Olivier Grasset

The extreme physical properties of the CoRoT-7b super-Earth

Photospheric stellar activity (i.e. dark spots or bright pl ages) might be an important source of noise and confusion in s tellar radialvelocity (RV) measurements. Radial-velocimetry planet se arch surveys as well as follow-up of photometric transit sur veys require a deeper understanding and characterization of the e ffects of stellar activities to di fferentiate them from planetary signals. We simulate dark spots on a rotating stellar photosphere. The variation s in the photometry, RV, and spectral line shapes are charact erized and analyzed according to the stellar inclination, the latitud e, and the number of spots. We show that the anti-correlation between RV and bisector span, known to be a signature of activity, requi s a good sampling to be resolved when there are several spot s on the photosphere. The Lomb-Scargle periodograms of the RV varia tions induced by activity present power at the rotational pe riod Prot of the star and its two first harmonics Prot/2 andProt/3. Three adjusted sinusoids fixed at the fundamental period a nd its two-first harmonics allow us to remove about 90% of the RV jitter amplit ude. We apply and validate our approach on four known active p lanethost stars: HD 189733, GJ 674, CoRoT-7, and ιHor. We succeed in fitting simultaneously activity and plane t ry signals on GJ674 and CoRoT-7. This simultaneous modeling of the activity and planetary parameters leads to slightly higher masses of CoR oT-7b and c of respectively, 5.7± 2.5 MEarth and 13.1± 4.1 MEarth. The larger uncertainties properly take into account the st ellar active jitter. We exclude short-period low-mass exoplanets around ιHor. For data with realistic time-sampling and white Gaussi an noise, we use simulations to show that our approach is e ffective in distinguishing reflex-motion due to a planetary co mpanion and stellar-activityinduced RV variations provided that 1) the planetary orbita l period is not close to that of the stellar rotation or one of i ts two first harmonics, 2) the semi-amplitude of the planet exceeds ∼30% of the semi-amplitude of the active signal, 3) the rotati nal period of the star is accurately known, and 4) the data cover more than o ne stellar rotational period.

Thermodynamic model for water and high-pressure ices up to 2.2GPa and down to the metastable domain

We propose a thermodynamic model of the properties of liquid water and ices I, III, V, and VI that can be used in the ranges of 0-2200 MPa and 180-360 K. This model is the first to be applicable to all H(2)O phases in these wide ranges, which exceed the stability domain of all phases. Developing empirical or semiempirical expressions for the specific volumes of liquid water or ices has been necessary. The model has been tested on available experimental data sets. The specific volume of liquid water is reproduced with an accuracy better than 1%. The error on the specific volume of ices remains within 2%. The model has also been used to describe the melting curves of high-pressure ice polymorphs and compared with new Simon equations fitting available data. Our calculations suggest a slight revision of the triple point positions in the H(2)O phase diagram. We have ensured the reliability of our model up to 1.5 GPa, and we have shown that it can be used with good confidence up to 2.2 GPa. In order to show the validity of this model in the low-temperature domains, the melting curve of ice Ih in the water-ammonia system has been modeled. This curve is reproduced with good accuracy down to 180 K, at a 1 bar pressure. It shows that this model can be used in further studies for modeling equilibriums involving liquid or solid phases of H(2)O under pressure and for investigating the effect of inhibitors in complex water-rich systems.

Formation, Habitability, and Detection of Extrasolar Moons

The diversity and quantity of moons in the Solar System suggest a manifold population of natural satellites exist around extrasolar planets. Of peculiar interest from an astrobiological perspective, the number of sizable moons in the stellar habitable zones may outnumber planets in these circumstellar regions. With technological and theoretical methods now allowing for the detection of sub-Earth-sized extrasolar planets, the first detection of an extrasolar moon appears feasible. In this review, we summarize formation channels of massive exomoons that are potentially detectable with current or near-future instruments. We discuss the orbital effects that govern exomoon evolution, we present a framework to characterize an exomoons stellar plus planetary illumination as well as its tidal heating, and we address the techniques that have been proposed to search for exomoons. Most notably, we show that natural satellites in the range of 0.1-0.5 Earth mass (i) are potentially habitable, (ii) can form within the circumplanetary debris and gas disk or via capture from a binary, and (iii) are detectable with current technology.

