Elke Pilat-Lohinger

Stability of S-type Orbits in Binaries

This numerical investigation is concerned with the stability of planets moving around one component of a double star system. Since the discovery of four extra solar planets moving in such orbits, there is a growing interest of stability studies thereto. We determined the stable regions in the elliptic restricted three body problem, for the whole range of mass-ratios from 0.1 to 0.9, by means of the Fast Lyapunov Indicators. The computations have been carried out for eccentricities of the binary and of the planet in the range 0–0.5. Therefore, we present for the first time the variation of the stable regions when the initial eccentricity of the planet is increased. We have found a correlation between the reduction of the stable zones if the eccentricity of the planet or of the binary is increased — of course the latter one is more effective.

Extrasolar Trojan planets close to habitable zones

We investigate the stability regions of hypothetical terrestrial planets around the Lagrangian equilibrium points L4 and L5 in some specific extrasolar planetary systems. The problem of their stability can be treated in the framework of the restricted three body problem where the host star and a massive Jupiter-like planet are the primary bodies and the terrestrial planet is regarded as being massless. From these theoretical investigations one cannot determine the extension of the stable zones around the equilibrium points. Using numerical experiments we determined their largeness for three test systems chosen from the table of the know extrasolar planets, where a giant planet is moving close to the so-called habitable zone around the host star in low eccentric orbits. The results show the dependence of the size and structure of this region, which shrinks significantly with the eccentricity of the known gas giant.

Planets in habitable zones: A study of the binary Gamma Cephei

The recently discovered planetary system in the binary y Cep was studied concerning its dynamical evolution. We confirm that the orbital parameters found by the observers are in a stable configuration. The primary aim of this study was to find stable planetary orbits in a habitable region in this system, which consists of a double star (a = 21.36 AU) and a relatively close (a = 2.15 AU) massive (1.7 m j u p sin i) planet. We did straightforward numerical integrations of the equations of motion in different dynamical models and determined the stability regions for a fictitious massless planet in the interval of the semimajor axis 0.5 AU < a < 1.85 AU around the more massive primary. To confirm the results we used the Fast Lyapunov Indicators (FLI) in separate computations, which are a common tool for determining the chaoticity of an orbit. Both results are in good agreement and unveiled a small island of stable motions close to I AU up to an inclination of about 15° (which corresponds to the 3:1 mean motion resonance between the two planets). Additionally we computed the orbits of earthlike planets (up to 90 earthmasses) in the small stable island and found out, that there exists a small window of stable orbits on the inner edge of the habitable zone in γ Cep even for massive planets.

AN ANALYTIC METHOD TO DETERMINE HABITABLE ZONES FOR S-TYPE PLANETARY ORBITS IN BINARY STAR SYSTEMS

With more and more extrasolar planets discovered in and around binary star systems, questions concerning the determination of the classical Habitable Zone arise. Do the radiative and gravitational perturbations of the second star influence the extent of the Habitable Zone significantly, or is it sufficient to consider the host-star only? In this article we investigate the implications of stellar companions with different spectral types on the insolation a terrestrial planet receives orbiting a Sun-like primary. We present time independent analytical estimates and compare these to insolation statistics gained via high precision numerical orbit calculations. Results suggest a strong dependence of permanent habitability on the binarys eccentricity, as well as a possible extension of Habitable Zones towards the secondary in close binary systems.

Pathways to Earth-like atmospheres. Extreme ultraviolet (EUV)-powered escape of hydrogen-rich protoatmospheres.

We discuss the evolution of the atmosphere of early Earth and of terrestrial exoplanets which may be capable of sustaining liquid water oceans and continents where life may originate. The formation age of a terrestrial planet, its mass and size, as well as the lifetime in the EUV-saturated early phase of its host star play a significant role in its atmosphere evolution. We show that planets even in orbits within the habitable zone of their host stars might not lose nebular- or catastrophically outgassed initial protoatmospheres completely and could end up as water worlds with CO2 and hydrogen- or oxygen-rich upper atmospheres. If an atmosphere of a terrestrial planet evolves to an N2-rich atmosphere too early in its lifetime, the atmosphere may be lost. We show that the initial conditions set up by the formation of a terrestrial planet and by the evolution of the host star’s EUV and plasma environment are very important factors owing to which a planet may evolve to a habitable world. Finally we present a method for studying the discussed atmosphere evolution hypotheses by future UV transit observations of terrestrial exoplanets.

CONDITIONS OF DYNAMICAL STABILITY FOR THE HD 160691 PLANETARY SYSTEM

In our previous paper we showed that the currently determined orbital parameters placed four recently announced planetary systems (HD 12661, HD 38529, HD 37124, and HD 160691) in very different situations from the point of view of dynamical stability. In the present paper we deal with the last of these systems, whose orbital parameters of the outer planet are yet uncertain. We discover a stabilizing mechanism that could be the key to its existence. The paper is devoted to the study of this mechanism by a global dynamics analysis in the orbital parameter space related to the HD 160691 system. We obtained our results using a new technique called the mean exponential growth factor of nearby orbits (MEGNO), and verified them with the fast Lyapunov indicator technique (FLI). In order to be dynamically stable, the HD 160691 planetary system has to satisfy the following conditions: (1) it should have a 2 : 1 mean motion resonance, (2) combined with an apsidal secular resonance, (3) in a configuration Pc(ap)-S-Pb(ap) (which means that the planets c and b may be considered as initially located at their apoastron around the central star S), and (4) it must satisfy specific conditions for the respective sizes of the eccentricities. High eccentricity for the outer orbit (ec > 0.52) is the most probable necessary condition, while the eccentricity of the inner orbit eb becomes relatively unimportant when ec > 0.7. We also show that there is an upper limit for planetary masses (in the interval permitted by the undetermined line-of-sight inclination factor sin il) due to the dynamical stability mechanism. More generally, in this original orbital topology, where the resonance variables θ1 and θ3 librate about 180° while θ2 librates about 0°, the HD 160691 system and its mechanism have revealed aspects of the 2 : 1 orbital resonances that have not been observed nor analyzed before. The present topology with antialigned apsidal lines, combined with the 2 : 1 resonance, is indeed more wide-ranging than the particular case of the HD 160691 planetary system. It is a new theoretical possibility that is suitable for a stable regime despite relatively small semimajor axes with respect to the important masses in interactions.

