The nature of the radius valley

Venturini, Julia; Guilera, O. M.; Haldemann, Jonas; Ronco, M. P.; Mordasini, Christoph

doi:10.1051/0004-6361/202039141

Cited by 140 publications

(127 citation statements)

References 70 publications

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“…We note that gas accretion stops earlier than this, due to the low accretion rates with gap opening given by Tanigawa & Ikoma (2007) (dashed-green lines). We study the migration scenario more in-depth in an accompanying letter (Venturini et al 2020a, hereafter, Paper II)Paper II.…”

Section: Migration From the Ice Linementioning

confidence: 99%

Most super-Earths formed by dry pebble accretion are less massive than 5 Earth masses

Venturini

Guilera

Ronco

et al. 2020

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Aims. The goal of this work is to study the formation of rocky planets by dry pebble accretion from self-consistent dust-growth models. In particular, we aim to compute the maximum core mass of a rocky planet that can sustain a thin H-He atmosphere to account for the second peak of the Kepler size distribution. Methods. We simulate planetary growth by pebble accretion inside the ice line. The pebble flux is computed self-consistently from dust growth by solving the advection–diffusion equation for a representative dust size. Dust coagulation, drift, fragmentation, and sublimation at the water ice line are included. The disc evolution is computed solving the vertical and radial structure for standard α-discs with photoevaporation from the central star. The planets grow from a moon-mass embryo by silicate pebble accretion and gas accretion. We perform a parameter study to analyse the effect of a different initial disc mass, α-viscosity, disc metallicity, and embryo location. We also test the effect of considering migration versus an in situ scenario. Finally, we compute atmospheric mass loss due to evaporation over 5 Gyr of evolution. Results. We find that inside the ice line, the fragmentation barrier determines the size of pebbles, which leads to different planetary growth patterns for different disc viscosities. We also find that in this inner disc region, the pebble isolation mass typically decays to values below 5 M⊕ within the first million years of disc evolution, limiting the core masses to that value. After computing atmospheric mass loss, we find that planets with cores below ~4 M⊕ become completely stripped of their atmospheres, and a few 4–5 M⊕ cores retain a thin atmosphere that places them in the “gap” or second peak of the Kepler size distribution. In addition, a few rare objects that form in extremely low-viscosity discs accrete a core of 7 M⊕ and equal envelope mass, which is reduced to 3–5 M⊕ after evaporation. These objects end up with radii of ~6–7 R⊕. Conclusions. Overall, we find that rocky planets form only in low-viscosity discs (α ≲ 10−4). When α ≥ 10−3, rocky objects do not grow beyond 1 Mars mass. For the successful low-viscosity cases, the most typical outcome of dry pebble accretion is terrestrial planets with masses spanning from that of Mars to ~4 M⊕.

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Section: Migration From the Ice Linementioning

confidence: 99%

Most super-Earths formed by dry pebble accretion are less massive than 5 Earth masses

Venturini

Guilera

Ronco

et al. 2020

A&A

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“…We note that if the presence of a valley seems robust, it could be due to effects that are not related to evaporation, e.g. core cooling(Gupta & Schlichting 2019), or from combined formation and evolution effects(Venturini et al 2020).…”

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confidence: 93%

Six transiting planets and a chain of Laplace resonances in TOI-178

Leleu

Alibert²,

Hara³

et al. 2021

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Determining the architecture of multi-planetary systems is one of the cornerstones of understanding planet formation and evolution. Resonant systems are especially important as the fragility of their orbital configuration ensures that no significant scattering or collisional event has taken place since the earliest formation phase when the parent protoplanetary disc was still present. In this context, TOI-178 has been the subject of particular attention since the first TESS observations hinted at the possible presence of a near 2:3:3 resonant chain. Here we report the results of observations from CHEOPS, ESPRESSO, NGTS, and SPECULOOS with the aim of deciphering the peculiar orbital architecture of the system. We show that TOI-178 harbours at least six planets in the super-Earth to mini-Neptune regimes, with radii ranging from 1.152−0.070+0.073 to 2.87−0.13+0.14 Earth radii and periods of 1.91, 3.24, 6.56, 9.96, 15.23, and 20.71 days. All planets but the innermost one form a 2:4:6:9:12 chain of Laplace resonances, and the planetary densities show important variations from planet to planet, jumping from 1.02−0.23+0.28 to 0.177−0.061+0.055 times the Earth’s density between planets c and d. Using Bayesian interior structure retrieval models, we show that the amount of gas in the planets does not vary in a monotonous way, contrary to what one would expect from simple formation and evolution models and unlike other known systems in a chain of Laplace resonances. The brightness of TOI-178 (H = 8.76 mag, J = 9.37 mag, V = 11.95 mag) allows for a precise characterisation of its orbital architecture as well as of the physical nature of the six presently known transiting planets it harbours. The peculiar orbital configuration and the diversity in average density among the planets in the system will enable the study of interior planetary structures and atmospheric evolution, providing important clues on the formation of super-Earths and mini-Neptunes.

