topographic fractures in the south polar region of Enceladus have been clearly identified during the Cassini-Huygens mission. Io, the famous moon of Jupiter, is by far the most active world in the Solar System in terms of volcanic activity. Tidal forces can have a major impact on the internal activity of many worlds even if those worlds evolve particularly far away from the Sun. The study of the multitude of worlds, planets or moons in the Solar System allows us to better evaluate the potential habitability of the numerous exoplanets we have identified in the Milky Way, in our own galaxy. The concept of Habitable Zone must be developed on the basis of many factors such as the environmental temperature, the atmospheric pressure or the composition of the atmosphere.

Most researchers believe that life can only develop in liquid water since our biosphere is mainly composed of water. That's why we try to identify exoplanets which look like the Earth in terms of mass and density and which evolve in the famous Habitable Zone around their star. Recently, we have identified the presence of an exoplanet around Proxima Centauri, the closest star to the Sun. That exoplanet known as Proxima b is a little more massive than the Blue Planet and evolves in the Habitable Zone of the small star which represents a Red Dwarf. The study of exoplanets has clearly shown that several types of worlds which are not found in the Solar System orbit other stars. For instance, one can find Gas Giants evolving extremely close to their star or even Terrestrial planets which are hot enough to allow the presence of oceans or layers of lava on their surface. That's why we have to try imagine what we can find elsewhere on the basis of our scientific knowledge. Even far away from any star, a world can contain a subsurface ocean of liquid water due to tidal forces, gravitational forces or physical processes.

We can't rule out active processes or a complex chemistry occurring on rogue planets in the Milky Way because tidal forces or gravitational forces can engender a lot of energy and fuel the internal activity of the world located extremely far away from any star. Planetologists are wondering whether an exoplanet like Proxima b can contain a biosphere or a significant atmosphere since the exoplanet is likely to receive relatively strong stellar flares which can represent sterilizing stellar flares. Can the hot Gas Giants host any lifeform in their clouds or cyclones ? The study of the planets, moons or Dwarf Planets of the Solar System allows us to gather clues regarding the chemistry of organics or complex molecules in the Universe. We are in a position to anticipate what we can find in other star systems on the basis of what we discover in the Solar System and on the basis of computer modeling. Varous factors must be seriously considered. Mary Voytek who is the senior scientist for astrobiology at NASA headquarters in Washington, D.C. pointed out :  It's very important to realize, it's not just the location.  She added :  There are many other factors that contribute to establishing habitable conditions.  The Moon which is devoid of any atmosphere is in the Habitable Zone but the world appears completely dead in terms of biology.

The atmosphere of Venus, like the Martian atmosphere, is dominated by carbon dioxide and contains clouds of sulfuric acid rather than clouds of water. Why is that planet so different from the Earth in terms of hydrology ? The conventional concept of Habitable Zone appears weak in that context. Mary Voytek argued :  The Moon is in the habitable zone in our solar system.  She added :  Is it habitable ? It can't retain an atmosphere. It doesn't have water on its surface. It's something in the habitable zone that isn't.  Beyond Mars, several icy worlds like Europa or Enceladus which are not in the Habitable Zone may contain a subsurface ocean where exotic lifeforms may thrive or develop. Tidal heating engendered by Gas Giants is likely to make the interior of some moons habitable. At a larger scale, we also have to consider the concept of Habitable Zone in our galaxy, from the core of the Milky Way to the edge or the limits of the arms, because the concentration of metal or heavy elements must play a key role. There may be fewer rocky worlds in the outer limits of our galaxy than near the galactic core.

The likelihood of supernovae in the stellar neighborhood must be considered as well for the development of any lifeform. The type of star will also play a key role for the environmental temperature at a given distance from the star. The Habitable Zone is particularly close to the star if that star is a Red Dwarf for instance. Planetologists mobilize their knowledge of the Solar System and resort to complex computer models, originally designed to simulate our climate, to simulate the potential climate of various exoplanets. Nancy Kiang who is an astrobiologist at NASA's Goddard Institute for Space Studies in New York advanced :  We're using our understanding from Earth to inform our search for life, for habitability, on other planets.  The models of planetologists like Aomawa Shields of the University of California, Irvine, reveal that small rocky exoplanets evolving around Red-Dwarf stars could have stable climates and could be warm enough for the development of any typical lifeform. That's the case for the seven roughly Earth-sized planets evolving around TRAPPIST-1.

