Figure 1. Left: Spectra of model atmospheres of giantplanets for different effective temperatures (colored solid lines, BT-Settl models from Allard et al. 2011). Each spectrum is normalized to its value at 1.6 µm and is vertically shifted by a constant. The transmission of the three SDI filters of NaCo are shown (black curves). Theoretical absorption bands of water and methane are also indicated. Right: Flux ratio as a function of the effective temperature derived for the NaCo SDI filters. F 1 , F 2 , and F 3 refer to the fluxes in the filters at 1.575, 1.6, and 1.625 µm. the analysis of SDI-processed images and instead we propose a straightforward method based on the injection of synthetic planets in the raw data, combined with the use of evolutionary models. This framework should serve as a basis for the data reduction and analysis of SPHERE and GPI. We present the observing strategy, the procedure used to analyze and interprete the data, and the detection limits of the survey.
Finally, hydrostatic balance also applies in the interior of giantplanets. To be stricto censu applicable, there would need to be no wind acceleration whatsoever in giantplanets. This is impossible, as we have shown above that Jupiter is dominated by convective motion, and the outgoing or ingoing convective plumes must turn over eventually. How- ever, if the planet is in a globally equilibrated state (a planet is never truly in equilibrium as long as it radiates more ﬂux than it receives from the sun), then the internal motions must average out overall. In that case, although locally this assumption can break, we have globally dP/dr = −gρ. This does not prevent the emergence of winds, but it supposes that this balance is instantaneously recovered if there is the slightest change: the pressure structure smoothes away any deviation. Said diﬀerently, any local perturbation will always be at the pressure of its sur- rounding, and other thermodynamic quantities (density and temperature mainly) will adjust more slowly. Globally, it means that the speed of sound is much larger than any other characteristic velocity in the interior of giantplanets. Hydrostatic balance in giantplanets can be understood further. Let’s assume that the gravity acceleration is not compensated by the pressure gradients. Necessarily, the winds are the only other phenomena that can prevent the star from collapsing (see the Navier-Stokes equation for the vertical momentum). With order of magnitude estimates, one would obtain that the global vertical motion must be, in absolute value, of the order of a few km.s −1
extending from about 1.5 AU beyond the last planet up to 30–35 AU. As a consequence of the interaction of the planets with the planetesimal disk, the giantplanets suffered orbital
migration, which slowly increased their orbital separation. As shown in their N-body
simulations, after a long quiescent phase (with a duration varying from 300 My to 1 Gy, depending on the exact initial conditions), Jupiter and Saturn were forced to cross their mutual 1:2 MMR. This event excited their orbital eccentricities to values similar to those presently observed. The acquisition of eccentricity by both gas giants destabilized Uranus and Neptune. Their orbits became very eccentric, so that they penetrated deep into the planetesimal disk. Thus, the planetesimal disk was dispersed, and the interaction between planets and planetesimals finally parked all four planets on orbits with separations, eccentricities and inclinations similar to what we currently observe. This model has a long list of successes. As already said, it explains the current orbital architecture of the giantplanets (Tsiganis et al., 2005). It also explains the origin of the so-called Late Heavy Bombardment (LHB), a spike in the cratering history of the terrestrial planets that occurred ∼ 650 My after planet formation. In the Nice model, the LHB is triggered by the dispersion of the planetesimal disk; both the timing, the duration and the intensity of the LHB deduced from Lunar constraints are well reproduced by the model (Gomes et al., 2005). Furthermore, the Nice model also explains the capture of planetesimals around the Lagrangian points of Jupiter, with a total mass and orbital distribution consistent with the observed Jupiter Trojans (Morbidelli et al., 2005). More recently, it has been shown to provide a framework for understanding the capture and orbital distribution of the irregular satellites of Saturn, Uranus and Neptune (Nesvorny et al., 2007). The main properties of the Kuiper belt (the relic of the primitive trans-planetary planetesimal disk) have also been explained in the context of the Nice model (Levison et al., 2007; see Morbidelli et al., 2007, for a review).
