With steadily mounting evidence that points to a cosmic origin of terrestrial life, a cultural barrier prevails against admitting that such a connection exists. Astronomy continues to reveal the presence of organic molecules and organic dust on a huge cosmic scale, amounting to a third of interstellar carbon tied up in this form. Just as the overwhelming bulk of organics on Earth stored over geological timescales are derived from the degradation of living cells, so it seems likely that interstellar organics in large measure also derive from biology. As we enter a new decade – the year 2010 – a clear pronouncement of our likely alien ancestry and of the existence of extraterrestrial life on a cosmic scale would seem to be overdue.

Low atmospheric pressures on Mars and the lack of substantial amounts of liquid water were suggested to be among the major limiting factors for the potential Martian biosphere. However, large amounts of ice were detected in the relatively shallow subsurface layers of Mars by the Odyssey Mission and when ice sublimates the water vapour can diffuse through the porous surface layer of the soil. Here we studied the possibility for the active growth of microorganisms in such a vapour diffusion layer. Our results showed the possibility of metabolism and the reproduction of non-extremophile terrestrial microorganisms (Vibrio sp.) under very low (0.01–0.1 mbar) atmospheric pressures in a Martian-like shallow subsurface regolith.

One of the key considerations when assessing the potential habitability of telluric worlds will be that of the impact regime experienced by the planet. In this work, we present a short review of our understanding of the impact regime experienced by the terrestrial planets within our own Solar system, describing the three populations of potentially hazardous objects which move on orbits that take them through the inner Solar system. Of these populations, the origins of two (the Near-Earth Asteroids and the Long-Period Comets) are well understood, with members originating in the Asteroid belt and Oort cloud, respectively. By contrast, the source of the third population, the Short-Period Comets, is still under debate. The proximate source of these objects is the Centaurs, a population of dynamically unstable objects that pass perihelion (closest approach to the Sun) between the orbits of Jupiter and Neptune. However, a variety of different origins have been suggested for the Centaur population. Here, we present evidence that at least a significant fraction of the Centaur population can be sourced from the planetary Trojan clouds, stable reservoirs of objects moving in 1:1 mean-motion resonance with the giant planets (primarily Jupiter and Neptune). Focussing on simulations of the Neptunian Trojan population, we show that an ongoing flux of objects should be leaving that region to move on orbits within the Centaur population. With conservative estimates of the flux from the Neptunian Trojan clouds, we show that their contribution to that population could be of order ~3%, while more realistic estimates suggest that the Neptune Trojans could even be the main source of fresh Centaurs. We suggest that further observational work is needed to constrain the contribution made by the Neptune Trojans to the ongoing flux of material to the inner Solar system, and believe that future studies of the habitability of exoplanetary systems should take care not to neglect the contribution of resonant objects (such as planetary Trojans) to the impact flux that could be experienced by potentially habitable worlds.

A high level of stellar activity in the form of frequent flaring and frequent mass ejections can lead to the total loss of exoplanetary atmospheres due to evaporation and erosion. Simulations have shown such scenarii for close-in exoplanets orbiting M-stars. Information on stellar flaring activity is accessible more easily than information on stellar mass ejections, simply due to the difference in detection. In the National Aeronautics and Space Administration/Far Ultraviolet Spectroscopic Explorer spectra of the dM star AD Leonis we find an interesting event lasting for only one spectrum. The first component of the OVI (103.19 nm, 103.76 nm) duplet shows an enhancement of the blue wing, shifted by about 90 km s−1. This event occurred one spectrum after a flare. We discuss several solar/stellar phenomena that might produce such a spectral feature and could therefore explain this event.

In the last few years several investigations, based on culture-dependent and -independent techniques, have shown that salt-attacked stone surfaces present a habitat for extremely salt tolerant and moderate halophilic microorganisms. The inner walls of the Chapel of St. Virgil in Vienna (Austria) are an example of this phenomenon. Salt crusts cover most of the wall surfaces and salt crystallization in the porous space of the stone is causing decohesion of material and destruction of the original medieval paintings. The salt, together with the oligotrophic conditions, creates a very special and extreme habitat for halotolerant and halophilic microorganisms.

