MAKE SUPER-EARTHS, NOT JUPITERS: ACCRETING NEBULAR GAS ONTO SOLID CORES AT 0.1 AU AND BEYOND

By Eve J. Lee, E. Chiang, C. Ormel. 2014

157 citations recorded by the source corpus.

Close-in super-Earths having radii 1–4 R⊕ may possess hydrogen atmospheres comprising a few percent by mass of their rocky cores. We determine the conditions under which such atmospheres can be accreted by cores from their parent circumstellar disks. Accretion from the nebula is problematic because it is too efficient: we find that 10 M⊕ cores embedded in solar metallicity disks tend to undergo runaway gas accretion and explode into Jupiters, irrespective of orbital location. The threat of runaway is especially dire at ∼0.1 AU, where solids may coagulate on timescales orders of magnitude shorter than gas clearing times; thus nascent atmospheres on close-in orbits are unlikely to be supported against collapse by planetesimal accretion. The time to runaway accretion is well approximated by the cooling time of the atmosphere's innermost convective zone, whose extent is controlled by where H2 dissociates. Insofar as the temperatures characterizing H2 dissociation are universal, timescales for core instability tend not to vary with orbital distance—and to be alarmingly short for 10 M⊕ cores. Nevertheless, in the thicket of parameter space, we identify two scenarios, not mutually exclusive, that can reproduce the preponderance of percent-by-mass atmospheres for super-Earths at ∼0.1 AU, while still ensuring the formation of Jupiters at ≳ 1 AU. Scenario (a): planets form in disks with dust-to-gas ratios that range from ∼20× solar at 0.1 AU to ∼2× solar at 5 AU. Scenario (b): the final assembly of super-Earth cores from mergers of proto-cores—a process that completes quickly at ∼0.1 AU once begun—is delayed by gas dynamical friction until just before disk gas dissipates completely. Both scenarios predict that the occurrence rate for super-Earths versus orbital distance, and the corresponding rate for Jupiters, should trend in opposite directions, as the former population is transformed into the latter: as gas giants become more frequent from ∼1 to 10 AU, super-Earths should become more rare.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/03b57bbfe3e88a55e6aa1df3dbb523da5625bfba

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In Astronomy and Astrophysics

1. J. Owen
2. E. Petigura
3. B. Fulton
4. Yanqin Wu
5. Timothy Morton
6. C. Mordasini
7. A. Morbidelli
8. Andrew W. Howard

In Instrumentation

1. J. Owen
2. Timothy Morton
3. E. Petigura
4. B. Fulton
5. Andrew W. Howard
6. G. Marcy
7. Yanqin Wu
8. J. Johnson

Citations in the corpus

• The Evaporation Valley in the Kepler Planets (2017, background): “It is roughly the mass of an adiabatic envelope maintained by a planet of a few Earth masses at 0.1 AU (but see Lee et al. 2014; Ginzburg et al. 2016, for modifications).”
• Formation of planetary systems by pebble accretion and migration (2018, background): “Isolated planets exceeding approximately 1 ME can accrete about 10% of their total mass in gas during the disc phase (Lee et al. 2014; Ormel et al. 2015; Lambrechts & Lega 2017).”
• IN SITU FORMATION AND DYNAMICAL EVOLUTION OF HOT JUPITER SYSTEMS (2015, methodology): “Following an initial phase of solid body accretion, the 2 We note that the calculations of Lee et al. (2014); Lee and Chiang (2015) are subject to a different set of physical assumptions than the calculations presented in this work.”
• Photoevaporation and high-eccentricity migration created the sub-Jovian desert (2018, background): “Many authors (e.g. Hansen & Murray 2012; Chatterjee & Tan 2014; Lee et al. 2014; Lee & Chiang 2016; Mohanty et al. 2017) suggested that low-mass super-Earths/mini-Neptunes formed in situ, close to their current orbital locations, while Boley et al. (2016); Batygin et al. (2016) suggested that…”
• Dearth of short-period Neptunian exoplanets - a desert in period-mass and period-radius planes (2016, background): “…for the lower boundary of the desert and for the existence of the super-Earth ridge, we mention the recent works that suggested in situ models for the formation of short-period super-Earths (Lee & Chiang (2016); see also Hansen & Murray (2012); Chiang & Laughlin (2013) and Lee et al. (2014)).”
• SUPER-EARTH ATMOSPHERES: SELF-CONSISTENT GAS ACCRETION AND RETENTION (2015, background): “E P] 1 6 A pr 2 01 6 2 of ≈ 10M⊕ is often cited; see, e.g., Rafikov 2006, 2011; Lee et al. 2014), which gathered their own mass in gas, and exploded into gas giants.”
• The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets (2017, background): “In the other scenario superEarths form late in the evolution of the protoplanetary disk, just as the gas dissipates (Lee et al. 2014).”
• Core-powered mass-loss and the radius distribution of small exoplanets (2017, methodology): “Theoretical models demonstrate that such atmospheres can be gravitationally accreted by the cores from the gaseous nebulae that surround young stars for their first few million years (Lee et al. 2014; Inamdar & Schlichting 2015; Lee & Chiang 2015; Ginzburg et al. 2016).”
• BREEDING SUPER-EARTHS AND BIRTHING SUPER-PUFFS IN TRANSITIONAL DISKS (2015, background): “…and too large to be purely rocky.3 Motivated by these discoveries, we have investigated in a series of papers the physics of nebular accretion: how rocky cores at stellocentric distances of 0.1–5 AU acquire gas from their parent nebulae (Lee et al. 2014, Paper I; Lee & Chiang 2015, Paper II).”
• 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): “From a formation point of view it seems rather unlikely that other effects like a late gas-poor formation (Lee et al. 2014; Lopez & Rice 2016) or envelope removal by giant impacts (e.g., Schlichting et al. 2015) should not have played a role as well.”
• About 30% of Sun-like Stars Have Kepler-like Planetary Systems: A Study of Their Intrinsic Architecture (2018, background): “One possibility for this is late assembly (Lee et al. 2014).”
• The California-Kepler Survey. VII. Precise Planet Radii Leveraging Gaia DR2 Reveal the Stellar Mass Dependence of the Planet Radius Gap (2018, background): “…mechanisms have been proposed that could potentially produce a gap in the size distribution including delayed formation in a gas-poor disk (e.g., Lee et al. 2014; Lee & Chiang 2016), and sculpting by giant impacts (e.g., Liu et al. 2015; Schlichting et al. 2015; Inamdar & Schlichting 2016).”