Aomawa Shields has clearly shown, on the basis of computer models, the effects of the stars on the potential habitability of any exoplanet orbiting those stars. At a recent American Astronomical Society conference taking place in Seattle, she pointed out :  Our solar system is not the standard model.  She added :  It's one of many possible configurations we're seeing out there. Some push the boundaries of the traditional habitable zone.  Several exoplanets around TRAPPIST-1 may be in the Habitable Zone. Thanks to future telescopes like the James Webb Space Telescope, we may be in a position to gather clues on the composition, the dynamics or the structure of the atmosphere of large exoplanets like mini-Neptunes or Super-Earths. Regarding the study of Earth-sized exoplanets, we'll probably have to wait for a better telescope due to the limited resolution of the James Webb Space Telescope. However, that telescope will probably be very useful to monitor the evolution of the dynamics or the meteorology of the atmosphere of worlds of the Solar System like Saturn or Titan. Stephen Kane from the University of California, Riverside, who is also a member of the NASA Astrobiology Institute and who is a specialist of Habitable Zones pointed out :  If something looks like a terrestrial planet within the habitable zone, what can we actually say about that ?  He added :  The answer, at the moment, is very little. 

He advanced that the notion of Habitable Zone is  one of the most misunderstood concepts  in planetology and that the development of technology may help us advance at a rapid pace in that field. Currently, that concept represents a key  target selection tool . He argued :  There are many candidates to choose from.  He added :  How do we prioritize our list ? The answer is the habitable zone.  Planetologists have developed an indicator to evaluate the potential for habitability of any world. That indicator represents a graph that contains two parameters, the amount of light the planetary body receives from the star and the speed at which the atmosphere can escape into space. An updated version of the indicator was proposed in 2017 by Kevin Zahnle of the NASA Ames Research Center in Moffett Field, California and David Catling of the University of Washington Astrobiology Program. The vertical axis represents the solar radiation whereas the horizontal axis represents the  escape velocity  of atmospheric particles. A roughly diagonal line in the graph represents the  cosmic shoreline .

The planetary bodies located below the line are more likely to contain an atmosphere whereas the planetary bodies located above the line are less likely to contain an atmosphere. The planetary bodies which are right on the line are likely to have a thin atmosphere like Mars or Pluto. The indicator appears useful to classify exoplanets. Kevin Zahnle argued :  It's certainly a testable hypothesis.  He added :  It would be kind of fun to know if this is actually a (physically based) general feature or just an accident of plotting two unlike things that coincidentally line up.  The study of worlds like Io, Europa, Enceladus, Titan, Triton or Pluto and the better resolution of future telescopes like the James Webb Space Telescope which will be much better than the Hubble Space Telescope will probably allow us to better evaluate the potential habitability of key exoplanets like Proxima b, Kepler-22b or Kepler-69c which evolve in the Habitable Zone of their star. If life can only develop in a liquid environment composed of water, the conventional concept of Habitable Zone is clearly a key indicator in astrobiology.

The image above reveals, from the left to the right, the Earth, Io and Titan at scale. Titan which is the largest moon of Saturn, Io which is a major moon of Jupiter and the Earth represent worlds containing liquid surfaces. Oceans of liquid water dominate the surface of our planet. Lakes, seas and rivers of methane and ethane can be found in the high latitudes of Titan. Io appears to be the most volcanically active world in the Solar System due to particularly strong tidal forces involving Jupiter and the other moons of the Gas Giant. Io also contains lakes but those lakes are made of lava. Those worlds reveal very different environments and illustrate the potential diversity of exoplanets. Credit for the original view of the Earth: DSCOVR. Credit for the original view of Io: NASA/JPL/University of Arizona. Credit for the original view of Titan: NASA/JPL-Caltech/Space Science Institute. Credit for the montage: Marc Lafferre, 2019.

- To get further information on that news, go to: https://exoplanets.nasa.gov/news/1583/oceans-beaches-cosmic-shorelines-our-changing-views-of-habitable-planets and https://www.nasa.gov/feature/goddard/2019/nasa-s-webb-telescope-will-survey-saturn-and-its-moon-titan.



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