184 research outputs found
Exoplanets - search methods, discoveries, and prospects for astrobiology
Whereas the Solar System has Mars and Europa as the best candidates for
finding fossil/extant life as we know it - based on complex carbon compounds
and liquid water - the 263 (non-pulsar) planetary systems around other stars as
known at 15 September 2008 could between them possess many more planets where
life might exist. Moreover, the number of these exoplanetary systems is growing
steadily, and with this growth there is an increase in the number of planets
that could bear carbon-liquid water life. In this brief review the main methods
by which exoplanets are being discovered are outlined, and then the discoveries
that have so far been made are presented. Habitability is then discussed, and
an outline presented of how a planet could be studied from afar to determine
whether it is habitable, and whether it is indeed inhabited. This review is
aimed at the astrobiology community, which spans many disciplines, few of which
involve exoplanets. It is therefore at a basic level and concentrates on the
major topics.Comment: 37 pages, 12 Figure
Mars before the Space Age
Mars has surely been scrutinised since the dawn of humankind. Its appearance every couple of years like a drop of blood in the sky led to warlike attributes in the ancient world. In the 16th century Tycho Brahe made accurate observations of the position of Mars that enabled Johannes Kepler to obtain his first two laws of planetary motion. These in turn were explained by Newton's laws of motion and gravity. In the 17th century the first telescope observations were made, but Mars is small and very little surface detail could be discerned.
Throughout the 18th and 19th centuries telescopes improved, revealing many dark areas on the red tinted surface. During the close opposition of 1877 sufficient detail could be seen that enabled Giovanni Schiaparelli to announce that he could see about 40 canali on Mars. This led to the saga of the canals of Mars, finally laid to rest in 1971 when Mariner 9 made observations from Martian orbit showing that the canali/canals do not exist.
Belief that there was life on Mars was widespread in the 19th century. However, the majority of astronomers never believed in Martian intelligence. Least controversial was the view that the dark areas were some form of plant life. This view persisted until Mariner 4 flew past Mars in 1965 and discovered a far thinner atmosphere than previously thought. This was a low point, with impact craters dominating the images. It was Mariner 9 that revealed much more promising landscapes, including volcanic features, and others indicating that water had flowed across the surface, particularly when Mars was young. Thus, the contemporary era of Mars exploration began.
Our picture of Mars today is not only much more complete than that before Mariner 4, in several ways it is quite different. The belief, however, that there might be life on Mars persists – subsurface life cannot be ruled out and, failing that, there might be ancient fossils on Mars
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
Effect of Gene Therapy on Visual Function in Leber's Congenital Amaurosis
Early-onset, severe retinal dystrophy caused by mutations in the gene encoding retinal pigment epithelium–specific 65-kD protein (RPE65) is associated with poor vision at birth and complete loss of vision in early adulthood. We administered to three young adult patients subretinal injections of recombinant adeno-associated virus vector 2/2 expressing RPE65 complementary DNA (cDNA) under the control of a human RPE65 promoter. There were no serious adverse events. There was no clinically significant change in visual acuity or in peripheral visual fields on Goldmann perimetry in any of the three patients. We detected no change in retinal responses on electroretinography. One patient had significant improvement in visual function on microperimetry and on dark-adapted perimetry. This patient also showed improvement in a subjective test of visual mobility. These findings provide support for further clinical studies of this experimental approach in other patients with mutant RPE65. (ClinicalTrials.gov number, NCT00643747.)Supported by grants from the U.K. Department of Health, the
British Retinitis Pigmentosa Society, and the Special Trustees of
Moorfields Eye Hospital, and by the Sir Jules Thorn Charitable
Trust, the Wellcome Trust, the European Union (EVI-Genoret
and Clinigene programs), the Medical Research Council, Foundation
Fighting Blindness, Fight for Sight, the Ulverscroft Foundation,
Fighting Blindness (Ireland), Moorfields Eye Hospital,
and Institute of Ophthalmology Biomedical Research Centre for
Ophthalmology, University College London
Utilisation of an operative difficulty grading scale for laparoscopic cholecystectomy
Background
A reliable system for grading operative difficulty of laparoscopic cholecystectomy would standardise description of findings and reporting of outcomes. The aim of this study was to validate a difficulty grading system (Nassar scale), testing its applicability and consistency in two large prospective datasets.
