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Register a new userHot-Jupiters and hot-Neptunes: a common origin?
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Added by
Isabelle Baraffe dr
Publication date
2005
fields
Physics
and research's language is
English
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We compare evolutionary models for close-in exoplanets coupling irradiation and evaporation due respectively to the thermal and high energy flux of the parent star with observations of recently discovered new transiting planets. The models provide an overall good agreement with observations, although at the very limit of the quoted error bars of OGLE-TR-10, depending on its age. Using the same general theory, we show that the three recently detected hot-Neptune planets (GJ436, $rho$ Cancri, $mu$ Ara) may originate from more massive gas giants which have undergone significant evaporation. We thus suggest that hot-Neptunes and hot-Jupiters may share the same origin and evolution history. Our scenario provides testable predictions in terms of the mass-radius relationships of these hot-Neptunes.
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Short period planets are subject to intense energetic irradiations from their stars. It has been shown that this can lead to significant atmospheric mass-loss and create smaller mass planets. Here, we analyse whether the evaporation mechanism can affect the orbit of planets. The orbital evolution of a planet undergoing evaporation is derived analytically in a very general way. Analytical results are then compared with the period distribution of two classes of inner exoplanets: Jupiter-mass planets and Neptune-mass planets. These two populations have a very distinct period distribution, with a probability lower than 10^-4 that they were derived from the same parent distribution. We show that mass ejection can generate significant migration with an increase of orbital period that matches very well the difference of distribution of the two populations. This would happen if the evaporation emanates from above the hottest region of planet surface. Thus, migration induced by evaporation is an important mechanism that cannot be neglected.
High resolution spectroscopy (HRS) has been used to detect a number of species in the atmospheres of hot Jupiters. Key to such detections is accurately and precisely modelled spectra for cross-correlation against the R$gtrsim$20,000 observations. There is a need for the latest generation of opacities which form the basis for high signal-to-noise detections using such spectra. In this study we present and make publicly available cross sections for six molecular species, H$_2$O, CO, HCN, CH$_4$, NH$_3$ and CO$_2$ using the latest line lists most suitable for low- and high-resolution spectroscopy. We focus on the infrared (0.95-5~$mu$m) and between 500-1500~K where these species have strong spectral signatures. We generate these cross sections on a grid of pressures and temperatures typical for the photospheres of super Earth, warm Neptunes and hot Jupiters using the latest H$_2$ and He pressure broadening. We highlight the most prominent infrared spectral features by modelling three representative exoplanets, GJ~1214~b, GJ~3470~b and HD~189733~b, which encompass a wide range in temperature, mass and radii. In addition, we verify the line lists for H$_2$O, CO and HCN with previous high resolution observations of hot Jupiters. However, we are unable to detect CH$_4$ with our new cross sections from HRS observations of HD~102195~b. These high accuracy opacities are critical for atmospheric detections with HRS and will be continually updated as new data becomes available.
Photoionization heating from UV radiation incident on the atmospheres of hot Jupiters may drive planetary mass loss. We construct a model of escape that includes realistic heating and cooling, ionization balance, tidal gravity, and pressure confinement by the host star wind. We show that mass loss takes the form of a hydrodynamic (Parker) wind, emitted from the planets dayside during lulls in the stellar wind. When dayside winds are suppressed by the confining action of the stellar wind, nightside winds might pick up if there is sufficient horizontal transport of heat. A hot Jupiter loses mass at maximum rates of ~2 x 10^12 g/s during its host stars pre-main-sequence phase and ~2 x10^10 g/s during the stars main sequence lifetime, for total maximum losses of ~0.06% and ~0.6% of the planets mass, respectively. For UV fluxes F_UV < 10^4 erg/cm^2/s, the mass loss rate is approximately energy-limited and is proportional to F_UV^0.9. For larger UV fluxes, such as those typical of T Tauri stars, radiative losses and plasma recombination force the mass loss rate to increase more slowly as F_UV^0.6. Dayside winds are quenched during the T Tauri phase because of confinement by overwhelming stellar wind pressure. We conclude that while UV radiation can indeed drive winds from hot Jupiters, such winds cannot significantly alter planetary masses during any evolutionary stage. They can, however, produce observable signatures. Candidates for explaining why the Lyman-alpha photons of HD 209458 are absorbed at Doppler-shifted velocities of +/- 100 km/s include charge-exchange in the shock between the planetary and stellar winds.
The observed low densities of gas giant planets with a high equilibrium temperature can be simulated in models when a fraction of the surface radiation is deposited deeper in the interior. Meanwhile migration theories suggest that hot Jupiters formed further away from their host-star and migrated inward. We incorporate disk migration in simulations of the evolving interior of hot Jupiters to determine whether migration has a long lasting effect on the inflation of planets. We quantify the difference between the radius of a migrated planet and the radius of a planet that formed in situ as the radius discrepancy. We remain agnostic about the physical mechanism behind interior heating, but assume it scales with the received stellar flux by a certain fraction. We find that the change in irradiation received from the host-star while the planet is migrating can affect the inflation and final radius of the planet. Models with a high fraction of energy deposited in the interior ( > 5%) show a significant radius discrepancy when the deposit is at higher pressures than P=1 bar. For a smaller fraction of 1%, there is no radius discrepancy for any deposit depth. We show that a uniform heating mechanism can cause different rates of inflation, depending on the migration history. If the forthcoming observations on mean densities and atmospheres of gas giants give a better indication of a potential heating mechanism, this could help to constrain the prior migration of such planets.
We provide a brief review of many aspects of the planetary physics of hot Jupiters. Our aim is to cover most of the major areas of current study while providing the reader with additional references for more detailed follow-up. We first discuss giant planet formation and subsequent orbital evolution via disk-driven torques or dynamical interactions. More than one formation pathway is needed to understand the population. Next, we examine our current understanding of the evolutionary history and current interior structure of the planets, where we focus on bulk composition as well as viable models to explain the inflated radii of the population. Finally we discuss aspects of their atmospheres in the context of observations and 1D and 3D models, including atmospheric structure and escape, spectroscopic signatures, and complex atmospheric circulation. The major opacity sources in these atmospheres, including alkali metals, water vapor, and others, are discussed. We discuss physics that control the 3D atmospheric circulation and day-to-night temperature structures. We conclude by suggesting important future work for still-open questions.
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