In this study we investigate and monitor the cultivable and non-cultivable members of the microbial community present on the stonework of the medieval Chapel of St. Virgil after several severe disturbances of the microbial environment caused by desalination and disinfection treatments. With this finality, a combination of culture-dependent and -independent techniques was selected. The genetic diversity of a total of 104 bacterial strains isolated from the stone samples was analysed by denaturing gradient gel electrophoresis (DGGE), random amplified polymorphic DNA (RAPD) analysis and 16S rRNA gene sequencing. Strains were distributed over 29 groups on the basis of their RAPD patterns. Only 19 groups were differentiated by DGGE. Comparative sequence analyses showed that the isolated strains belong to related species of the genera Halobacillus (47.1%), Bacillus (35.6%), Acinetobacter (4.8%), Halomonas (3.9%), Nesterenkonia (2.9%), Paucisalibacillus (2.9%), Paenibacillus (1%), Staphylococcus (1%) and Exiguobacterium (1%).

In addition, polymerase chain reaction DGGE fingerprints, in combination with the creation of clone libraries and sequencing analyses, were used to monitor and identify Archaea, the non-cultivable fraction of the microbial community. The detected archaeal sequences were closely related to different uncultured archaeons as well as to the cultured genera Halococcus and Halalkalicoccus and Halobacterium.

Cultivation and molecular analyses revealed the presence of highly specialized microorganisms that were able to thrive and survive after several desalination and disinfection treatments in the extreme environment presented by the salt-attacked Chapel of St. Virgil.

We have found that the principles of dialetheism, which state that some contradictions (typically at the limits of a system) may be true, and which amply demonstrate the limits of thought and conception, can be valuable in sorting out and clarifying some astrobiological problems that impede our ability to define life. The examples include the classification of viruses as alive or not alive, and the description of the transition zone for the abiotic-to-biotic transition. Dialetheism gives us the philosophical tool to state that the viruses may be both alive and not alive, and that chemical systems may exist that are both abiotic and biotic.

We have extracted some philosophical principles of the identity and have applied them to the identity of living organisms and their life forms. The first and most important idea is that we should define an individual organism via its numerical identity. For each organism its identity will be in relation to itself. As the organism undergoes various changes during its development, and as it transitions from one to the next of its life forms, one can observe numerous qualitative differences between these life forms. Although the life forms change and the organism is in a flux, what remains constant is the numerical identity of the organism. If the organism reproduces, for example by a fission mode, then the daughter cells will have their own numerical identity. We can state that the life of an organism is a sum of all its life forms over the period of time of the existence of the organism. Reproduction, particularly by fission, represents an identity dilemma, but it can be resolved by Gallois' occasional identities theory.

Stellar X-ray and extreme ultraviolet (XUV) radiation is an important driver of the escape of planetary atmospheres. Young stars emit high XUV fluxes that decrease as they age. Since the XUV emission of a young star can be orders of magnitude higher compared to an older one, this evolution has to be taken into account when studying the mass-loss history of a planet. The temporal decrease of activity is closely related to the operating magnetic dynamo, which depends on rotation and convection in Sun-like stars. Using a sample of nearby M dwarfs, we study the relations between age, rotation and activity and discuss the influence on planets orbiting these low-mass stars.