Planet formation scenarios (Alibert, Mordasini & Benz 2011 ) and simulations extrapolating RV planet populations (Johnson et al. 2007 ; Crepp & Johnson 2011 ) suggest that massive stars ( >1.3 M) are the most favourable targets for directly imaging planets, since they have proportionally more material to form giantplanets. Indeed many directly imaged planetary mass companions have been found around A or F stars: HR8799 bcde (Marois et al. 2008 ; Marois, Macintosh & V´eran 2010 ), β Pic b (Lagrange et al. 2009 , 2010 ), HD 95086 b (Rameau et al. 2013b , c ), HD 106906 b (Bailey et al. 2014 ). The detection of the HR8799 planets was the result of the Vigan et al. ( 2012 ) International Deep Planet Survey. Most surveys, however, have yielded null results (Kasper et al. 2007 ; Lafreni`ere et al. 2007 ; Chauvin et al. 2010 , 2015 ; Heinze et al. 2010 ; Biller
Giantplanets are expected to be born on quasi-circular orbits because low relative velocities with respect to the planetesimals in the disk are a necessary condition to allow the rapid formation of their cores (Kokubo & Ida, 1996, 1998; Goldreich et al., 2004). Once the giantplanets have formed, their eccentricities evolve under the effects of their interactions with the disc of gas. These interactions can in principle enhance the eccentricities of very massive planets (Goldreich & Sari, 2003), but for moderate-mass planets they have a damping effect. In fact, numerical hydro-dynamical simulations (Kley & Dirksen, 2006; D’Angelo et al., 2006) show that only planets of masses larger than 2–3 Jupiter masses that are initially on circular orbits are able to excite an eccentricity in the disk and, in response, to become eccentric themselves. Planets of Jupiter-mass or less have their eccentricities damped. Accounting for turbulence should not change the result significantly: the eccentricity excitation due to turbulence is only of the order of 0.01 for a 10 Earth mass planet and decreases rapidly with increasing mass of said planet (Nelson, 2005). By comparison, the mean ec- centricities of Jupiter and Saturn are 0.045 and 0.05 respectively. In addition, the interactions between Jupiter and Saturn, as they evolve and migrate in the disk of gas, should not lead to a significant enhancement of their eccentricities. Figure 1 shows a typical evolution of the Jupiter-Saturn pair, from Masset &
DOI: 10.1103/PhysRevB.75.024206 PACS number共s兲: 61.20.Ja, 61.25.Em, 61.25.Mv, 61.20.⫺p
The discovery of the first extrasolar planet in 1995 共Ref. 1 兲 marked the beginning of a new era in planetary science, which is characterized by great improvements in observa- tional techniques and a rapidly expanding set of known ex- trasolar planets. Most of the about 200 known planets are giant gas planets in small orbits since the primary tool for detection, radio velocity measurement, is most sensitive for finding heavy planets that rapidly orbit their parent star. 2,3 From radius measurements of transient extrasolar planets, we know that most of the discovered planets consist primarily of hydrogen and helium. Therefore, there is a great need for accurate equation of state 共EOS兲 data for these elements un- der giant gas planet conditions. 4 Knowledge of the equilib- rium properties of mixtures of hydrogen and helium will help to clarify questions concerning the inner structure, origin, and evolution of such astrophysical objects. Open questions are whether or not hydrogen and helium phase separate in- side giantplanets, whether or not a plasma phase transition 72 under the influence of helium can be found, and whether or not a solid rocky core exists in Jupiter. 4,5
