Current Population Statistics Do Not Favor Photoevaporation over Core-powered Mass Loss as the Dominant Cause of the Exoplanet Radius Gap

Loyd, R. O. Parke; Shkolnik, Evgenya L.; Schneider, Adam C.; Richey-Yowell, Tyler; Barman, Travis; Peacock, Sarah; Pagano, I.

doi:10.3847/1538-4357/ab6605

Cited by 55 publications

(48 citation statements)

References 36 publications

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“…Instead, the amount of mass lost depends on the equilibrium temperature of the planet. Simulations of core-powered mass loss from Loyd et al (2020) produce a radius gap that is comparable to the the radius gap observed in the data, differing only in location. This may be explained by differences in stellar mass, though they find no stellar mass dependence on the exoplanet radius gap of their sample.…”

Section: Atmosphere Retentionsupporting

confidence: 68%

Rapid Formation of Super-Earths Around Low-Mass Stars

Zawadzki¹

2024

Preprint

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<p>NASA's TESS mission is expected to discover hundreds of M dwarf planets. However, few studies focus on how planets form around low-mass stars. We aim to better characterize the formation process of M dwarf planets to fill this gap and aid in the interpretation of TESS results. We use six sets of N-body planet formation simulations which vary in whether a gas disc is present, initial range of embryo semi-major axes, and initial solid surface density profile. Each simulation begins with 147 equal-mass embryos around a 0.2 solar mass star and runs for 100 Myr. We find that planets form rapidly, with most collisions occurring within the first 1 Myr. The presence of a gas disc reduces the final number of planets relative to a gas-free environment and causes planets to migrate inward. Because planet formation occurs significantly faster than the disc lifetime, super-Earths have plenty of time to accrete extended gaseous envelopes, though these may later be removed by collisions or a secondary process like photo-evaporation. In addition, we find that the final distribution of planets does not retain a memory of the slope of the initial surface density profile, regardless of whether or not a gas disc is present. Thus, our results suggest that present-day observations are unlikely to provide sufficient information to accurately reverse-engineer the initial distribution of solids.</p>

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Section: Atmosphere Retentionsupporting

confidence: 68%

Rapid Formation of Super-Earths Around Low-Mass Stars

Zawadzki¹

2024

Preprint

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“…The red lines show atmosphere retention thresholds after 3.0 Gy for the case where all volatiles are in the atmosphere initially and there is no primary atmosphere; the 16th and 84th percentiles are shown, for varying XUV flux (by ±0.4 dex, 1 σ; ref. 48) relative to the baseline model following the results of refs. 49 and 50 (SI Appendix, section 1a).…”

Section: Gj 357bmentioning

confidence: 99%

Exoplanet secondary atmosphere loss and revival

Kite

Barnett

2020

Proc. Natl. Acad. Sci. U.S.A.

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The next step on the path toward another Earth is to find atmospheres similar to those of Earth and Venus—high–molecular-weight (secondary) atmospheres—on rocky exoplanets. Many rocky exoplanets are born with thick (>10 kbar) H2-dominated atmospheres but subsequently lose their H2; this process has no known Solar System analog. We study the consequences of early loss of a thick H2 atmosphere for subsequent occurrence of a high–molecular-weight atmosphere using a simple model of atmosphere evolution (including atmosphere loss to space, magma ocean crystallization, and volcanic outgassing). We also calculate atmosphere survival for rocky worlds that start with no H2. Our results imply that most rocky exoplanets orbiting closer to their star than the habitable zone that were formed with thick H2-dominated atmospheres lack high–molecular-weight atmospheres today. During early magma ocean crystallization, high–molecular-weight species usually do not form long-lived high–molecular-weight atmospheres; instead, they are lost to space alongside H2. This early volatile depletion also makes it more difficult for later volcanic outgassing to revive the atmosphere. However, atmospheres should persist on worlds that start with abundant volatiles (for example, water worlds). Our results imply that in order to find high–molecular-weight atmospheres on warm exoplanets orbiting M-stars, we should target worlds that formed H2-poor, that have anomalously large radii, or that orbit less active stars.

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“…Regardless, this gap has been noted in both the Kepler (Fulton et al 2017) and K2 (Hardegree-Ullman et al 2020) planet samples, indicating that an underlying formation mechanism is at play. As we continue to increase the known planet sample, we will be able to better constrain the underlying mechanism (Loyd et al 2020).…”

Section: Deviations From the Modelmentioning

confidence: 99%

Scaling K2. III. Comparable Planet Occurrence in the FGK Samples of Campaign 5 and Kepler

Zink¹,

Hardegree-Ullman²,

Christiansen³

et al. 2020

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Using our K2 Campaign 5 fully automated planet-detection data set (43 planets), which has corresponding measures of completeness and reliability, we infer an underlying planet population model for the FGK dwarf sample (9257 stars). Implementing a broken power law for both the period and radius distributions, we find an overall planet occurrence of-+ 1.00 0.51 1.07 planets per star within a period range of 0.5-38 days. Making similar cuts and running a comparable analysis on the Kepler sample (2318 planets; 94,222 stars), we find an overall occurrence of 1.10±0.05 planets per star. Since the Campaign 5 field is nearly 120 angular degrees away from the Kepler field, this occurrence similarity offers evidence that the Kepler sample may provide a good baseline for Galactic inferences. Furthermore, the Kepler stellar sample is metal-rich compared to the K2 Campaign 5 sample, so a finding of occurrence parity may reduce the role of metallicity in planet formation. However, a weak (1.5σ) difference, in agreement with metal-driven formation, is found when assuming the Kepler model power laws for the K2 Campaign 5 sample and optimizing only the planet occurrence factor. This weak trend indicates that further investigation of metallicity-dependent occurrence is warranted once a larger sample of uniformly vetted K2 planet candidates is made available.

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Current Population Statistics Do Not Favor Photoevaporation over Core-powered Mass Loss as the Dominant Cause of the Exoplanet Radius Gap

Cited by 55 publications

References 36 publications

Rapid Formation of Super-Earths Around Low-Mass Stars

Rapid Formation of Super-Earths Around Low-Mass Stars

Exoplanet secondary atmosphere loss and revival

Scaling K2. III. Comparable Planet Occurrence in the FGK Samples of Campaign 5 and Kepler

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