Earth and Planetary Astrophysics (astro-ph.EP)

Abstract: We model the evaporation histories of the three planets around K2-136, a K-dwarf in the Hyades open cluster with an age of 700 Myr. The star hosts three transiting planets, with radii of 1.0, 3.0 and 1.5 Earth radii, where the middle planet lies above the radius-period valley and the inner and outer planets are below. We use an XMM-Newton observation to measure the XUV radiation environment of the planets, finding that the X-ray activity of K2-136 is lower than predicted by models but typical of similar Hyades members. We estimate the internal structure of each planet, and model their evaporation histories using a range of structure and atmospheric escape formulations. While the precise X-ray irradiation history of the system may be uncertain, we exploit the fact that the three planets must have shared the same history. We find that the Earth-sized K2-136b is most likely rocky, with any primordial gaseous envelope being lost within a few Myr. The sub-Neptune, K2-136c, has an envelope contributing 1-1.7% of its mass that is stable against evaporation thanks to the high mass of its rocky core, whilst the super-Earth, K2-136d, must have a mass at the upper end of the allowed range in order to retain any of its envelope. Our results are consistent with all three planets beginning as sub-Neptunes that have since been sculpted by atmospheric evaporation to their current states, stripping the envelope from planet b and removing most from planet d whilst preserving planet c above the radius-period valley.

Abstract: Observations point to a correlation between outer giants and inner sub-Neptunes, unexplained by simulations so far. We utilize N-body simulations including pebble and gas accretion as well as planetary migration to investigate how the gas accretion rates influence the formation of systems of inner sub-Neptunes and outer gas giants as well as the eccentricity distribution of the outer giant planets. Less efficient envelope contraction rates allow a more efficient formation of systems with inner sub-Neptunes and outer giants. This is caused by the fact that the cores formed in the inner disc are too small to accrete large envelopes and only cores growing in the outer disc can become giants. As a result, instabilities between the outer giant planets do not necessarily destroy the inner systems of sub-Neptunes unlike simulations where giant planets can form closer in. Our simulations show that up to 50% of the systems of cold Jupiters could have inner sub-Neptunes, in agreement with observations. Our simulations show a good agreement with the eccentricity distribution of giants, even though we find a slight mismatch to the mass and semi-major axes distributions. Synthetic transit observations of the inner systems (r<0.7 AU) reveal an excellent match to the Kepler observations, where our simulations match the period ratios of adjacent planet pairs. Thus, the breaking the chains model for super-Earth and sub-Neptune formation remains consistent with observations even when outer giant planets are present. However, simulations with outer giant planets produce more systems with mostly only one inner planet and with larger eccentricities, in contrast to simulations without outer giants. We thus predict that systems with truly single close-in planets are more likely to host outer gas giants and we consequently suggest RV follow-up observations of these systems to constrain the formation pathway.

Abstract: The characterisation of giant exoplanets is crucial to constrain giant planet formation and evolution theory and for putting the solar-system's giant planets in perspective. Typically, mass-radius (M-R) measurements of moderately irradiated warm Jupiters are used to estimate the planetary bulk composition, which is an essential quantity for constraining giant planet formation, evolution and structure models. The successful launch of the James Webb Space Telescope (JWST) and the upcoming ARIEL mission open a new era in giant exoplanet characterisation as atmospheric measurements provide key information on the composition and internal structure of giant exoplanets. In this review, we discuss how giant planet evolution models are used to infer the planetary bulk composition, and the connection between the compositions of the interior and atmosphere. We identify the important theoretical uncertainties in evolution models including the equations of state, atmospheric models, chemical composition, interior structure and main energy transport processes. Nevertheless, we show that that atmospheric measurements by JWST and ARIEL and the accurate determination of stellar ages by PLATO can significantly reduce the degeneracy in the inferred bulk composition. Furthermore, we discuss the importance of evolution models for the characterisation of direct-imaged planets. We conclude that giant planet theory has a critical role in the interpretation of observation and emphasise the importance of advancing giant planet theory.

Abstract: In the core accretion scenario of planet formation, rocky cores grow by first accreting solids until they are massive enough to accrete gas. For giant planet formation this means that a massive core must form within the lifetime of the gas disk. The accretion of roughly km-sized planetesimals and the accretion of mm-cm sized pebbles are typically discussed separately as the main solid accretion mechanisms. We investigate the interplay between the two accretion processes in a disk containing both pebbles and planetesimals for planet formation in general and in the context of giant planet formation specifically. The goal is to disentangle and understand the fundamental interactions that arise in such hybrid pebble-planetesimal models. We combine a simple model of pebble formation and accretion with a global model of planet formation which considers the accretion of planetesimals. We compare synthetic populations of planets formed in disks composed of different amounts of pebbles and 600 meter sized planetesimals. On a system-level, we study the formation pathway of giant planets in these disks. We find that, in hybrid disks containing both pebbles and planetesimals, the formation of giant planets is strongly suppressed whereas in a pebbles-only or planetesimals-only scenario, giant planets can form. We identify the heating associated with the accretion of up to 100 km sized planetesimals after the pebble accretion period to delay the runaway gas accretion of massive cores. Coupled with strong inward type-I migration acting on these planets, this results in close-in icy sub-Neptunes originating from the outer disk. We conclude that, in hybrid pebble-planetesimal scenarios, the late accretion of planetesimals is a critical factor in the giant planet formation process and that inward migration is more efficient for planets in increasingly pebble dominated disks.