The Stability of the orbits of Earth-mass planets in and near the habitable zones of known exoplanetary systems

We have shown that Earth-mass planets could survive in variously restricted regions of the habitable zones (HZs) of most of a sample of nine of the 93 main-sequence exoplanetary systems confirmed by May 2003. In a preliminary extrapolation of our results to the other systems, we estimate that roughly a third of the 93 systems might be able to have Earth-mass planets in stable, confined orbits somewhere in their HZs. Clearly, these systems should be high on the target list for exploration for terrestrial planets. We have reached this conclusion by launching putative Earth-mass planets in various orbits and following their fate with a mixed-variable symplectic integrator

Habitability of known exoplanetary systems based on measured stellar properties

At present, because of observational selection effects, we know of no exoplanetary systems with any planetary masses close to that of the Earth. We have therefore used computer models to see whether such planets could be dynamically stable in the presence of the more massive planets known to be present, and in particular whether planets with roughly an Earth mass could remain confined to the classical habitable zone (HZ) for long enough for life to have emerged. Measured stellar properties have been used to determine for each system the present location of the HZ. We have also determined the critical distances from the orbit of each giant planet within which an Earth-mass planet would suffer large orbital changes. We then evaluated the present habitability of each and every exoplanetary system by examining the penetration of these critical distances into the HZ. The critical distances can be obtained by extensive computer modelling of an exoplanetary system. This is far too time consuming to apply to all of the 150 or so systems already known, and to keep up with the latest discoveries. Therefore, in earlier work we studied a few systems in great detail, and developed a speedier means of obtaining the critical distances. We summarize this comparatively quick method here. We can then evaluate comparatively quickly the present habitability of each exoplanetary system by examining the penetration of the critical distance(s) into the HZ. The results are encouraging for astrobiology.Comment: Accepted for publication by The Astrophysical Journal. A few revisions have been made following suggestions by the refere

Habitability of exoplanetary systems with planets observed in transit

(Shortened) We have used the measured properties of the stars in the 79 exoplanetary systems with one or more planets that have been observed in transit, to estimate each system's present habitability. The measured stellar properties have been used to determine the present location of the classical habitable zone (HZ). To establish habitability we use the estimated distances from the giant planet(s) within which an Earth-like planet would be inside the gravitational reach of the giant. Of the 79 transiting systems known in April 2010, only 2 do not offer safe havens to Earth-like planets in the HZ, and thus could not support life today. We have also estimated whether habitability is possible for 1.7 Gyr into the past i.e. 0.7 Gyr for a heavy bombardment, plus 1.0 Gyr for life to emerge and thus be present today. We find that, for the best estimate of each stellar age, an additional 28 systems do not offer such sustained habitability. If we reduce 1.7 Gyr to 1.0 Gyr this number falls to 22. However, if giant planets orbiting closer to the star than the inner boundary of the HZ, have got there by migration through the HZ, and if this ruled out the subsequent formation of Earth-like planets, then, of course, none of the presently known transiting exoplanetary systems offers habitability. Fortunately, this bleak conclusion could well be wrong. As well as obtaining results on the 79 transiting systems, this paper demonstrates a method for determining the habitability of the cornucopia of such systems that will surely be discovered over the next few years.Comment: 20 pages, 2 Figures, 4 Tables. Accepted for publication by MNRA

Can terrestrial planets exist in the habitable zones of known exoplanetary systems?

The habitable zone (HZ) is defined as the range of distances from a star within which water at the surface of a terrestrial planet would be in the liquid phase. We have investigated whether terrestrial planets could exit in the HZs of known exoplanetary systems long enough for life to have emerged and to have evolved. Four contrasting systems in which giant planets have been detected have been examined, and HZs have been defined for each system using conservative definitions for the HZ boundaries. Mixed- variable sympletic numerical integration has ben used to investigate the orbits of putative terrestrial planets launched within the HZ of each system. In Rho CrB the HZ is exterior to the giant, and in 47 UMa it is interior. We have shown that in each of these two systems terrestrial planets could have orbits with semimajor axes that remain confined to the HZ for biologically significant lengths of time. We have also shown that the Gliese 876 and Ups And systems are very unlikely to have such orbits

Prospects for 'Earths' in the Habitable Zones of Known Exoplanetary Systems

We have shown that Earth-mass planets could survive in variously restricted regions of the habitable zones (HZs) of most of a sample of nine of the 104 main-sequence exoplanetary systems confirmed by mid-November 2003. In a preliminary extrapolation of our results to the other systems, we estimate that roughly a half of these systems could have had an Earth-mass planet confined to the HZ for at least the most recent 1000 Ma. The HZ migrates outwards during the main-sequence lifetime, and so this proportion varies with stellar age — about two thirds of the systems could have such a planet confined to the HZ for at least 1000 Ma at sometime during the main-sequence lifetime. Clearly, these systems should be targeted for exploration for terrestrial planets. We have reached this conclusion by launching putative Earth-mass planets in various orbits and following their fate with mixed-variable symplectic and hybrid integrators. Whether the Earth-mass planets could form in the HZs of the exoplanetary systems is an urgent question that needs further study

Insights into the fate of a 13C labelled phenol pulse for stable isotope probing (SIP) experiments

Stable isotope probing (SIP) using DNA or RNA as a biomarker has proven to be a useful method for attributing substrate utilisation to specific microbial taxa. In this study we followed the transfer of a 13C6-phenol pulse in an activated sludge micro-reactor to examine the resulting distribution of labelled carbon in the context of SIP. Most of the added phenol was metabolically converted within the first 100 min after 13C6-phenol addition, with 49% incorporated into microbial biomass and 6% respired as CO2. Less than 1% of the total 13C labelled carbon supplied was incorporated into microbial RNA and DNA, with RNA labelling 6.5 times faster than DNA. The remainder of the added 13C was adsorbed and/or complexed to suspended solids within the sludge. The 13C content of nucleic acids increased beyond the initial consumption of the 13C-phenol pulse. This study confirms that RNA labels more efficiently than DNA and reveals that only a small proportion of a pulse is incorporated into nucleic acids. Evidence of continued 13C incorporation into nucleic acids suggests that cross-feeding of the SIP substrate was rapid. This highlights both the benefits of using a biomarker that is rapidly labelled and the importance of sampling within appropriate timescales to avoid or capture the effects of cross-feeding, depending on the goal of the study