DIX Planetary Science Seminar

Stephen Markham

Sufficiently abundant condensible species in hydrogen atmospheres may inhibit convection by introducing molecular weight gradients near the cloud deck associated with condensation. In this work we place constraints on the effect this can have on the thermal histories of Uranus and Neptune. We demonstrate that convective inhibition should lead to a state of radiative-convective equilibrium with finite activation energy to disrupt. We further constrain possible meteorological consequences on Saturn as well as the ice giants. Finally we extend the argument beyond our solar system and investigate the "infinite reservoir" problem, when a hydrogen envelope overlies an unlimited supply of condensates, for example a super-Earth. We provide some preliminary comments on the implications for the thermal evolution of such bodies.

Shreyas Vissapragada

Abstract: "The early evolution of planetary atmospheres, especially those enduring extreme amounts of ionizing radiation, has profound consequences for the observed distribution of exoplanetary masses and radii. With the recent discovery of metastable helium, a tracer of tenuous gas in escaping atmospheres, both ground- and space-based facilities are sensitive to atmospheric outflows near the wind-launching radius. However, such observations have previously been limited to planets midway through their lives (at a few Gyr of age), after the majority of atmospheric evolution has concluded. Here, we present a search for metastable helium on three of the planets orbiting the young (~23 Myr) solar analogue V1298 Tau. Using a beam-shaping diffuser and a narrowband filter centered on the metastable helium feature, we observed two transits of planet d (P = 12.4 days) and one transit each of planets b (P = 24.1 days) and c (P = 8.2 days) on Palomar/WIRC. We tentatively detect excess absorption for planets b and d in our bandpass. We consider the confounding impacts of stellar activity, and outline the highest-priority measurements needed to confirm these signals. If the observed excesses are due entirely to metastable helium associated with the planets, then the atmospheres of these planets likely overflow their Roche lobes. These may therefore be the first young planets exhibiting long-theorized signs of vigorous atmospheric escape."