Geophysical and atmospheric evolution of habitable planets

A Metamorphic Origin for Europa's Ocean

Europa likely contains an iron-rich metal core. For it to have formed, temperatures within Europa reached ≳ 1250 K. Going up to that temperature, accreted chondritic minerals - for example, carbonates and phyllosilicates - would partially devolatilize. Here, we compute the amounts and compositions of exsolved volatiles. We find that volatiles released from the interior would have carried solutes, redox-sensitive species, and could have generated a carbonic ocean in excess of Europa's present-day hydrosphere, and potentially an early CO 2 atmosphere. No late delivery of cometary water was necessary. Contrasting with prior work, CO 2 could be the most abundant solute in the ocean, followed by Ca 2 + , SO 4 2 - , and HCO 3 - . However, gypsum precipitation going from the seafloor to the ice shell decreases the dissolved S/Cl ratio, such that Cl > S at the shallowest depths, consistent with recently inferred endogenous chlorides at Europa's surface. Gypsum would form a 3-10 km thick sedimentary layer at the seafloor.

Review on the Role of Planetary Factors on Habitability

In this work various factors on the habitability were considered, focusing on conditions irrespective of the central star's radiation, to see the role of specific planetary body related effects. These so called planetary factors were evaluated to identify those trans-domain issues where important information is missing but good chance exit to be filled by new knowledge that might be gained in the next decade(s). Among these strategic knowledge gaps, specific issues are listed, like occurrence of radioactive nucleides in star forming regions, models to estimate the existence of subsurface liquid water from bulk parameters plus evolutionary context of the given system, estimation on the existence of redox gradient depending on the environment type etc. These issues require substantial improvement of modelling and statistical handling of various cases, as "planetary environment types". Based on our current knowledge it is probable that subsurface habitability is at least as frequent, or more frequent than surface habitability. Unfortunately it is more difficult from observations to infer conditions for subsurface habitability, but specific argumentation might help with indirect ways, which might result in new methods to approach habitability in general.

Physicochemical Requirements Inferred for Chemical Self-Organization Hardly Support an Emergence of Life in the Deep Oceans of Icy Moons

An approach to the origin of life, focused on the property of entities capable of reproducing themselves far from equilibrium, has been developed recently. Independently, the possibility of the emergence of life in the hydrothermal systems possibly present in the deep oceans below the frozen crust of some of the moons of Jupiter and Saturn has been raised. The present report is aimed at investigating the mutual compatibility of these alternative views. In this approach, the habitability concept deduced from the limits of life on Earth is considered to be inappropriate with regard to emerging life due to the requirement for an energy source of sufficient potential (equivalent to the potential of visible light). For these icy moons, no driving force would have been present to assist the process of emergence, which would then have had to rely exclusively on highly improbable events, thereby making the presence of life unlikely on these Solar System bodies, that is, unless additional processes are introduced for feeding chemical systems undergoing a transition toward life and the early living organisms.

KEY WORDS

Icy moon-Bioenergetics-Chemical evolution-Habitability-Origin of life. Astrobiology 16, 328-334.

Formation, habitability, and detection of extrasolar moons

The diversity and quantity of moons in the Solar System suggest a manifold population of natural satellites exist around extrasolar planets. Of peculiar interest from an astrobiological perspective, the number of sizable moons in the stellar habitable zones may outnumber planets in these circumstellar regions. With technological and theoretical methods now allowing for the detection of sub-Earth-sized extrasolar planets, the first detection of an extrasolar moon appears feasible. In this review, we summarize formation channels of massive exomoons that are potentially detectable with current or near-future instruments. We discuss the orbital effects that govern exomoon evolution, we present a framework to characterize an exomoon's stellar plus planetary illumination as well as its tidal heating, and we address the techniques that have been proposed to search for exomoons. Most notably, we show that natural satellites in the range of 0.1-0.5 Earth mass (i) are potentially habitable, (ii) can form within the circumplanetary debris and gas disk or via capture from a binary, and (iii) are detectable with current technology.