The work reported here represents a study into the total fluorescence exhibited by a broad selection of model, extremophilic and photosynthetic bacterial strains, over a great range of excitation and emission wavelengths from ultraviolet (UV) through visible to near infrared. The aim is to identify distinctive fluorescent features that may serve as detectable biosignatures of remnant microbial life on the Martian surface. A lab-bench fluorescence spectrometer was used to generate an excitation–emission matrix (EEM) for the unpigmented Escherichia coli, radiation-resistant Deinococcus radiodurans, Antarctic Dry Valley isolates Brevundimonas sp. MV.7 and Rhodococcus sp. MV.10, and the cyanobacterium Synechocystis sp. PCC 6803. Detailed EEMs, representing the fluorescence signature of each organism, are presented, and the most significant features suitable for biosignature surveys are identified, including small-molecule cellular metabolites, light-harvesting photosynthetic pigments and extracellular UV-screening compounds. E. coli exhibits the most intense emission from tryptophan, presumably due to the absence of UV-screening pigments that would shield the organism from short-wavelength light-exciting intracellular fluorescence. The efficacy of commonly available laser diodes for exciting cellular fluorescence is treated, along with the most appropriate filter wavelengths for imaging systems. The best combination of available laser diodes and PanCam filters aboard the ExoMars probe is proposed. The possibility of detecting fluorescence excited by solar UV radiation in freshly exposed surface samples by imaging when both sunlit and shadowed, perhaps by the body of the rover itself, is discussed. We also study how these biological fluorophore molecules may be degraded, and thus the potential biosignatures erased, by the high flux of far-ultraviolet light on Mars.

HR8799 is a young (20–160 Myr) A-dwarf main sequence star with a debris disc detected by IRAS (InfraRed Astronomical Satellite). In 2008, it was one of two stars around which exoplanets were directly imaged for the first time. The presence of three Jupiter-mass planets around HR8799 provoked much interest in modelling the dynamical stability of the system. Initial simulations indicated that the observed planetary architecture was unstable on timescales much shorter than the lifetime of the star (~105 yr). Subsequent models suggested that the system could be stable if the planets were locked in a 1:2:4 mutual mean motion resonance (MMR). In this work, we have examined the influence of varying orbital eccentricity and the semi-major axis on the stability of the three-planet system, through dynamical simulations using the MERCURY n-body integrator. We find that, in agreement with previous work on this system, the 1:2:4 MMR is the most stable planetary configuration, and that the system stability is dominated by the interaction between the inner pair of planets. In contrast to previous results, we find that with small eccentricities, the three-planet system can be stable for timescales comparable to the system lifetime and, potentially, much longer.

Europa is a prime target for astrobiology and has been prioritized as the next target for a National Aeronautics and Space Administration flagship mission. It is important, therefore, that we advance our understanding of Europa, its ocean and physical environment as much as possible. Here, we describe observations of Europa obtained during its orbital eclipse by Jupiter using the Hubble Space Telescope. We obtained Advanced Camera for Surveys Solar Blind Channel far ultraviolet low-resolution spectra that show oxygen line emission both in and out of eclipse. We also used the Wide-Field and Planetary Camera-2 and searched for broad-band optical emission from fluorescence of the surface material, arising from the very high level of incident energetic particle radiation on ices and potentially organic substances. The high-energy particle radiation at the surface of Europa is extremely intense and is responsible for the production of a tenuous oxygen atmosphere and associated FUV line emission. Approximately 50% of the oxygen emission lasts at least a few hours into the eclipse. We discuss the detection limits of the optical emission, which allow us to estimate the fraction of incident energy reradiated at optical wavelengths, through electron-excited emission, Cherenkov radiation in the ice and fluorescent processes.

Within the next few years, the first Earth-mass planets will be discovered around other stars. Some of those worlds will certainly lie within the classical ‘habitable zone’ of their parent stars, and we will quickly move from knowing of no exoEarths to knowing many. For the first time, we will be in a position to carry out a detailed search for the first evidence of life beyond our Solar System. However, such observations will be hugely taxing and time consuming to perform, and it is almost certain that far more potentially habitable worlds will be known than it is possible to study. It is therefore important to catalogue and consider the various effects that make a promising planet more or less suitable for the development of life. In this work, we review the various planetary, dynamical and stellar influences that could influence the habitability of exoEarths. The various influences must be taken in concert when we attempt to decide where to focus our first detailed search for life. While there is no guarantee that any given planet will be inhabited, it is vitally important to ensure that we focus our time and effort on those planets most likely to yield a positive result.