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Despite being one of the weakest dimers in nature, low-spectral-resolution Voyager/IRIS observations revealed the presence of (H$_2$)$_2$ dimers on Jupiter and Saturn in the 1980s. However, the collision-induced H$_2$-H$_2$ opacity databases widely used in planetary science (Borysow et al., 1985; Orton et al., 2007; Richard et al., 2012) have thus far only included free-to-free transitions and have neglected the contributions of dimers. Dimer spectra have both fine-scale structure near the S$(0)$ and S$(1)$ quadrupole lines (354 and 587 cm$^{-1}$, respectively), and broad continuum absorption contributions up to $pm50$ cm$^{-1}$ from the line centres. We develop a new ab initio model for the free-to-bound, bound-to-free and bound-to-bound transitions of the hydrogen dimer for a range of temperatures (40-400 K) and para-hydrogen fractions (0.25-1.0). The model is validated against low-temperature laboratory experiments, and used to simulate the spectra of the giant planets. The new collision-induced opacity database permits high-resolution (0.5-1.0 cm$^{-1}$) spectral modelling of dimer spectra near S$(0)$ and S$(1)$ in both Cassini Composite Infrared Spectrometer (CIRS) observations of Jupiter and Saturn, and in Spitzer Infrared Spectrometer (IRS) observations of Uranus and Neptune for the first time. Furthermore, the model reproduces the dimer signatures observed in Voyager/IRIS data near S$(0)$ (McKellar et al., 1984) on Jupiter and Saturn, and generally lowers the amount of para-H$_2$ (and the extent of disequilibrium) required to reproduce IRIS observations.
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Planet-planet scattering best explains the eccentricity distribution of extrasolar giant planets. Past literature showed that the orbits of planets evolve due to planet-planet scattering. This work studies the spin evolution of planets in planet-planet scattering in 2-planet systems. Spin can evolve dramatically due to spin-orbit coupling made possible by the evolving spin and orbital precession during the planet-planet scattering phase. The main source of torque to planet spin is the stellar torque, and the total planet-plane torque contribution is negligible. As a consequence of the evolution of the spin, planets can end up with significant obliquity (the angle between a planets own orbit normal and spin axis) like planets in our Solar System.
Interior models of giant planets traditionally assume that at a given radius (i.e. pressure) the density should be larger than or equal to the one corresponding to a homogeneous, adiabatic stratification throughout the planet (referred to as the outer adiabat). The observations of Jupiters gravity field by Juno combined with the constraints on its atmospheric composition appear to be incompatible with such a profile. In this letter, we show that the above assumption stems from an incorrect understanding of the Schwarzschild-Ledoux criterion, which is only valid on a local scale. In order to fulfil the buoyancy stability condition, the density gradient with pressure in a non-adiabatic region must indeed rise more steeply than the {it local} adiabatic density gradient. However, the density gradient can be smaller than the one corresponding to the outer adiabat at the same pressure because of the higher temperature in an inhomogeneously stratified medium. Deep enough, the density can therefore be lower than the one corresponding to the outer adiabat. We show that this is permitted only if the slope of the local adiabat becomes shallower than the slope of the outer adiabat at the same pressure, as found in recent Jupiter models due to the increase of both specific entropy and adiabatic index with depth. We examine the dynamical stability of this structure and show that it is stable against non-adiabatic perturbations. The possibility of such unconventional density profile in Jupiter complicates further our understanding of the internal structure and evolution of (extrasolar) giant planets.
The terrestrial planets are believed to have formed by violent collisions of tens of lunar- to Mars-size protoplanets at time t<200 Myr after the protoplanetary gas disk dispersal (t_0). The solar system giant planets rapidly formed during the protoplanetary disk stage and, after t_0, radially migrated by interacting with outer disk planetesimals. An early (t<100 Myr) dynamical instability is thought to have occurred with Jupiter having gravitational encounters with a planetary-size body, jumping inward by ~0.2-0.5 au, and landing on its current, mildly eccentric orbit. Here we investigate how the giant planet instability affected formation of the terrestrial planets. We study several instability cases that were previously shown to match many solar system constraints. We find that resonances with the giant planets help to remove solids available for accretion near ~1.5 au, thus stalling the growth of Mars. It does not matter, however, whether the giant planets are placed on their current orbits at t_0 or whether they realistically evolve in one of our instability models; the results are practically the same. The tight orbital spacing of Venus and Earth is difficult to reproduce in our simulations, including cases where bodies grow from a narrow annulus at 0.7-1 au, because protoplanets tend to radially spread during accretion. The best results are obtained in the narrow-annulus model when protoplanets emerging from the dispersing gas nebula are assumed to have (at least) the Mars mass. This suggests efficient accretion of the terrestrial protoplanets during the first ~10 Myr of the solar system.
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In the standard model of core accretion, the formation of giant planets occurs by two main processes: first, a massive core is formed by the accretion of solid material; then, when this core exceeds a critical value (typically greater than 10 Earth masses) a gaseous runaway growth is triggered and the planet accretes big quantities of gas in a short period of time until the planet achieves its final mass. Thus, the formation of a massive core has to occur when the nebular gas is still available in the disk. This phenomenon imposes a strong time-scale constraint in giant planet formation due to the fact that the lifetimes of the observed protoplanetary disks are in general lower than 10 Myr. The formation of massive cores before 10 Myr by accretion of big planetesimals (with radii > 10 km) in the oligarchic growth regime is only possible in massive disks. However, planetesimal accretion rates significantly increase for small bodies, especially for pebbles, particles of sizes between mm and cm, which are strongly coupled with the gas. In this work, we study the formation of giant planets incorporating pebble accretion rates in our global model of planet formation.
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