Architecture of Planetary Systems Predicted from Protoplanetary Disks Observed with ALMA. I. Mass of the Possible Planets Embedded in the Dust Gap

Recent ALMA observations have identified a variety of dust gaps in protoplanetary disks, which are commonly interpreted to be generated by unobserved planets. Predicting mass of such embedded planets is of fundamental importance in comparing those disk architectures with the observed diversity of exoplanets. The prediction, however, depends on the assumption that whether the same gap structure exists in the dust component alone or in the gas component as well. We assume a planet can only open a gap in the gas component when its mass exceeds the pebble isolation mass by considering the core-accretion scenario. We then propose two criteria to distinguish if a gap is opened in the dust disk alone or the gas gap as well when observation data on the gas profile is not available. We apply the criteria to 35 disk systems with a total of 55 gaps compiled from previous studies and classify each gap into four different groups. The classification of the observed gaps allows us to predict the mass of embedded planets in a consistent manner with the pebble isolation mass. We find that outer gaps are mostly dust alone, while inner gaps are more likely to be associated with a gas gap as well. The distribution of such embedded planets is very different from the architecture of the observed planetary systems, suggesting that significant inward migration is required in their evolution.

No Sub-Saturn-mass Planet Desert in the CORALIE/HARPS Radial-velocity Sample

We analyze the CORALIE/HARPS sample of exoplanets found by the Doppler radial-velocity method for signs of the predicted gap or “desert” at 10–100 M ⊕ caused by runaway gas accretion at semimajor axes of <3 au. We find that these data are not consistent with this prediction. This result is similar to the finding by the MOA gravitational microlensing survey that found no desert in the exoplanet distribution for exoplanets in slightly longer period orbits and somewhat lower host masses (Suzuki et al. 2018). Together, these results imply that the runaway gas accretion scenario of the core accretion theory does not have a large influence on the final mass and semimajor axis distribution of exoplanets.

Dynamical stability of giant planets: the critical adiabatic index in the presence of a solid core

ABSTRACT The dissociation and ionization of hydrogen, during the formation of giant planets via core accretion, reduce the effective adiabatic index γ of the gas and could trigger dynamical instability. We generalize the analysis of Chandrasekhar, who determined that the threshold for instability of a self-gravitating hydrostatic body lies at γ = 4/3, to account for the presence of a planetary core, which we model as an incompressible fluid. We show that the dominant effect of the core is to stabilize the envelope to radial perturbations, in some cases completely (i.e. for all γ &gt; 1). When instability is possible, unstable planetary configurations occupy a strip of γ values whose upper boundary falls below γ = 4/3. Fiducial evolutionary tracks of giant planets forming through core accretion appear unlikely to cross the dynamical instability strip that we define.

Global 3D radiation hydrodynamic simulations of proto-Jupiter’s convective envelope

The core accretion model of giant planet formation has been challenged by the discovery of recycling flows between the planetary envelope and the disc that can slow or stall envelope accretion. We carry out 3D radiation hydrodynamic simulations with an updated opacity compilation to model the proto-Jupiter’s envelope. To isolate the 3D effects of convection and recycling, we simulate both isolated spherical envelopes and envelopes embedded in discs. The envelopes are heated at given rates to achieve steady states, enabling comparisons with 1D models. We vary envelope properties to obtain both radiative and convective solutions. Using a passive scalar, we observe significant mass recycling on the orbital timescale. For a radiative envelope, recycling can only penetrate from the disc surface until ∼0.1-0.2 planetary Hill radii, while for a convective envelope, the convective motion can ‘dredge up’ the deeper part of the envelope so that the entire convective envelope is recycled efficiently. This recycling, however, has only limited effects on the envelopes’ thermal structure. The radiative envelope embedded in the disc has identical structure as the isolated envelope. The convective envelope has a slightly higher density when it is embedded in the disc. We introduce a modified 1D approach which can fully reproduce our 3D simulations. With our updated opacity and 1D model, we recompute Jupiter’s envelope accretion with a 10 M⊕ core, and the timescale to runaway accretion is shorter than the disc lifetime as in prior studies. Finally, we discuss the implications of the efficient recycling on the observed chemical abundances of the planetary atmosphere (especially for super-Earths and mini-Neptunes).

Medium-resolution spectrum of the exoplanet HIP 65426 b

Medium-resolution integral-field spectrographs (IFS) coupled with adaptive-optics such as Keck/OSIRIS, VLT/MUSE, or SINFONI are appearing as a new avenue for enhancing the detection and characterization capabilities of young, gas giant exoplanets at large heliocentric distances (>5 au). We analyzed K-band VLT/SINFONI medium-resolution (Rλ ~5577) observations of the young giant exoplanet HIP 65426 b. Our dedicated IFS data analysis toolkit (TExTRIS) optimized the cube building, star registration, and allowed for the extraction of the planet spectrum. A Bayesian inference with the nested sampling algorithm coupled with the self-consistent forward atmospheric models BT-SETTL15 and Exo-REM using the ForMoSA tool yields Teff = 1560 ± 100 K, log(g) ≤ 4.40 dex, [M/H] = 0.05−0.22+0.24 dex, and an upper limit on the C/O (≤0.55). The object is also re-detected with the so-called “molecular mapping” technique. The technique yields consistent atmospheric parameters, but the loss of the planet pseudo-continuum in the process degrades or modifies the constraints on these parameters. The solar to sub-solar C/O ratio suggests an enrichment by solids at formation if the planet was formed beyond the water snowline (≥20 au) by core accretion (CA hereafter). However, a formation by gravitational instability (GI hereafter) cannot be ruled out. The metallicity is compatible with the bulk enrichment of massive Jovian planets from the Bern planet population models. Finally, we measure a radial velocity of 26 ± 15 km s−1 compatible with our revised measurement on the star. This is the fourth imaged exoplanet for which a radial velocity can be evaluated, illustrating the potential of such observations for assessing the coevolution of imaged systems belonging to star forming regions, such as HIP 65426.