The current picture we have consists of bottom-up formation of the solar system bodies from micrometric dust all the way to the large bodies we see today through chemical sticking and gravitational attraction. More massive bodies will form farther out from the sun due to the presence of ices, unavailable in the inner hot nebula. In these regions, planetary bodies ten times heavier than Earth will then gravitationally capture the sur- rounding gas and transform into giant gaseous planets. The solar system's small bodies are the remaining debris once planets formation has ceased. However, the devil is in the detail, since measurements showed a large discrepancy in the chemical compositions of giantplanets. This necessitate invoking additional mechanisms and formation steps to explain the unique chemical composition of individual bodies. Jupiter for example was visited in 1995 by the space probe Galileo, who revealed the planet to be more rich in volatiles elements (such as carbon, nitrogen, noble gases and others) than expected. This motivated a lot of research on the origin of giantplanets, considering Jupiter as a bench- mark and test lab for the theories. Another surprise came recently with the inference of a near unity Carbon-to-Oxygen (C/O) ratio in the hot Jupiter WASP 12b, indicating a chemical composition dierent than its host star. This discovery lead me to the rst question I tried answering during my PhD thesis:
2. Auroral Characteristics
[ 4 ] The auroral emissions on the giantplanets in the UV
(700–1800 Å) stem from the emission of atomic H lines from the Lyman series and H 2 vibronic lines + continuum from the B 1 ∑ g + →X 1 ∑ g + , C 1 ∏ u → X 1 ∑ g + , B′ 1 ∑ u + → X 1 ∑ g + , D 1 ∏ u → X 1 ∑ g + , B″ 1 ∑ u + → X 1 ∑ g + and D′ 1 ∏ u → X 1 ∑ g + system bands. These bands are produced by the excitation of H 2 ground-state molecules by electrons of magnetospheric origin precipitating into the atmosphere. In the FUV (1200– 1800 Å), the signal is dominated by H Lyman-a (Ly-a) and the Lyman (B → X) and Werner (C → X) bands. An iono- spheric hydrocarbon layer interacts with the aurora and attenuates the emission in specific wavelength ranges. Three hydrocarbons have a significant optical depth in the UV: methane (CH 4 ), ethane (C 2 H 6 ) and acetylene (C 2 H 2 ), with a clear domination of CH 4 . Methane attenuates the H 2 emis- sion shortward of 1350 Å, leaving the emission longward of 1350 Å unattenuated. This absorption is measured by the color ratio CR = I (1550–1620 Å)/I(1230/1300 Å) with I the intensity in photon units [Yung et al., 1982]. It relates the attenuation of the auroral emission to the amount of hydro- carbons overlying the emission layer. It is thus an indicator of the relative penetration depth of the primary electrons into the hydrocarbon layer and can be related to their incident energy. The conversion factors derived hereafter consider that all the auroral emission is due to electron precipitation, as demon- strated by the study by Trafton et al.  and Liu and Schultz . By contrast, the X-ray aurora is principally due to sulfur and oxygen ions precipitation [Waite et al., 1994; Kharchenko et al., 2006].
Cite this article as: C.S. Arridge, N. Achilleos, J. Agarwal, C.B. Agnor, R. Ambrosi, N. André, S.V. Badman, K. Baines, D. Banfield, M. Barthélémy, M. Bisi, J. Blum, T. Bocanegra-Bahamon, B. Bonfond, C. Bracken, P. Brandt, C. Briand, C. Briois, S. Brooks, J. Castillo-Rogez, T. Cavalié, B. Christophe, A. Coates, G. Collinson, J.F. Cooper, M. Costa-Sitja, R. Courtin, I.A. Daglis, I. de Pater, M. Desai, D. Dirkx, M. K. Dougherty, R.W. Ebert, G. Filacchione, L.N. Fletcher, J. Fortney, I. Gerth, D. Grassi, D. Grodent, E. Grün, J. Gustin, M. Hedman, R. Helled, P. Henri, S. Hess, J. K. Hillier, M.H. Hofstadter, R. Holme, M. Horanyi, G. Hospodarsky, S. Hsu, P. Irwin, C.M. Jackman, O. Karatekin, S. Kempf, E. Khalisi, K. Konstantinidis, H. Krüger, W.S. Kurth, C. Labrianidis, V. Lainey, L.L. Lamy, M. Laneuville, D. Lucchesi, A. Luntzer, J. MacArthur, A. Maier, A. Masters, S. McKenna-Lawlor, H. Melin, A. Milillo, G. Moragas-Klostermeyer, A. Morschhauser, J.I. Moses, O. Mousis, N. Nettelmann, F.M. Neubauer, T. Nordheim, B. Noyelles, G.S. Orton, M. Owens, R. Peron, C. Plainaki, F. Postberg, N. Rambaux, K. Retherford, S. Reynaud, E. Roussos, C.T. Russell, A.M. Rymer, R. Sallantin, A. Sánchez-Lavega, O. Santolik, J. Saur, K. Sayanagi, P. Schenk, J. Schubert, N. Sergis, E.C. Sittler, A. Smith, F. Spahn, R. Srama, T. Stallard, V. Sterken, Z. Sternovsky, M. Tiscareno, G. Tobie, F. Tosi, M. Trieloff, D. Turrini, E.P. Turtle, S. Vinatier, R. Wilson, P. Zarka, The science case for an orbital mission to Uranus: Exploring the origins and evolution of ice giantplanets, Planetary and Space Science, http://dx.doi.org/ 10.1016/j.pss.2014.08.009