Circumstellar Habitable Zones of Binary Star Systems in the Solar Neighborhood

Binary and multiple systems constitute more than half of the total stellar population in the Solar neighborhood (Kiseleva-Eggleton & Eggleton 2001). Their frequent occurrence as well as the fact that more than 70 (Schneider et al. 2011) planets have already been discovered in such configurations - most noteably the telluric companion of α Cen B (Dumusque et al. 2012) - make them interesting targets in the search for habitable worlds. Recent studies (Eggl et al. 2012b; Forgan 2012) have shown, that despite the variations in gravitational and radiative environment, there are indeed circumstellar regions where planets can stay within habitable insolation limits on secular dynamical timescales. In this article we provide habitable zones for 19 near S-Type binary systems from the Hipparchos and WDS catalogues with semimajor axes between 1 and 100 AU. Hereby, we accounted for the combined dynamical and radiative influence of the second star on the Earth-like planet. Out of the 19 systems presented, 17 offer dynamically stable habitable zones around at least one component. The 17 potentially habitable systems contain 5 F, 3 G, 7 K and 16 M class stars. As their proximity to the Solar System (d < 31 pc) makes the selected binary stars exquisite targets for observational campaigns, we offer estimates on radial velocity, astrometric and transit signatures produced by habitable Earth-like planets in eccentric circumstellar orbits.

AstRoMap European Astrobiology Roadmap

Abstract The European AstRoMap project (supported by the European Commission Seventh Framework Programme) surveyed the state of the art of astrobiology in Europe and beyond and produced the first European roadmap for astrobiology research. In the context of this roadmap, astrobiology is understood as the study of the origin, evolution, and distribution of life in the context of cosmic evolution; this includes habitability in the Solar System and beyond. The AstRoMap Roadmap identifies five research topics, specifies several key scientific objectives for each topic, and suggests ways to achieve all the objectives. The five AstRoMap Research Topics are • Research Topic 1: Origin and Evolution of Planetary Systems • Research Topic 2: Origins of Organic Compounds in Space • Research Topic 3: Rock-Water-Carbon Interactions, Organic Synthesis on Earth, and Steps to Life • Research Topic 4: Life and Habitability • Research Topic 5: Biosignatures as Facilitating Life Detection It is strongly recommended that steps be taken towards the definition and implementation of a European Astrobiology Platform (or Institute) to streamline and optimize the scientific return by using a coordinated infrastructure and funding system. Key Words: Astrobiology roadmap—Europe—Origin and evolution of life—Habitability—Life detection—Life in extreme environments. Astrobiology 16, 201–243.

Prospects of the detection of circumbinary planets with Kepler and CoRoT using the variations of eclipse timing

In close eclipsing binaries, measurements of the variations in binarys eclipse timing may be used to infer information about the existence of circumbinary objects. To determine the possibility of the detection of such variations with CoRoT and Kepler space telescopes, we have carried out an extensive study of the dynamics of a binary star system with a circumbinary planet, and calculated its eclipse timing variations (ETVs) for different values of the mass ratio and orbital elements of the binary and the perturbing body. Here, we present the results of our study and assess the detectability of the planet by comparing the resulting values of ETVs with the temporal sensitivity of CoRoT and Kepler. Results point to extended regions in the parameter space where the perturbation of a planet may be large enough to create measurable variations in the eclipse timing of the secondary star. Many of these variations point to potentially detectable ETVs and the possible inference of Jovian-type planets.

THE INFLUENCE OF GIANT PLANETS NEAR A MEAN MOTION RESONANCE ON EARTH-LIKE PLANETS IN THE HABITABLE ZONE OF SUN-LIKE STARS

We present a numerical study of several two-planet systems based on the motions of Jupiter and Saturn, in which the two giant planets move in low eccentric orbits close to a mean motion resonance. It is more likely to find two planets with similar characteristics in a system than a clone of the Jupiter-Saturn pair of our solar system. Therefore, we vary the distance between the two planets and their mass ratio by changing Saturns semimajor axis from 8 to 11 AU and increasing its mass by factors of 2-40. The different two-planets were analyzed for the interacting perturbations due to the mean motion resonances of the giant planets. We select several mass ratios for the gas giants, for which we study their influence on test bodies (with negligible mass) moving in the habitable zone (HZ) of a Sun-like star. The orbits are calculated for 2 × 107 yr. In all cases the HZ is dominated by a significant curved band, indicating higher eccentricity, which corresponds to a secular resonance with Jupiter. Interesting results of this study are finding (1) an increase of Venuss eccentricity for the real Jupiter and Saturn masses and the actual semimajor axis of Saturn; (2) an increase of the eccentricity of a test planet at Earths position when Saturns mass was increased by a factor of 3 or more; and (3) if the two giant planets are in 2:1 resonance, we observe a strong influence on the outer region of the HZ.