Results: Broad range of retained atmospheric masses Observational implications Residual H/He Atmospheres of Super-Earths William Misener & Hilke Schlichting (University of California, Los Angeles) Full paper: MNRAS 503:5658 TESS Science Conference 2 – August 2021 Figure 4: Evolution in atmospheric mass fraction f with time for two sets of planets: (a) with set Teq and varying Mc and (b) with set Mc and varying Teq. Dots mark time at which tcool=tloss Figure 6: Contours of log10fret, in orange, for finit=0.03. Super-imposed are planets with well-measured radii and masses. TESS planets are marked by crosses, those discovered by others are marked by dots. Planets are colored by their K band magnitude (planet data from Exoplanet Archive) • Fig. 6 demonstrates many known planets, including TESS discoveries, could have retained substantial residual atmospheres after core-powered mass loss • A number of these planets are amenable to further characterization with transmission spectroscopy • Such studies will be able to distinguish residual H/He atmospheres from those composed of heavier outgassed species • Mass fraction retained varies orders of magnitude across super- Earth regime, from <10-8 (negligible) to 10-2 (sub-Neptune) • Generally matches analytic trends • Exact values vary due to variation in Rrcb at which tcool=tloss • After spontaneous mass loss, if planet maintains atmosphere with larger heat capacity than its core, atmosphere will cool and contract, cutting off atmospheric mass loss →sub-Neptune (Fig. 1a) • If core has larger heat capacity than atmosphere, Ccore > Catm, its cooling will inhibit contraction, leading to continued mass loss →super-Earth • If planet can never cool more quickly than it loses mass →atmosphere entirely stripped (Fig. 1b) • If it can cool more quickly, atmosphere resumes contraction and mass loss ceases →thin H/He atmosphere saved (Fig. 1c, Fig. 2) Figure 1: schematic of the evolution of sub-Neptune and super-Earth planets from disk dispersal. Super-Earth atmosphere evolution References • Retained atmospheric mass is f at which tcool becomes shorter than tloss • Analytic form (plotted in Fig. 3): • Higher mass and lower equilibrium temperature planets retain more massive primordial atmospheres • Assumes Rrcb=2Rc when timescales become equal fret / M 3/2 c T −1/2 eq exp [M 3/4 c T −1 eq ] Main Takeaways Motivation • Previous work has shown core-powered mass loss can reproduce the observed dichotomy of super-Earths and sub-Neptunes (e.g., Ginzburg et al. (2016), Gupta & Schlichting (2019), Berger et al. (2020)) • But what is the long-term imprint of this mass loss process on super-Earth atmospheres? Mechanism • As core-powered mass loss unbinds overlying atmospheres, super-Earth cores can cool more quickly • These planets’ cooling timescales eventually become shorter than their mass loss timescales, allowing super-Earths to keep small residual H/He envelopes Results • The mass of these retained envelopes increases with planet mass and semi-major axis • The retained atmospheric mass fraction, fret, ranges from <10-8 to 1% of the planet’s total mass and is of order 10-3 for a 5 Earth mass planet at Teq = 1000 K Importance • Retaining such quantities of H/He reduces the atmosphere's mean molecular weight compared to an outgassed secondary atmosphere • This signature could be observable today or in the near future via transmission spectroscopy (e.g., Benneke & Seager (2012), Fortney et al. (2013), Greene et al. (2016)) • Large amounts of retained H/He after core-powered mass loss would affect the early geochemistry and rock-atmosphere interactions of this common class of planet (e.g., Wordsworth et al. (2018), Doyle et al. (2019), Seager et al. (2020)) • It therefore affects their potential habitability • Such tenuous atmospheres may be susceptible to further processing, e.g. by long-term photo- evaporation • TESS and related missions have found and continue to find excellent candidates to test these