VLT Very Large Telescope
WFIRST Wide-Field Infrared Survey Telescope
The canonical architecture of the Solar System often groups the Gas Giantplanets, Jupiter and Saturn, together with the Ice Giants, Uranus and Neptune, and refers to them as the giantplanets. However, the importance of volatile materials (known as ices) such as methane in the interiors and atmospheres of Uranus and Neptune, the highly asymmetric configuration of their magnetic fields, and their different internal structure (amongst other things) clearly distinguish the Ice Giants as a very different class of planet. In order to unravel the origin and evolution of the Solar System one must understand all of its components. In this regard Uranus and Neptune are enigmatic objects with very poorly constrained interiors, magnetic fields, atmospheres, ring and satel- lite systems and magnetospheres, among just a few of the intriguing aspects of these systems. The importance of filling these gaps in our knowledge of the Solar System is particularly acute when trying to apply our understanding to the numerous planetary systems that have been discovered around other stars. Uranus occupies a unique place in the history of the Solar System and the fundamental processes occurring within the uranian system confirm that its scientific exploration is essential in meeting ESA’s Cosmic Vision goals (see Section 2 , particularly Section 2.4 and Table 2 ). Table 1 illustrates the key properties of the uranian system. Uranus Pathfinder (UP) was proposed to the European Space Agency’s Cosmic Vision 2015–2025 call for medium “M”
In order to study the habitability of icy moons around giantplanets, we look at the atmosphere-surface-interior connections with their similarities with the Earth as a starting point. The discovery of the water jets on Enceladus, the possibility for cryovolcanic processes on Titan and the hypothetically active mantle of Europa suggest that icy moons around giantplanets may well contain subsurface oceans.
3. In between the hot jupiter and cool/temperate giants resides a transition popu- lation of planets. They are called the period-valley giants or even warm giants. They have an orbital period roughly in the range 10 – 100 days. Their occurrence rate has been reported up to 1.6 % in the solar neighbourhood and down to 0.9 % in the Kepler field, albeit the uncertainties are large enough to prevent this differ- ence to be statistically significant. The origin of these EGPs is still unclear and they are probably in the tail of the distribution of the other two main populations. The properties of the host star, proxy of the properties of the protoplanetary disk, has been found to have an impact on the formation of EGPs. The more metal-rich stars are, the more likely they form EGPs. The metal-poor stars still form EGPs but these planets are observed with a much wider orbital separation than those orbiting metal-rich stars. This is explained by the fact the core-accretion mechanism is less efficient to form giantplanets in metal-poor disks. Therefore, when EGPs form in metal-poor disk, they need more time to accrete materials and hence migrate less efficiently. The mass of the star also impacts the formation of EGPs: the more mas- sive the central star is, the more likely they formed with EGPs. As a consequence, EGPs are extremely rare in orbit around the low-mass M dwarfs.
