IN SITU ACCRETION OF HYDROGEN-RICH ATMOSPHERES ON SHORT-PERIOD SUPER-EARTHS: IMPLICATIONS FOR THE KEPLER-11 PLANETS

By M. Ikoma, Y. Hori. 2012

138 citations recorded by the source corpus.

Motivated by recent discoveries of low-density super-Earths with short orbital periods, we have investigated in situ accretion of H-He atmospheres on rocky bodies embedded in dissipating warm disks, by simulating quasi-static evolution of atmospheres that connect to the ambient disk. We have found that the atmospheric evolution has two distinctly different outcomes, depending on the rocky body's mass: while the atmospheres on massive rocky bodies undergo runaway disk-gas accretion, those on light rocky bodies undergo significant erosion during disk dispersal. In the atmospheric erosion, the heat content of the rocky body that was previously neglected plays an important role. We have also realized that the atmospheric mass is rather sensitive to disk temperature in the mass range of interest in this study. Our theory is applied to recently detected super-Earths orbiting Kepler-11 to examine the possibility that the planets are rock-dominated ones with relatively thick H-He atmospheres. The application suggests that the in situ formation of the relatively thick H-He atmospheres inferred by structure modeling is possible only under restricted conditions, namely, relatively slow disk dissipation and/or cool environments. This study demonstrates that low-density super-Earths provide important clues to understanding of planetary accretion and disk evolution.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/06d98c50c7813d2310c6aadf28bc54ac46fe4784

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THE KEPLER-10 PLANETARY SYSTEM REVISITED BY HARPS-N: A HOT ROCKY WORLD AND A SOLID NEPTUNE-MASS PLANET (2014, background): “Therein lies the challenge, as Kepler-10c is significantly above (A color version of this figure is available in the online journal.) the theoretical critical mass of 10–12 M⊕ enabling envelope accretion (Ikoma & Hori 2012; Lopez & Fortney 2013), but has none.”
BULK COMPOSITION OF GJ 1214b AND OTHER SUB-NEPTUNE EXOPLANETS (2013, background): “A study by (Ikoma & Hori 2012) investigates the formation of single low-mass planets with H/He envelopes by invoking in-situ accretion that they then apply to Kepler-11.”
A FRAMEWORK FOR CHARACTERIZING THE ATMOSPHERES OF LOW-MASS LOW-DENSITY TRANSITING PLANETS (2013, background): “Formation models are now aiming to understand the frequency of such planets, their formation timescale, and how often they end up in compact orbital configurations (Hansen & Murray 2012; Rogers et al. 2011; Ikoma & Hori 2012; Chiang & Laughlin 2013).”
Compositional Imprints in Density–Distance–Time: A Rocky Composition for Close-in Low-mass Exoplanets from the Location of the Valley of Evaporation (2017, background): “The colors show that already during formation, there is also a positive correlation of envelope mass and orbital distance (Ikoma & Hori 2012; Lee & Chiang 2015).”
BREEDING SUPER-EARTHS AND BIRTHING SUPER-PUFFS IN TRANSITIONAL DISKS (2015, background): “5M⊕ planets accreting enough nebular gas in-situ at ∼0.1 AU (Ikoma & Hori 2012; Inamdar & Schlichting 2015).”
Origin and evolution of the atmospheres of early Venus, Earth and Mars (2018, background): “5MEarth, gravitational energy, short-lived radioactive elements together with captured H2-envelopes enhance the temperature of the planetary surface to ≥2000 K, resulting in deep global magma oceans (e.g., Mizuno et al. 1980; Mizuno and Wetherill 1984; Sasaki and Nakazawa 1989; Ikoma et al. 2000; Ik”
MOST 1.6 EARTH-RADIUS PLANETS ARE NOT ROCKY (2014, methodology): “Calculations by Ikoma & Hori (2012) estimate that a 6 M⊕ core could accrete & 0.3% H/He by mass when imbedded in a minimummass-solar-nebula protoplanetary disk with a nebular temperature Td = 550 K and a disk lifetime of 100 kyr.”
The naked planet Earth: Most essential pre-requisite for the origin and evolution of life (2013, background): “Whereas such large radii may be accounted for by the presence of hydrogen-rich atmospheres instead of water, the masses of the hydrogen-rich atmospheres seem to be incompatible with recent formation theories (Ikoma and Hori, 2012).”
The minimum-mass extrasolar nebula: in situ formation of close-in super-Earths (2012, background): “Planetary accretion luminosities are so high (see §3.1), and dust-laden gas in the dense inner disk so optically thick, that the energy is likely transported by convection — i.e., the atmospheres are adiabatic, not isothermal (e.g., Rafikov 2006; Ikoma & Hori 2012).”
MAKE SUPER-EARTHS, NOT JUPITERS: ACCRETING NEBULAR GAS ONTO SOLID CORES AT 0.1 AU AND BEYOND (2014, background): “Although Ikoma & Hori (2012) set the planetesimal accretion luminosity Lacc = 0 (as we have explained is realistic), their models still feature a large internal luminosity: one that emanates from the rocky core with its finite heat capacity.”
HOW THERMAL EVOLUTION AND MASS-LOSS SCULPT POPULATIONS OF SUPER-EARTHS AND SUB-NEPTUNES: APPLICATION TO THE KEPLER-11 SYSTEM AND BEYOND (2012, background): “Combined with the results of Ikoma & Hori (2012), this disfavors in situ formation of Kepler-11b.”
Core-powered mass-loss and the radius distribution of small exoplanets (2017, background): “We thus identify (see also Ikoma & Hori 2012; Owen & Wu 2016) a process of spontaneous mass loss that is essentially different from (and complementary to) the high-energy photoevaporation that is usually discussed in the literature (Murray-Clay et al. 2009; Lopez et al. 2012; Owen & Jackson 2012;…”
PLANETARY POPULATION SYNTHESIS COUPLED WITH ATMOSPHERIC ESCAPE: A STATISTICAL VIEW OF EVAPORATION (2014, background): “For a fixed core mass under these circumstances, the envelope mass increases with orbital distance (see Ikoma & Hori 2012) because more gas can be bound at lower nebula temperatures, which translates into a larger radius and makes the hollow higher at larger distances (to approximately 2 AU).”
SUPER-EARTH ATMOSPHERES: SELF-CONSISTENT GAS ACCRETION AND RETENTION (2015, methodology): “Quantitatively, we parametrize the disk dispersal over time as ρd(t) = ρ 0 d exp[−(t/ttrans)α], allowing for an exponential depletion as modeled by Ikoma & Hori (2012), as well as more or less gradual processes (see, e.g. Rogers et al. 2011, who incorporate a linear decrease with time).”
UNDERSTANDING THE MASS–RADIUS RELATION FOR SUB-NEPTUNES: RADIUS AS A PROXY FOR COMPOSITION (2013, background): “…measured mass. Planet mass is critical for understanding how volatile rich planets accrete their initial H/He envelope (Bodenheimer et al. 2000; Ikoma & Hori 2012) and whether they can retain it against X-ray and EUV driven photo-evaporation (Lopez et al. 2012; Lopez & Fortney 2013; Owen &…”
Kepler Planet Masses and Eccentricities from TTV Analysis (2016, background): “A number of studies have examined envelope accretion by planetary cores embedded in a gaseous protoplanetary disk (e.g., Ikoma & Hori 2012; Bodenheimer & Lissauer 2014; Ginzburg et al. 2016; Lee & Chiang 2016).”