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“…Planets above ≈0.1 Earth masses start to migrate through the disk (for a review, see Baruteau et al 2014 (Bitsch et al 2019a;Izidoro et al 2021;Schoonenberg et al 2019;Venturini et al 2020). The exact water fraction of the sub-Neptunes therefore depends on a complex interplay between the planetary growth rate, planetary migration, and the evolution of the water ice line over time (Bitsch et al 2019a), as well as on the scattering and merging of planets (Raymond et al 2018).…”

Section: Planet Migration and The Timing Of Planet Formationmentioning

confidence: 99%

“…This could lead to sub-Neptunes formed in the inner disk that accrete water vapor from the disk. Alternatively, if planetary cores grow too slowly at the water ice line, they are subject to type-I migration for a long time and might migrate into the inner disk before they become gas giants, forming very water-rich sub-Neptunes (Izidoro et al 2021;Bitsch et al 2019b;Venturini et al 2020, see scenario C in Fig. 1).…”

Section: Planet Migration and The Timing Of Planet Formationmentioning

confidence: 99%

“…If the sub-Neptune forms outside the water ice line, it can accrete large amounts of water-rich pebbles and planetesimals (Ormel et al 2017;Bitsch et al 2019a;Schoonenberg et al 2019;Liu et al 2019;Venturini et al 2020) before it migrates to the inner edge of the protoplanetary disk. As a result, a very wet sub-Neptune is formed (scenario C in Fig.…”

Section: Introductionmentioning

confidence: 99%

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Dry or water world? How the water contents of inner sub-Neptunes constrain giant planet formation and the location of the water ice line

Bitsch

Raymond

Buchhave

et al. 2021

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In the pebble accretion scenario, the pebbles that form planets drift inward from the outer disk regions, carrying water ice with them. At the water ice line, the water ice on the inward drifting pebbles evaporates and is released into the gas phase, resulting in water-rich gas and dry pebbles that move into the inner disk regions. Large planetary cores can block the inward drifting pebbles by forming a pressure bump outside their orbit in the protoplanetary disk. Depending on the relative position of a growing planetary core relative to the water ice line, water-rich pebbles might be blocked outside or inside the water ice line. Pebbles blocked outside the water ice line do not evaporate and thus do not release their water vapor into the gas phase, resulting in a dry inner disk, while pebbles blocked inside the water ice line release their water vapor into the gas phase, resulting in water vapor diffusing into the inner disk. As a consequence, close-in sub-Neptunes that accrete some gas from the disk should be dry or wet, respectively, if outer gas giants are outside or inside the water ice line, assuming that giant planets form fast, as has been suggested for Jupiter in our Solar System. Alternatively, a sub-Neptune could form outside the water ice line, accreting a large amount of icy pebbles and then migrating inward as a very wet sub-Neptune. We suggest that the water content of inner sub-Neptunes in systems with giant planets that can efficiently block the inward drifting pebbles could constrain the formation conditions of these systems, thus making these sub-Neptunes exciting targets for detailed characterization (e.g., with JWST, ELT, or ARIEL). In addition, the search for giant planets in systems with already characterized sub-Neptunes can be used to constrain the formation conditions of giant planets as well.

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The nature of the radius valley

Cited by 140 publications

References 70 publications

Most super-Earths formed by dry pebble accretion are less massive than 5 Earth masses

Most super-Earths formed by dry pebble accretion are less massive than 5 Earth masses

Six transiting planets and a chain of Laplace resonances in TOI-178

Dry or water world? How the water contents of inner sub-Neptunes constrain giant planet formation and the location of the water ice line

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