The booming study of exoplanets allow us to assess the diver- sity of the planetary systems of the Milky Way and to put our own solar system in perspective. Notably, ground-based transit surveys targeting relatively bright (V < 13) stars are detecting at an increasing rate short-period giantplanets amenable for a thorough characterization (orbit, structure, atmosphere), thanks to the brightness of their host star, the favorable planet-star size ratio and their high stellar irradiation (e.g. Winn 2010). With its very high detection efficiency, the WASP transit survey (Pollacco et al. 2006) is one of the most productive projects in that domain. In this context, we report here the detection by WASP of two new giantplanets, WASP-64 b and WASP-72 b, transiting relatively bright Southern stars. Section 2 presents the WASP discovery photometry, and high-precision follow-up observa- tions obtained from La Silla ESO Observatory (Chile) by the TRAPPIST and Euler telescopes to confirm the transits and planetary nature of both objects and to determine precisely the systems parameters. In Sect. 3, we present the spectroscopic de- termination of the stellar properties and the derivation of the sys- tems parameters through a combined analysis of the follow-up
Context. Tidal dissipation in planetary interiors is one of the key physical mechanisms that drive the evolution of star-planet and planet-moon systems. New constraints on this dissipation are now obtained both in the solar and exo-planetary systems.
Aims. Tidal dissipation in planets is intrinsically related to their internal structure. Indeed, the dissipation behaves very differently when we compare its properties in solid and fluid planetary layers. Since planetary interiors consist of both types of regions, it is necessary to be able to assess and compare the respective intensity of the reservoir of dissipation in each type of layers. Therefore, in the case of giantplanets, the respective contribution of the potential central dense rocky /icy core and of the deep convective fluid envelope must be computed as a function of the mass and the radius of the core. This will allow us to obtain their respective strengths.
location of Jupiter’s real tropopause where T ≈ 110 K. Actually, the properties of the opacities of important absorbing chemical species like water and their pressure dependence imply that photospheres around 0.1 bar should be common (Robinson and Catling, 2014).
However, the Eddington boundary condition should not be used in the case of irradiated atmospheres because it does not properly account for both the incoming flux (mostly at visible wavelengths for planets around solar-type stars) and the intrinsic flux (in the infrared). The fact that opacities differ at these wavelengths yields the possibility of thermal inversions (higher visible than infrared opacities) or a greenhouse effect (lower visible than infrared opacities) and thus a hotter interior, something that cannot be captured without accounting for the different fluxes. Analytical solutions of the radiative transfer problem exist in the semi-grey case (two opacities for the visible and infrared, respectively) (Hansen, 2008; Guillot, 2010), and can even be extended to include non-grey effects (Parmentier and Guillot, 2014). Numerical solutions in the non-irradiated, solar-composition case are provided by Saumon et al. (1996), and for the irradiation levels and compositions relevant for the solar system giantplanets by Fortney et al. (2011). In that case, a grid is used to relate the atmospheric temperature and pressure at a given level to the radius
the planet. This is for example believed to be the case of Neptune’s largest moon, Triton, which has a retrograde orbit.
A few satellites stand out by having relatively large masses: it is the case of Jupiter’s Io, Europa, Ganymede and Callisto, of Saturn’s Titan, and of Neptune’s Triton. Ganymede is the most massive of them, being about twice the mass of our Moon. However, compared to the mass of the central planet, these moons and satellites have very small weights: 10 −4 and less for Jupiter, 1/4000 for Saturn, 1/25000 for Uranus and 1/4500 for Neptune. All these satellites orbit relatively closely to their planets. The farthest one, Callisto revolves around Jupiter in about 16 Earth days. The four giantplanets also have rings, whose material is probably constantly resupplied from their satellites. The ring of Saturn stands out as the only one directly visible with binoculars. In this particular case, its enormous area allows it to reﬂect a sizable fraction of the stellar ﬂux arriving at Saturn, and makes this particular ring as bright as the planet itself. The occurrence of such rings would make the detection of extrasolar planets slightly easier, but it is yet unclear how frequent they can be, and how close to the stars rings can survive both the increased radiation and tidal forces.
