SUPER-EARTH ATMOSPHERES: SELF-CONSISTENT GAS ACCRETION AND RETENTION

By S. Ginzburg, H. Schlichting, R. Sari. 2015

133 citations recorded by the source corpus.

Some recently discovered short-period Earth- to Neptune-sized exoplanets (super-Earths) have low observed mean densities that can only be explained by voluminous gaseous atmospheres. Here, we study the conditions allowing the accretion and retention of such atmospheres. We self-consistently couple the nebular gas accretion onto rocky cores and the subsequent evolution of gas envelopes following the dispersal of the protoplanetary disk. Specifically, we address mass-loss due to both photo-evaporation and cooling of the planet. We find that planets shed their outer layers (dozens of percent in mass) following the disk's dispersal (even without photo-evaporation), and their atmospheres shrink in a few Myr to a thickness comparable to the radius of the underlying rocky core. At this stage, atmospheres containing less particles than the core (equivalently, lighter than a few percent of the planet's mass) can be blown away by heat coming from the cooling core, while heavier atmospheres cool and contract on a timescale of Gyr at most. By relating the mass-loss timescale to the accretion time, we analytically identify a Goldilocks region in the mass-temperature plane in which low-density super-Earths can be found: planets have to be massive and cold enough to accrete and retain their atmospheres, but not too massive or cold, such that they do not enter runaway accretion and become gas giants (Jupiters). We compare our results to the observed super-Earth population and find that low-density planets are indeed concentrated in the theoretically allowed region. Our analytical and intuitive model can be used to investigate possible super-Earth formation scenarios.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/d8675a21ee989f2c18c5170a3c4afc3a11599fea

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In Astronomy and Astrophysics

1. J. Owen
2. Yanqin Wu
3. E. Petigura
4. B. Fulton
5. C. Mordasini
6. Sheng Jin
7. D. Huber
8. J. V. van Saders

In Instrumentation

1. J. Owen
2. B. Fulton
3. Yanqin Wu
4. D. Huber
5. J. V. van Saders
6. E. Petigura
7. Sheng Jin
8. S. Hadden

Citations in the corpus

• 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).”
• The Evaporation Valley in the Kepler Planets (2017, methodology): “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).”
• 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): “Moreover, besides impacts there could be additional loss mechanisms like for example mass loss driven by magnetohydrodynamic waves (Tanaka et al. 2014) or core-powered mass loss (Ginzburg et al. 2016, 2017, see also Owen & Wu 2016).”
• The California-Kepler Survey. VII. Precise Planet Radii Leveraging Gaia DR2 Reveal the Stellar Mass Dependence of the Planet Radius Gap (2018, background): “Ginzburg et al. (2016) developed the theory of core-powered mass loss and computed mass loss rates.”
• Revised Radii of Kepler Stars and Planets Using Gaia Data Release 2 (2018, background): “This supports XUV-driven photoevaporation as the dominant mechanism sculpting the radii of the exoplanet population, while other mechanisms such as core-powered mass loss (Ginzburg et al. 2016, 2018) seem less important.”
• Core-powered mass-loss and the radius distribution of small exoplanets (2017, background): “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).”