Theoretical seismic models of Mars : the importance of the iron content of the mantle

Abstract Present-day averaged temperature profiles of the mantle of Mars are computed through numerical convection experiments performed with axisymmetrical geometry, for different values of core radii and different boundary conditions at the core-mantle boundary. Internal heating of the mantle is considered in each case. It is found that the temperature profiles of the mantle are very stable whatever the imposed conditions at the core-mantle boundary. A 300 km thick thermal lithosphere, displaying a temperature gradient equal to 4.4 K km−1 is followed at greater depths by a quasi-isothermal mantle, the temperature of which is found in a 1200–1600 K temperature range. A mean temperature equal to 1400 K is in a good agreement with the low Q of Mars at tidal frequencies. These characteristics, together with the small increase of pressure with depth, of the order of 0.01 GPa km−1, induce the presence of a low-velocity zone similar to the Earths one, down to 300 km depth. Densities and seismic velocities corresponding to these thermodynamical conditions are computed using Gruneisens and third-order finite strain theory for different values of the iron content of mantle minerals. Below 300 km depth, the values of magnitude as within the Earths transition zone. An increase of the iron content of the Martian mantle with respect to the Earths one results (1) in an increase of density, and a decrease of seismic velocities, which can reach more than 2% of the values expected from an Earth like composition, (2) in a homogenization of mantle structure through the smoothing out of seismic discontinuities over a thickness of a few hundred kilometres. This smoothing process is due to the large pressure domains of coexistence between different phases of olivine when the iron content of this latter mineral increases. Plausible domains of core density and core radius are finally checked back for each of the computed models of mantle density. These tests show that the principal moment of inertia ratio of Mars should not be lower than 0.355 if the iron content of the Martian mantle is at least equal to that of the Earth, and that the thickness of the liquid core should be small, of the order of 300–400 km, if a solid core is present at the centre of the planet. This small thickness might explain the weakness (or absence?) of an internally generated magnetic field on Mars.

Thermodynamic data and modeling of the water and ammonia-water phase diagrams up to 2.2 GPa for planetary geophysics

We present new experimental data on the liquidus of ice polymorphs in the H(2)O-NH(3) system under pressure, and use all available data to develop a new thermodynamic model predicting the phase behavior in this system in the ranges (0-2.2 GPa; 175-360 K; 0-33 wt % NH(3)). Liquidus data have been obtained with a cryogenic optical sapphire-anvil cell coupled to a Raman spectrometer. We improve upon pre-existing thermodynamic formulations for the specific volumes and heat capacities of the solid and liquid phase in the pure H(2)O phase diagram to ensure applicability of the model in the low-temperature metastable domain down to 175 K. We compute the phase equilibria in the pure H(2)O system with this new model. Then we develop a pressure-temperature dependent activity model to describe the effect of ammonia on phase transitions. We show that aqueous ammonia solutions behave as regular solutions at low pressures, and as close-to-ideal solutions at pressure above 600 MPa. The computation of phase equilibria in the H(2)O-NH(3) system shows that ice III cannot exist at concentrations above 5-10 wt % NH(3) (depending on pressure), and ice V is not expected to form above 25%-27% NH(3). We eventually address the applications of this new model for thermal and evolution models of icy satellites.

Origin and Formation of Planetary Systems

To estimate the occurrence of terrestrial exoplanets and maximize the chance of finding them, it is crucial to understand the formation of planetary systems in general and that of terrestrial planets in particular. We show that a reliable formation theory should not only explain the formation of the Solar System, with small terrestrial planets within a few AU and gas giants farther out, but also the newly discovered exoplanetary systems with close-in giant planets. Regarding the presently known exoplanets, we stress that our current knowledge is strongly biased by the sensitivity limits of current detection techniques (mainly the radial velocity method). With time and improved detection methods, the diversity of planets and orbits in exoplanetary systems will definitely increase and help to constrain the formation theory further. In this work, we review the latest state of planetary formation in relation to the origin and evolution of habitable terrestrial planets.

