Atmospheres & Climate

Under The Light Of A New Star: Evolution Of Planetary Atmospheres Through Protoplanetary Disc Dispersal And Boil-off

By Keith Cowing
Status Report
November 22, 2023
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Under The Light Of A New Star: Evolution Of Planetary Atmospheres Through Protoplanetary Disc Dispersal And Boil-off
A summary of planet properties at the onset of XUV photoevaporation from models in Figure 6 with constant equilibrium temperatures of 600K, 900K, 1200K, and 1500K in blue, green, orange and red respectively. We assume the disc disperses at 𝑡disp = 3 × 106 yrs with a disc dispersal timescale of 𝜏disp = 105 yrs, all of which lead to XUV photoevaporation dominating at ∼ 4 Myrs, which coincides with the local protoplanetary disc having completely dispersed in these models. The left-hand panel shows the atmospheric mass fraction. Green lines with squares represent results from Owen & Wu (2016), for which boil-off was performed at 900K with initial atmospheric mass fractions of 0.3 and 0.1 in dot-dashed and dotted lines, respectively (note that initial atmospheric mass fractions in our models are calculated self-consistently through gaseous accretion). The right-hand panel shows planet sizes, including those that have been stripped of their atmosphere (atmospheric mass fraction < 10−4 ) and sit on an Earth-like composition line (grey dashed). This panel can be interpreted as a primordial mass-radius distribution. -- astro-ph.EP

The atmospheres of small, close-in exoplanets are vulnerable to rapid mass-loss during protoplanetary disc dispersal via a process referred to as `boil-off’, in which confining pressure from the local gas disc reduces, inducing atmospheric loss and contraction. We construct self-consistent models of planet evolution during gaseous core accretion and boil-off.

As the surrounding disc gas dissipates, we find that planets lose mass via subsonic breeze outflows which allow causal contact to exist between disc and planet. Planets initially accrete of order ∼10% in atmospheric mass, however, boil-off can remove ≳90% of this mass during disc dispersal. We show that a planet’s final atmospheric mass fraction is strongly dictated by the ratio of cooling timescale to disc dispersal timescale, as well as the planet’s core mass and equilibrium temperature.

With contributions from core cooling and radioactivity, we show that core luminosity eventually leads to the transition from boil-off to core-powered mass-loss. We find that smaller mass planets closest to their host star may have their atmospheres completely stripped through a combination of boil-off and core-powered mass-loss during disc dispersal, implying the existence of a population-level radius gap emerging as the disc disperses.

We additionally consider the transition from boil-off/core-powered mass-loss to X-ray/EUV (XUV) photoevaporation by considering the penetration of stellar XUV photons below the planet’s sonic surface. Finally, we show that planets may open gaps in their protoplanetary discs during the late stages of boil-off, which may enhance mass-loss rates.

James G. Rogers, James E. Owen, Hilke E. Schlichting

Comments: 18 pages, 8 figures. Re-submitted to MNRAS after moderate revisions. Comments welcome
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2311.12295 [astro-ph.EP] (or arXiv:2311.12295v1 [astro-ph.EP] for this version)
Submission history
From: James Rogers

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