Measurements by Probe and Orbiter Critical for Models of Formation and Evolution of Uranus and Neptune

&lt;p&gt;Core accretion is the conventional model of the formation of gas giants, Jupiter and Saturn. According to this model, a core of 10-15 Earth-mass forms in 1-5 Myr from non-gravitational collisions between submicron size grains of dust &amp;#8722; ice, rock, metals, and trapped gases. Most volatile of the gases, hydrogen, helium, and neon, can then be gravitationally captured, completing the planetary formation. Unlike gas giants, formation timescale of the icy giant planets (IGPs), Uranus, and Neptune by core accretion at their present orbital distance exceed the typical lifetime of the protoplanetary nebula. Thus, there are two alternatives: IGPs begin their formation also in the neighborhood of Jupiter and Saturn (5-10 AU) and then migrate out to their present orbital distances (20 and 30 AU), or they form by a fast process, called the gravitational instability model that requires only 1000&amp;#8217;s of years for to form them from clumps in massive protoplanetary disks at their present orbital distances. Core accretion followed by migration is still the favored scenario for the IGPs, considering the latter model does not satisfactorily explain the measured elemental abundances in the giant planets. Moreover, the exoplanet observations also support the core accretion theory. The heavy elements are key constraints to formation and migration models. Those found in the condensible, reactive, and disequilibrium species (C, N, S, O) require measurements in the deep well-mixed atmosphere, which is below kilobar levels at the IGPs, according to our thermochemical models. Extension of the models deeper shows formation of alkali metal and rock clouds at several kilobars and greater. These cloud aerosols provide extensive sites for adsorption of volatiles, irrespective of any volatile loss by sequestration or clustering in a purported water ocean or ionic-superionic ocean proposed previously [1]. Fortunately, abundances and isotopic ratios of the noble gases, He, Ne, Ar, Kr and Xe, will provide necessary constraints to the formation and evolution models of the IGPs [1,2], and entry probes deployed to only a few bars can measure them precisely. In addition, complementary measurements of gravity, magnetic field, stratospheric composition, and depth profiles of certain condensible gases from an orbiter are important to make [1,3]. Atmospheric temperature vs. pressure from exosphere to the probe depth of 5-10 bars is essential also for the interpretation of the measurements. An orbiter-probe mission that makes use of a Jupiter gravity-assisted trajectory to deliver affordable payload mass requires launch between 2030-2034 for Uranus and 2029-2031 to Neptune [1]. Such a mission requires no new technology. This presentation will discuss the new models mentioned above and possible mission scenarios. The US Astrobiology and Planetary Science Decadal Survey committee is presently reviewing the White Papers submitted in support of a mission to the icy giants in the 2023-2032 decade [e.g., 4], and would make a recommendation of mission priorities for NASA in 2022. [1]Atreya et al. Space Sci. Rev. 216:18; [2]Mousis et al. Space Sci. Rev. 216:77, 2020; [3] Fletcher et al. Trans. R. Soc. A 378: 20190473, 2020; [4]Beddingfield et al. arXiv.2007.11063, 2020.&lt;/p&gt;

Unveiling the planet population at birth

ABSTRACT The radius distribution of small, close-in exoplanets has recently been shown to be bimodal. The photoevaporation model predicted this bimodality. In the photoevaporation scenario, some planets are completely stripped of their primordial H/He atmospheres, whereas others retain them. Comparisons between the photoevaporation model and observed planetary populations have the power to unveil details of the planet population inaccessible by standard observations, such as the core mass distribution and core composition. In this work, we present a hierarchical inference analysis on the distribution of close-in exoplanets using forward models of photoevaporation evolution. We use this model to constrain the planetary distributions for core composition, core mass, and initial atmospheric mass fraction. We find that the core-mass distribution is peaked, with a peak-mass of ∼4M⊕. The bulk core-composition is consistent with a rock/iron mixture that is ice-poor and ‘Earth-like’; the spread in core-composition is found to be narrow ($\lesssim 16{{\ \rm per\ cent}}$ variation in iron-mass fraction at the 2σ level) and consistent with zero. This result favours core formation in a water/ice poor environment. We find the majority of planets accreted a H/He envelope with a typical mass fraction of $\sim 4{{\ \rm per\ cent}}$; only a small fraction did not accrete large amounts of H/He and were ‘born-rocky’. We find four times as many super-Earths were formed through photoevaporation, as formed without a large H/He atmosphere. Finally, we find core-accretion theory overpredicts the amount of H/He cores would have accreted by a factor of ∼5, pointing to additional mass-loss mechanisms (e.g. ‘boil-off’) or modifications to core-accretion theory.

Self-Sustaining Vortices in Protoplanetary Disks: Setting the Stage for Planetary System Formation

The core accretion scenario of planet formation assumes that planetesimals and planetary embryos are formed during the primordial, gaseous phases of the protoplanetary disk. However, how the dust particles overcome the traditional growth barriers is not well understood. Recently proposed viscous ring-instability explains the concentric rings observed in protoplanetary disks by assuming that the dust grains can reduce the gas conductivity, which can weaken the magneto-rotational instability. We present an analysis of this model with the help of GPU-based numerical hydrodynamic simulations of coupled gas and dust in the thin-disk limit. During the evolution of the disk the dusty rings become Rossby unstable and break up into a cascade of small-scale vortices. The vortices form secularly stable dusty structures, where the rapid dust growth can occur via the streaming instability. The phenomenon of self-sustaining vortices is consistent with observational constraints of exoplanets and sets a favorable environment for planetary system formation.