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Register a new userModons on Tidally Synchronised Extrasolar Planets
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We investigate modons on tidally synchronised extrasolar planets. Modons are highly dynamic, coherent flow structures composed of a pair of storms with opposite signs of vorticity. They are important because they divert flows on the large-scale; and, powered by the intense irradiation from the host star, they are planetary-scale sized and exhibit quasi-periodic life-cycles -- chaotically moving around the planet, breaking and reforming many times over long durations (e.g. thousands of planet days). Additionally, modons transport and mix planetary-scale patches of hot and cold air around the planet, leading to high-amplitude and quasi-periodic signatures in the disc-averaged temperature flux. Hence, they induce variations of the hot spot longitude to either side of the planets sub-stellar point -- consistent with observations at different epoch. The variability behaviour in our simulations broadly underscores the importance of accurately capturing vortex dynamics in extrasolar planet atmosphere modelling, particularly in understanding current observations.
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The search for extrasolar rocky planets has already found the first transiting rocky super-Earth, Corot 7b, with a surface temperature that allows for magma oceans. Here we ask if we could distinguish rocky planets with recent major volcanism by remote observation. We develop a model for volcanic eruptions on an Earth-like exoplanet based on the present day Earth, derive the observable features in emergent and transmission spectra for multiple scenarios of gas distribution and cloudcover. We calculate the observation time needed to detect explosive volcanism on exoplanets in primary as well as secondary eclipse and discuss the likelihood of observing volcanism on transiting Earth to super-Earth sized exoplanets. We find that sulfur dioxide from large explosive eruptions does present a spectral signal that is remotely detectable especially for secondary eclipse measurements around the closest stars using ground based telescopes, and report the frequency and magnitude of the expected signatures. Transit probability of planet in the habitable zone decreases with distance to the host star, making small, close by host stars the best targets
We use the Met Office Unified Model to explore the potential of a tidally locked M dwarf planet, nominally Proxima Centauri b irradiated by a quiescent version of its host star, to sustain an atmospheric ozone layer. We assume a slab ocean surface layer, and an Earth-like atmosphere of nitrogen and oxygen with trace amounts of ozone and water vapour. We describe ozone chemistry using the Chapman mechanism and the hydrogen oxide (HO$_x$, describing the sum of OH and HO$_2$) catalytic cycle. We find that Proxima Centauri radiates with sufficient UV energy to initialize the Chapman mechanism. The result is a thin but stable ozone layer that peaks at 0.75 parts per million at 25 km. The quasi-stationary distribution of atmospheric ozone is determined by photolysis driven by incoming stellar radiation and by atmospheric transport. Ozone mole fractions are smallest in the lowest 15 km of the atmosphere at the sub-stellar point and largest in the nightside gyres. Above 15 km the ozone distribution is dominated by an equatorial jet stream that circumnavigates the planet. The nightside ozone distribution is dominated by two cyclonic Rossby gyres that result in localized ozone hotspots. On the dayside the atmospheric lifetime is determined by the HO$_x$ catalytic cycle and deposition to the surface, with nightside lifetimes due to chemistry much longer than timescales associated with atmospheric transport. Surface UV values peak at the substellar point with values of 0.01 W/m$^2$, shielded by the overlying atmospheric ozone layer but more importantly by water vapour clouds.
Over large timescales, a terrestrial planet may be driven towards spin-orbit synchronous rotation by tidal forces. In this particular configuration, the planet exhibits permanent dayside and nightside, which may induce strong day-night temperature gradients. The nightside temperature depends on the efficiency of the day-night heat redistribution and determines the stability of the atmosphere against collapse. To better constrain the atmospheric stability, climate, and surface conditions of rocky planets located in the habitable zone of their host star, it is thus crucial to understand the complex mechanism of heat redistribution. Building on early works and assuming dry thermodynamics, we developed a hierarchy of analytic models taking into account the coupling between radiative transfer, dayside convection, and large-scale atmospheric circulation in the case of slowly rotating planets. There are two types of these models: a zero-dimensional two-layer approach and a two-column radiative-convective-subsiding-upwelling (RCSU) model. They yield analytical solutions and scaling laws characterising the dependence of the collapse pressure on physical features, which are compared to the results obtained by early works using 3D global climate models (GCMs). The analytical theory captures (i) the dependence of temperatures on atmospheric opacities and scattering in the shortwave and in the longwave, (ii) the behaviour of the collapse pressure observed in GCM simulations at low stellar fluxes that are due to the non-linear dependence of the atmospheric opacity on the longwave optical depth at the planets surface, (iii) the increase of stability generated by dayside sensible heating, and (iv) the decrease of stability induced by the increase of the planet size.
Using the period and mass data of two hundred and seventy-nine extrasolar planets, we have constructed a coupled period-mass function through the non-parametric approach. This analytic expression of the coupled period-mass function has been obtained for the first time in this field. Moreover, due to a moderate period-mass correlation, the shapes of mass/period functions vary as a function of period/mass. These results of mass and period functions give way to two important implications: (1) the deficit of massive close-in planets is confirmed, and (2) the more massive planets have larger ranges of possible semi-major axes. These interesting statistical results will provide important clues into the theories of planetary formation.
We revisit the tidal stability of extrasolar systems harboring a transiting planet and demonstrate that, independently of any tidal model, none but one (HAT-P-2b) of these planets has a tidal equilibrium state, which implies ultimately a collision of these objects with their host star. Consequently, conventional circularization and synchronization timescales cannot be defined because the corresponding states do not represent the endpoint of the tidal evolution. Using numerical simulations of the coupled tidal equations for the spin and orbital parameters of each transiting planetary system, we confirm these predictions and show that the orbital eccentricity and the stellar obliquity do not follow the usually assumed exponential relaxation but instead decrease significantly, reaching eventually a zero value, only during the final runaway merging of the planet with the star. The only characteristic evolution timescale of {it all} rotational and orbital parameters is the lifetime of the system, which crucially depends on the magnitude of tidal dissipation within the star. These results imply that the nearly circular orbits of transiting planets and the alignment between the stellar spin axis and the planetary orbit are unlikely to be due to tidal dissipation. Other dissipative mechanisms, for instance interactions with the protoplanetary disk, must be invoked to explain these properties.
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