Evolution of Titan and implications for its hydrocarbon cycle

Measurements of the carbon and nitrogen isotopic ratios as well as the detection of 40Ar and 36Ar by the gas chromatograph mass spectrometer (GCMS) instrument on board the Huygens probe have provided key constraints on the origin and evolution of Titans atmosphere, and indirectly on the evolution of its interior. Those data combined with models of Titans interior can be used to determine the story of volatile outgassing since Titans formation. In the absence of an internal source, methane, which is irreversibly photodissociated in Titans stratosphere, should be removed entirely from the atmosphere in a time-span of a few tens of millions of years. The episodic destabilization of methane clathrate reservoir stored within Titans crust and subsequent methane outgassing could explain the present atmospheric abundance of methane, as well as the presence of argon in the atmosphere. The idea that methane is released from the interior through eruptive processes is also supported by the observations of several cryovolcanic-like features on Titans surface by the mapping spectrometer (VIMS) and the radar on board Cassini. Thermal instabilities within the icy crust, possibly favoured by the presence of ammonia, may explain the observed features and provide the conditions for eruption of methane and other volatiles. Episodic resurfacing events associated with thermal and compositional instabilities in the icy crust can have major consequences on the hydrocarbon budget on Titans surface and atmosphere.

On the Possible Properties of Small and Cold Extrasolar Planets: Is OGLE 2005-BLG-390Lb Entirely Frozen?

Extrasolar planets as light as a few Earths are now being detected. Such planets are likely not gas or ice giants. Here, we present a study on the possible properties of the small and cold extrasolar planets, applied to the case of the recently discovered planet OGLE-2005-BLG-390Lb (Beaulieu et al. 2006). This planet (5.5[+5.5/-2.7] Earth masses) orbits 2.6[+1.5/-0.6]-astronomical units away from an old M-type star of the Galactic Bulge. The planet should be entirely frozen given the low surface temperature (35 to 47 K). However, depending on the rock-to-ice mass ratio in the planet, the radiogenic heating could be sufficient to make the existence of liquid water within an icy crust possible. This possibility is estimated as a function of the planetary mass and the illumination received from the parent star, both being strongly related by the observational constraints. The results are presented for water-poor and water-rich planets. We find that no oceans can be present in any cases at 9-10 Gyr, a typical age for a star of the Bulge. However, we find that, in the past when the planet was < 5-billion-years old, liquid water was likely present below an icy surface. Nevertheless, the planet is now likely to be entirely frozen.

Very high-density planets: a possible remnant of gas giants.

Data extracted from the Extrasolar Planets Encyclo- paedia (see http://exoplanet.eu) show the existence of planets that are more massive than iron cores that would have the same size. After meticulous verification of the data, we conclude that the mass of the smallest of these planets is actually not known. However, the three largest planets, Kepler-52b, Kepler-52c and Kepler-57b, which are between 30 and 100 times the mass of the Earth, have indeed density larger than an iron planet of the same size. This observation triggers this study that investigates under which conditions these planets could represent the naked cores of gas giants that would have lost their atmospheres during their migration towards the star. This study shows that for moderate viscosity values (1025 Pa s or lower), large values of escape rate and associated unloading stress rate during the atmospheric loss process lead to the explosion of extremely massive planets. However, for moderate escape rate, the bulk viscosity and finite-strain incompressibility of the cores of giant planets can be large enough to retain a very high density during geological time scales. This would make those a new kind of planet, which would help in understanding the interior structure of the gas giants. However, this new family of exoplanets adds some degeneracy for characterizing terrestrial exoplanets.