Akash Gupta and Allie Doyle - UCLA

Nov. 14, 2019, noon - 1 p.m.
Slichter 3853

Presented By:
Akash Gupta and Allie Doyle,
UCLA

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ALLIE DOYLE: Exoplanetary Cosmochemistry using Polluted White Dwarf Stars; AKASH GUPTA: Understanding the Radius Valley as a by-product of Planet Formation: the Physics and Observational Predictions of the Core-Powered Mass-Loss Mechanism

ALLIE DOYLE: Abstract: Studies of exoplanets are on the rise, and we want to constrain the geochemistry of the rocky exoplanetary bodies. Using white dwarf stars that are polluted by rocky exoplanetary material, we have measured the oxidation state of ~15 of these bodies. The oxidation state, as measured by its bulk oxygen fugacity (fO2), is a critical factor that determines a planets structure and evolution. The intrinsic oxygen fugacity of a planet can determine the relative size of its metallic core, the geochemistry of its mantle and crust, the composition of its primordial atmosphere, and the forces responsible for mountain building. Most rocky bodies in our solar system formed with oxygen fugacities approximately five orders of magnitude higher than that corresponding to a hydrogen-rich gas of solar composition. We find that the intrinsic oxygen fugacities of rocks accreted by polluted white dwarf stars are similar to those of terrestrial planets and asteroids in our solar system. This result suggests that at least some rocky exoplanets are geochemically, and could be geophysically, similar to Earth. AKASH GUPTA: Abstract: In the last decade, NASA's Kepler mission has revolutionized the field of planetary science by discovering more than 4000 planetary candidates. As one of its key findings, it revealed that the most common planets, observed to date, are of the size 1 to 4 Earth radii in size. Interestingly, further observations have revealed a 'radius valley' in the size distribution of small, short-period exoplanets, i.e., a lack of planets of the size 1.5 to 2 Earth radii (by a factor of ~2). Furthermore, studies have observed that smaller planets (<1.5 Earth radii) have higher densities consistent with rocky compositions while larger planets (>2.0 Earth radii) have lower densities, suggesting they are engulfed in significant H/He atmospheres. Typically, this radius valley has been attributed to atmospheric mass-loss because of photoevaporation due to high energy radiation from host stars. However, in recent work, it has been demonstrated that atmospheric mass-loss, powered by the cooling luminosity of the planetary core, can explain this radius valley in the exoplanet size distribution, even without photoevaporation. In my talk, I will describe the key physical processes that drive this core-powered mass-loss mechanism and discuss our recent work where we present comparisons with a multitude of latest observations and present testable predictions for the core-powered mass-loss mechanism as a function of stellar mass, metallicity and age.