Setting the stage for habitable planets

Our understanding of the processes that are relevant to the formation and maintenance of habitable planetary systems is advancing at a rapid pace, both from observation and theory. The present review focuses on recent research that bears on this topic and includes discussions of processes occurring in astrophysical, geophysical and climatic contexts, as well as the temporal evolution of planetary habitability. Special attention is given to recent observations of exoplanets and their host stars and the theories proposed to explain the observed trends. Recent theories about the early evolution of the Solar System and how they relate to its habitability are also summarized. Unresolved issues requiring additional research are pointed out, and a framework is provided for estimating the number of habitable planets in the Universe.

Tidal Venuses: triggering a climate catastrophe via tidal heating

Traditionally, stellar radiation has been the only heat source considered capable of determining global climate on long timescales. Here, we show that terrestrial exoplanets orbiting low-mass stars may be tidally heated at high-enough levels to induce a runaway greenhouse for a long-enough duration for all the hydrogen to escape. Without hydrogen, the planet no longer has water and cannot support life. We call these planets "Tidal Venuses" and the phenomenon a "tidal greenhouse." Tidal effects also circularize the orbit, which decreases tidal heating. Hence, some planets may form with large eccentricity, with its accompanying large tidal heating, and lose their water, but eventually settle into nearly circular orbits (i.e., with negligible tidal heating) in the habitable zone (HZ). However, these planets are not habitable, as past tidal heating desiccated them, and hence should not be ranked highly for detailed follow-up observations aimed at detecting biosignatures. We simulated the evolution of hypothetical planetary systems in a quasi-continuous parameter distribution and found that we could constrain the history of the system by statistical arguments. Planets orbiting stars with masses<0.3 MSun may be in danger of desiccation via tidal heating. We have applied these concepts to Gl 667C c, a ∼4.5 MEarth planet orbiting a 0.3 MSun star at 0.12 AU. We found that it probably did not lose its water via tidal heating, as orbital stability is unlikely for the high eccentricities required for the tidal greenhouse. As the inner edge of the HZ is defined by the onset of a runaway or moist greenhouse powered by radiation, our results represent a fundamental revision to the HZ for noncircular orbits. In the appendices we review (a) the moist and runaway greenhouses, (b) hydrogen escape, (c) stellar mass-radius and mass-luminosity relations, (d) terrestrial planet mass-radius relations, and (e) linear tidal theories.

A two-tiered approach to assessing the habitability of exoplanets

In the next few years, the number of catalogued exoplanets will be counted in the thousands. This will vastly expand the number of potentially habitable worlds and lead to a systematic assessment of their astrobiological potential. Here, we suggest a two-tiered classification scheme of exoplanet habitability. The first tier consists of an Earth Similarity Index (ESI), which allows worlds to be screened with regard to their similarity to Earth, the only known inhabited planet at this time. The ESI is based on data available or potentially available for most exoplanets such as mass, radius, and temperature. For the second tier of the classification scheme we propose a Planetary Habitability Index (PHI) based on the presence of a stable substrate, available energy, appropriate chemistry, and the potential for holding a liquid solvent. The PHI has been designed to minimize the biased search for life as we know it and to take into account life that might exist under more exotic conditions. As such, the PHI requires more detailed knowledge than is available for any exoplanet at this time. However, future missions such as the Terrestrial Planet Finder will collect this information and advance the PHI. Both indices are formulated in a way that enables their values to be updated as technology and our knowledge about habitable planets, moons, and life advances. Applying the proposed metrics to bodies within our Solar System for comparison reveals two planets in the Gliese 581 system, GJ 581 c and d, with an ESI comparable to that of Mars and a PHI between that of Europa and Enceladus.

A scientometric prediction of the discovery of the first potentially habitable planet with a mass similar to Earth

BACKGROUND

The search for a habitable extrasolar planet has long interested scientists, but only recently have the tools become available to search for such planets. In the past decades, the number of known extrasolar planets has ballooned into the hundreds, and with it, the expectation that the discovery of the first Earth-like extrasolar planet is not far off.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we develop a novel metric of habitability for discovered planets and use this to arrive at a prediction for when the first habitable planet will be discovered. Using a bootstrap analysis of currently discovered exoplanets, we predict the discovery of the first Earth-like planet to be announced in the first half of 2011, with the likeliest date being early May 2011.

CONCLUSIONS/SIGNIFICANCE

Our predictions, using only the properties of previously discovered exoplanets, accord well with external estimates for the discovery of the first potentially habitable extrasolar planet and highlight the the usefulness of predictive scientometric techniques to understand the pace of scientific discovery in many fields.