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Register a new userCharacterization of potentially habitable planets: Retrieval of atmospheric and planetary properties from emission spectra
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An increasing number of potentially habitable terrestrial planets and planet candidates are found by ongoing planet search programs. The search for atmospheric signatures to establish planetary habitability and the presence of life might be possible in the future. We want to quantify the accuracy of retrieved atmospheric parameters which might be obtained from infrared emission spectroscopy. We use synthetic observations of hypothetical habitable planets, constructed with a parametrized atmosphere model, a high-resolution radiative transfer model and a simplified noise model. Classic statistical tools such as chi2 statistics and least-square fits were used to analyze the simulated observations. When adopting the design of currently planned or proposed exoplanet characterization missions, we find that emission spectroscopy could provide weak limits on surface conditions of terrestrial planets, hence their potential habitability. However, these mission designs are unlikely to allow to characterize the composition of the atmosphere of a habitable planet, even though CO2 is detected. Upon increasing the signal-to-noise ratios by about a factor of 2-5 (depending on spectral resolution) compared to current mission designs, the CO2 content could be characterized to within two orders of magnitude. The detection of the O3 biosignature remains marginal. The atmospheric temperature structure could not be constrained. Therefore, a full atmospheric characterization seems to be beyond the capabilities of such missions when using only emission spectroscopy during secondary eclipse or target visits. Other methods such as transmission spectroscopy or orbital photometry are probably needed in order to give additional constraints and break degeneracies. (abridged)
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We simulate the coupled stellar and tidal evolution of short-period binary stars (orbital period $P_{orb} lsim$8 days) to investigate the orbital oscillations, instellation cycles, and orbital stability of circumbinary planets (CBPs). We consider two tidal models and show that both predict an outward-then-inward evolution of the binarys semi-major axis $a_{bin}$ and eccentricity $e_{bin}$. This orbital evolution drives a similar evolution of the minimum CBP semi-major axis for orbital stability. By expanding on previous models to include the evolution of the mass concentration, we show that the maximum in the CBP orbital stability limit tends to occur 100 Myr after the planets form, a factor of 100 longer than previous investigations. This result provides further support for the hypothesis that the early stellar-tidal evolution of binary stars has removed CBPs from short-period binaries. We then apply the models to Kepler-47 b, a CBP orbiting close to its host stars stability limit, to show that if the binarys initial $e_{bin} gsim$0.24, the planet would have been orbiting within the instability zone in the past and probably wouldnt have survived. For stable, hypothetical cases in which the stability limit does not reach a planets orbit, we find that the amplitudes of $a_{bin}$ and $e_{bin}$ oscillations can damp by up to 10% and 50%, respectively. Finally, we consider equal-mass stars with $P_{orb} =$ 7.5 days and compare the HZ to the stability limit. We find that for stellar masses $lsim0.12M_{odot}$, the HZ is completely unstable, even if the binary orbit is circular. For $e_{bin} lsim$0.5, that limit increases to $0.17M_{odot}$, and the HZ is partially destabilized for stellar masses up to $0.45M_{odot}$. These results may help guide searches for potentially habitable CBPs, as well as characterize their evolution and likelihood to support life after they are found.
As part of a national scientific network Pathways to Habitability the formation of planets and the delivery of water onto these planets is a key question as water is essential for the development of life. In the first part of the paper we summarize the state of the art of planet formation - which is still under debate in the astronomical community - before we show our results on this topic. The outcome of our numerical simulations depends a lot on the choice of the initial distribution of planetesimals and planetary embryos after gas disappeared in the protoplanetary disk. We also take into account that some of these planetesimals of sizes in the order of the mass of the Moon already contained water; the quantity depends on the distance from the Sun - close-by bodies are dry, but starting from a distance of about 2 AU they can contain substantial amounts of water. We assume that the gas giants and terrestrial planets are already formed when we check the collisions of the small bodies containing water (in the order of a few percent) with the terrestrial planets. We thus are able to give an estimate of the respective contribution to the actual water content (of some Earth-oceans) in the mantle, in the crust and on the surface of Earth. In the second part we discuss in more detail how the formation of larger bodies after a collision may happen as the outcome depends on parameters like collision velocity, impact angle, and the materials involved. We present results obtained by SPH (Smooth Particle Hydrodynamics) simulations. We briefly describe this method and show different scenarios with respect to the formed bodies, possible fragmentation and the water content before and after the collision. In an appendix we discuss detection methods for extrasolar planets (close to 2000 such objects have been discovered so far).
The competition between the torques induced by solid and thermal tides drives the rotational dynamics of Venus-like planets and super-Earths orbiting in the habitable zone of low-mass stars. The tidal responses of the atmosphere and telluric core are related to their respective physical properties and strongly depend on the tidal frequency. The resulting torque determines the possible equilibrium states of the planets spin. We compute here an analytic expression for the total tidal torque exerted on a Venus-like planet. This expression is used to characterize the equilibrium rotation of the body. Close to the star, the solid tide dominates. Far from it, the thermal tide drives the rotational dynamics of the planet. The transition regime corresponds to the habitable zone, where prograde and retrograde equilibrium states appear. We demonstrate the strong impact of the atmospheric properties and of the rheology of the solid part on the rotational dynamics of Venus-like planets, highlighting the key role played by dissipative mechanisms in the stability of equilibrium configurations.
The satellites of extrasolar planets (exomoons) have been recently proposed as astrobiological targets. Since giant planets in the habitable zone are thought to have migrated there, it is possible that they may have captured a former terrestrial planet or planetesimal. We therefore attempt to model the dynamical evolution of a terrestrial planet captured into orbit around a giant planet in the habitable zone of a star. We find that approximately half of loose elliptical orbits result in stable circular orbits over timescales of less than a few million years. We also find that those orbits are mostly low-inclination, but have no prograde/retrograde preference. In addition, we calculate the transit timing and duration variations for the resulting systems, and find that potentially habitable Earth-mass exomoons should be detectable.
We study the magnetospheric structure and the ionospheric Joule Heating of planets orbiting M-dwarf stars in the habitable zone using a set of magnetohydrodynamic (MHD) models. The stellar wind solution is used to drive a model for the planetary magnetosphere, which is coupled with a model for the planetary ionosphere. Our simulations reveal that the space environment around close-in habitable planets is extreme, and the stellar wind plasma conditions change from sub- to super-Alfvenic along the planetary orbit. As a result, the magnetospheric structure changes dramatically with a bow shock forming in the super-Alfvenic sectors, while no bow shock forms in the sub-Alfvenic sectors. The planets reside most of the time in the sub-Alfvenic sectors with poor atmospheric protection. A significant amount of Joule Heating is provided at the top of the atmosphere as a result of the planetary interaction with the stellar wind. For the steady-state solution, the heating is about 0.1-3% of the total incoming stellar irradiation, and it is enhanced by 50% for the time-dependent case. The significant Joule Heating obtained here should be considered in models for the atmospheres of habitable planets in terms of the thickness of the atmosphere, the top-side temperature and density, the boundary conditions for the atmospheric pressure, and particle radiation and transport.
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