The radius anomaly in the planet/brown dwarf overlapping mass regime

The recent detection of the transit of very massive substellar companions (CoRoT-3b, Deleuil et al. 2008; CoRoT-15b, Bouchy et al. 2010; WASP-30b, Anderson et al. 2010; Hat-P-20b, Bakos et al. 2010) provides a strong constraint to planet and brown dwarf formation and migration mechanisms. Whether these objects are brown dwarfs originating from the gravitational collapse of a dense molecular cloud that, at the same time, gave birth to the more massive stellar companion, or whether they are planets that formed through core accretion of solids in the protoplanetary disk can not always been determined unambiguously and the mechanisms responsible for their short orbital distances are not yet fully understood. In this contribution, we examine the possibility to constrain the nature of a massive substellar object from the various observables provided by the combination of Radial Velocity and Photometry measurements (e.g. M_p, R_p, M_s, Age, a, e...). In a second part, developments in the modeling of tidal evolution at high eccentricity and inclination - as measured for HD 80 606 with e=0.9337 (Naef et al. 2001), XO-3 with a stellar obliquity >37.3+-3.7 deg (H\'ebrard et al. 2008; Winn et al. 2009) and several other exoplanets - are discussed along with their implication in the understanding of the radius anomaly problem of extrasolar giant planets.Comment: Proceedings of the conference: "Detection and dynamics of transiting exoplanets" held at the OHP, 23-27 August 2010. 7 pages, 3 figure

A long term numerical solution for the insolation quantities of the Earth

22 may 2004We present here a new solution for the astronomical computation of the insolation quantities on Earth spanning from -250 Myr to 250 Myr. This solution has been improved with respect to La93 (Laskar et al, 1993) by using a direct integration of the gravitational equations for the orbital motion, and by improving the dissipative contributions, in particular in the evolution of the Earth-Moon System. The orbital solution has been used for the calibration of the Neogene period (Lourens \etal, 2004), and is expected to be used for age calibrations of paleoclimatic data over 40 to 50 Myr, eventually over the full Palaeogene period (65 Myr) with caution. Beyond this time span, the chaotic evolution of the orbits prevents a precise determination of the Earth motion. However, the most regular components of the orbital solution could still be used over a much longer time span, which is why we provide here the solution over 250 Myr. Over this time interval, the most striking feature of the obliquity solution, apart from a secular global increase due to tidal dissipation, is a strong decrease of about $0.38$ degree in the next few millions of years, due to the crossing of the $s_6+g_5-g_6$ resonance (Laskar et al, 1993). For the calibration of the Mesozoic time scale (about 65 to 250 Myr), we propose to use the term of largest amplitude in the eccentricity, related to $g_2-g_5$, with a fixed frequency of $3.200$ ''/yr, corresponding to a period of 405000 yr. The uncertainty of this time scale over 100 Myr should be about $0.1\%$, and $0.2\%$ over the full Mesozoic era

Long term evolution and chaotic diffusion of the insolation quantities of Mars.

As the obliquity of Mars is strongly chaotic, it is not possible to give a solution for its evolution over more than a few million of years. Using the most recent data for the rotational state of Mars, and a new numerical integration of the Solar System, we provide here a precise solution for the evolution of Mars spin over 10 to 20 Myr.Over 250 Myr, we present a statistical study of its possible evolution, when considering the uncertainties in the present rotational state. Over much longer time span, reaching 5 Gyr, the chaotic diffusion prevails, and we have performed an extensive statistical analysis of the orbital and rotational evolution of Mars, relying on Laskar's secular solution of the Solar System, based on more than 600 orbital and 200 000 obliquity solutions over 5 Gyr.The density function of the eccentricity and obliquity are explicited with simple analytical formulas.We found an averaged eccentricity of Mars over 5 Gyr of e=0.0690 with standard deviation s_e=0.0299, while the averaged value of the obliquity is 37.62 deg with a standard deviation of 13.82 deg and a maximal value of 82.035 deg.We find that the probability for Mars obliquity to have reached more than 60 degin the past 1 Gyr is 63.0%, and 89.3% in 3 Gyr. Over 4 Gyr, the position of Mars axis is given by a uniform distribution on a spherical cap limited by the obliquity 58.62 deg, with the addition of a random noise allowing a slow diffusion beyond this limit. We can also define a standard model of Mars insolation parameters over 4 Gyr with the most probable values for the eccentricity 0.068 and 41.80 deg for the obliquity