∗ Radii from Davies et al. ( 1996 ), rotation periods from de Pater and Lissauer ( 2010 )
∗∗ The internal rotation period of Saturn is not known. This value is based on indirect evidence
the mass of typical plasma ions is an order of magnitude larger than the mass of the pro- tons that normally dominate terrestrial plasmas. Consequently inertial effects are far more important at Jupiter and Saturn than at Earth. Furthermore the large spatial dimensions and rapid rotation of the giantplanets (see Table 1 ) mean that the solar wind flows past only a portion of these magnetospheres in one planetary rotation period. Interaction with the solar wind does not dominate their dynamics. Magnetospheric and ionospheric plasmas interact through signals carried through the system by magnetohydrodynamic (MHD) waves, and the large travel distances through the giant planet magnetospheres introduce phase delays that are not readily recognizable at Earth. Furthermore, the plasma density drops rapidly as one follows a flux tube from the equator to the ionosphere, inhibiting the coupling of dif- ferent parts of the system. Indeed, the outer parts of these magnetospheres may be unable to communicate with their ionospheres. Rapid rotation of the heavy ion plasma modifies the geometry and dynamics of the entire magnetosphere and controls aspects of plasma heating and loss through mechanisms that differ from processes significant at Earth.
Among the various exoplanet detection methods, direct imag- ing is the best suited to look for sub-stellar companions orbit- ing at large orbital separation (>10 AU) around nearby stars ( <100 pc). Over the past 15 yr, multiple surveys targeting a variety of stars with di fferent spectral type, distance, age, or metallicity have been successful at placing tight constraints on the frequency of giantplanets and brown dwarfs in the 50– 1000 AU range (see Bowler 2016 , for a recent review). De- spite the development of optimized observing strategies and data analysis methods, only a limited number of sub-stellar companions have been detected, leading to the conclusion that the frequency of these objects is very low (typically .5%; Galicher et al. 2016 ). At closer orbital separations, in the 5– 50 AU range, the question remains largely unanswered, although initial results ( Macintosh et al. 2015 ; Wagner et al. 2016 ) from on-going large-scale direct imaging surveys with the new gen- eration of high-contrast imagers and spectrographs ( Beuzit et al. 2008 ; Macintosh et al. 2014 ; Guyon et al. 2010 ) suggest that the frequency of young giantplanets is within a few percent.
2. Science Themes
2.1 Elemental and Isotopic Composition as a Win- dow on the GiantPlanets Formation
The giantplanets in the solar system formed 4.55 Gyr ago from the same material that engendered the Sun and the entire solar system. Protoplanetary disks, composed of gas and dust, are almost ubiquitous when stars form, but their typical lifetimes do not exceed a few million years. This implies that the gas giants Jupiter and Saturn had to form rapidly to capture their hydrogen and helium envelopes, more rapidly than the tens of millions of years needed for terrestrial planets to reach their present masses [ 11 , 12 , 13 ]. Due to formation at fairly large radial distances from the Sun, where the solid surface density is low, the ice giants Uranus and Neptune had longer formation timescales (slow growth rates) and did not manage to capture large amounts of hydrogen and helium before the disk gas dissipated [ 14 , 15 ]. As a result, the masses of their gaseous envelopes are small compared to their ice/rock cores. A comparative study of the properties of these giantplanets thus gives information on spatial gradients in the physical and chemical properties of the solar nebula as well as on stochastic effects that led to the formation of the solar system. Data on the composition and structure of the giantplanets, which hold more than 95% of the mass of the solar system outside of the Sun, remain scarce, despite the importance of such knowledge. The formation of giantplanets is now largely thought to have taken place via the core accretion model in which a dense core is first formed by accretion and the hydrogen-helium envelope is captured after a critical mass is reached [ 16 , 11 ]. When the possibility of planet migration is included [ 17 , 18 ], such a model may be able to explain the orbital properties of exoplanets, although lots of unresolved issues remain [ 19 , 20 ]. An alternative giantplanets formation scenario is also the gravitational instability model [ 21 , 22 ], in which the giantplanets form from the direct contraction of a gas clump resulting from local gravitational instability in the disk.