Methods
Patient and disease-related variables and 30-day outcomes were identified in two prospective cholecystectomy databases: the multi-centre prospective cohort of 8820 patients from the recent CholeS Study and the single-surgeon series containing 4089 patients. Operative data and patient outcomes were correlated with Nassar operative difficultly scale, using Kendall’s tau for dichotomous variables, or Jonckheere–Terpstra tests for continuous variables. A ROC curve analysis was performed, to quantify the predictive accuracy of the scale for each outcome, with continuous outcomes dichotomised, prior to analysis.
Results
A higher operative difficulty grade was consistently associated with worse outcomes for the patients in both the reference and CholeS cohorts. The median length of stay increased from 0 to 4 days, and the 30-day complication rate from 7.6 to 24.4% as the difficulty grade increased from 1 to 4/5 (both p < 0.001). In the CholeS cohort, a higher difficulty grade was found to be most strongly associated with conversion to open and 30-day mortality (AUROC = 0.903, 0.822, respectively). On multivariable analysis, the Nassar operative difficultly scale was found to be a significant independent predictor of operative duration, conversion to open surgery, 30-day complications and 30-day reintervention (all p < 0.001).
Conclusion
We have shown that an operative difficulty scale can standardise the description of operative findings by multiple grades of surgeons to facilitate audit, training assessment and research. It provides a tool for reporting operative findings, disease severity and technical difficulty and can be utilised in future research to reliably compare outcomes according to case mix and intra-operative difficulty
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Minimally Symptomatic Infection in an Ebola ‘Hotspot’: A Cross-Sectional Serosurvey
Introduction: Evidence for minimally symptomatic Ebola virus (EBOV) infection is limited. During the 2013–16 outbreak in West Africa, it was not considered epidemiologically relevant to published models or projections of intervention effects. In order to improve our understanding of the transmission dynamics of EBOV in humans, we investigated the occurrence of minimally symptomatic EBOV infection in quarantined contacts of reported Ebola virus disease cases in a recognized ‘hotspot.’ Methodology/Principal Findings We conducted a cross-sectional serosurvey in Sukudu, Kono District, Sierra Leone, from October 2015 to January 2016. A blood sample was collected from 187 study participants, 132 negative controls (individuals with a low likelihood of previous exposure to Ebola virus), and 30 positive controls (Ebola virus disease survivors). IgG responses to Ebola glycoprotein and nucleoprotein were measured using Alpha Diagnostic International ELISA kits with plasma diluted at 1:200. Optical density was read at 450 nm (subtracting OD at 630nm to normalize well background) on a ChroMate 4300 microplate reader. A cutoff of 4.7 U/mL for the anti-GP ELISA yielded 96.7% sensitivity and 97.7% specificity in distinguishing positive and negative controls. We identified 14 seropositive individuals not known to have had Ebola virus disease. Two of the 14 seropositive individuals reported only fever during quarantine while the remaining 12 denied any signs or symptoms during quarantine. Conclusions/Significance: By using ELISA to measure Zaire Ebola virus antibody concentrations, we identified a significant number of individuals with previously undetected EBOV infection in a ‘hotspot’ village in Sierra Leone, approximately one year after the village outbreak. The findings provide further evidence that Ebola, like many other viral infections, presents with a spectrum of clinical manifestations, including minimally symptomatic infection. These data also suggest that a significant portion of Ebola transmission events may have gone undetected during the outbreak. Further studies are needed to understand the potential risk of transmission and clinical sequelae in individuals with previously undetected EBOV infection
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