Rotation et habitabilité des planètes extra-solaires

National audienc

Sur certains aspects de la théorie astronomique des paléoclimats terrestres et martiens

This thesis is devoted to the study of some aspects of the relationship between the astronomical forcing of the Earth and Mars and their respective paleoclimates. In a first part, we study in detail the influence of a dissipative phenomenon called “climate friction” on the Earth’s mean obliquity, caused by a resonant feedback between the obliquity oscillations and the lagged variations of the Earth’s oblateness (ice-mass transport, glacial isotatic adjustment). Its impact has been estimated during the last major Earth’s glaciations : the Plio-Pleistocene (the last 3 million years), the Permo-Carboniferous (∼260-340 Ma) and the Neoproterozoic (750± 200 Ma), taking available constraints on glacial data (duration, paleogeography, variability) into account. We show that climate friction has not changed the Earth’s obliquity by more than 3-4˚ over the last 800 Ma, which tends to exclude the past high obliquity scenario proposed by G.E. Williams (1993) to explain the paradoxical Neoproterozoic law-latitude glaciations. The second part is devoted to the evolution of the northen Martian polar cap. The layered deposits exposed in the throughs of the cap have been analyzed with the recent mars Global Surveyor data. A comparison between stratigraphic profiles and new orbital martian solutions has been made in the temporal and frequency domains to constrain their age and their average formation rate. We propose that the first 250 meters of the northern cap have been formed with a rate close to 0.05 cm/yr and that the polar cap could have mainly been formed during the last five millions years after the transition between a high mean obliquity regime and a lower mean obliquity regime (Laskar, J., Levrard, B., and Mustard, J.F., Naure, 419). Independently, the stability of the northern cap as been studied with the LMD General Circulation Model (GCM) for higher obliquities than the present one. The cap could be unstable for obliquities higher than 40˚ leading to an accumulation of water ice in equatorial areas which coincide with some observational zones of paleo-glaciers. The important speed would imply its quick disappearance during the high mean obliquity regime and would tend to support its possible youthfulness.Cette thèse consacrée à l’étude de certains aspects du lien entre la dynamique planétaire terrestre et martienne et leurs paléoclimats respectifs. Dans une première partie, nous étudions en détail l’influence d’un phénomène dissipatif appelé « friction climatique » sur l’obliquité moyenne terrestre, engendré par la rétroaction entre les oscillations de l’obliquité et les variations retardées de l’aplatissement terrestre (transport de masse glaciaire, ajustement visco-élastique). Son impact a été estimé durant les dernières grandes glaciations terrestres : le Plio-Pléistocène (les derniers 3 millions d’années), le Permo-Carbonifère (∼260-340 Ma) et le Néoprotérozoïque (750 ± 200 Ma), en tenant compte des contraintes disponibles sur les paléo-glaciations (durée, paléogéographie, variabilité). Nous montrons que cet effet n’a pas pu engendrer une dérive de l’obliquité terrestre supérieure à ∼ 3-4˚ durant les derniers 800 Ma, tendant ainsi à exclure le scénario de haute obliquité proposé par G.E. Williams (1993) pour expliquer les glaciations paradoxales à basses latitudes du Néoprotérozoïque. La seconde partie est dédiée à l’évolution de la calotte polaire nord martienne. Les dépôts sédimentaires en couches exposés dans les crevasses de la calotte ont été d’abord analysées grâce aux données de la sonde Mars Global Surveyor. Des corrélations temporelles et fréquentielles entre les profils stratigraphiques et de nouvelles solutions orbitales martiennes ont été cherchées pour contraindre leur âge et leur vitesse de formation. Nous proposons que les 250 premiers mètres de la calotte nord se sont déposés avec un taux moyen d’environ 0.05 cm/an et que celle-ci aurait pu se former principalement durant les cinq derniers millions d’années après la transition entre un régime de haute obliquité moyenne et un régime de plus basse obliquité moyenne (Laskar, J., Levrard, B., et Mustard, J.F., Naure, 419). Indépendamment, la stabilité de la calotte nord a été étudiée grâce aux Modèles de Circulation Générale martiens du LMD stabilité de la calotte nord a été étudiée grâce aux Modèles de Circulation Générale martien du LMD (Paris VI) pour des obliquités supérieures à la valeur actuelle. La calotte apparait instable pour des obliquités supérieures à 40˚ conduisant à l’accumulation de glace dans certaines régions équatoriales qui coïncident avec des zones d’observations de paléo-glaciers. Les vitesses importantes de perte de la calotte impliqueraient sa disparition rapide durant le régime de haute obliquité moyenne et tendrait à soutenir la possibilité de sa formation

Rotation et habitabilité des planètes extra-solaires

National audienc

La friction climatique a-t-elle changé l'obliquité terrestre

National audienc

La friction climatique a-t-elle changé l'obliquité terrestre

National audienc

Evolution of the Martian Northern cap Over the Last 10 Myr Inferred From GCM LMD Water Cycle Simulations: Implications for Layered Deposits and Cap Formation.

International audienceLayered deposits are exposed in the walls of the troughs cutting the north polar cap of Mars. They consist of alternating ice and dust layers or layers of an ice-dust mixture with varying proportions and are found throughout the cap. However, the details of their formation process remain unknown. Extensive simulations of the LMD GCM water cycle (Forget et al., 1999; Montmessin et al., 2004) have been performed at various obliquities (15o to 45o with a 5o step), eccentricities (0 to 0.12) and longitude of perihelion, to investigate the rates of water ice exchange between the northern cap and potential equatorial ice reservoirs. The corresponding rates were propagated over the last 10 Myr martian orbital and axial history (Laskar et al, 2002; 2004) in order to track the evolution of polar and equatorial ice thicknesses and test simple models for layers formation over orbital cycles. We found that the annual cap stability mainly depends on the summer solstice insolation. Above a critical insolation, the annual loss rates appear to be a marked exponential function of the insolation. Conversely, beyond this value, polar accumulation rates estimated from various sizes and locations of equatorial sources have found to be nearly unsensitive both to insolation values and remain close to 2 mm/yr. The evolution of the northern cap thickness was then tested for three scenarii.: (1) an infinite equatorial reservoir (2) The formation of a protecting dust lag reducing the further ice sublimation by a variable factor since ~ 10 meters of ice are sublimed (assuming that ice deposited at low obliquity contains about 10% of dust and that a ~1 m dust thickness is sufficient to strongly reduce the ice sublimation) (3) The previous scenario coupled with a realistic history of the equatorial reservoir. The exchange of ice between high-latitude deposits and equatorial reservoir (but not between the polar cap and high-latitudes reservoir) was thus simulated to ensure the global ice mass conservation. Our three-box model is then able to track the evolution of polar, high-latitudes and tropical ice reservoirs for arbitrary initial ice distributions. In the three scenarios, ice rapidly goes to the equator in the high mean obliquity regime (5-10 Myr) and the onset of the polar cap formation begins around 4 Myr during the transition towards the low-mean obliquity period. In the third scenario, more than 600 meters of ice could have accumulated at the north pole from the equator since 4 Myr, without major erosionnal episodes, allowing the formation of ~27 ice layers (or 54 dust-ice layers) with an averaged 22,0 m thickness and a 13.8 m standard deviation, consistent with current thickness observations (Milkovich and Head, 2004). These properties are not modified for moderate variations of model parameters. Our results implies that most of the current 3-km polar ice thickness must originate from other subsurface or surface reservoirs (south cap, high-latitudes ice deposits) acting when the equatorial reservoir disappears. Our model predicts that such periods could have occured recently (0-350 kyr and around 2.4 Myr) corresponding to periods of mininal obliquity variations.Conversely, we discuss the possibility of transient equatorial ice reservoirs during recent large insolation excursions and erosionnal periods for the northern cap