ExoLunch - winter-2024

Feb. 2, 2024
noon - 1 p.m.
3814 Geology

Presented By:

• Ethan Schreyer - Imperial College London

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Atmospheric escape plays a significant role in the evolution of close-in small exoplanets. Due to the proximity of these planets to their host star, their upper atmosphere is extremely hot and escapes in the form of a hydrodynamic wind. This process can strip these planets of their primordial hydrogen/helium envelopes and has been used to explain the lack of short period Neptune sized planets (e.g. hot Neptune desert) and the bimodal radius distribution of small planets (e.g. radius valley). Despite much theoretical progress, there is still uncertainty in the specifics of the escape process. For example, whether mass loss is driven by high energy or bolometric irradiation or the role of planetary magnetic fields in controlling escape. Observations of atmospheric escape, via the transit method, provide an opportunity to directly test different escape models. In this talk, I will outline both what we can learn from these observations, and the challenges involved in translating these observations into constraints on the properties of the outflow. I will focus on my work modelling Lyman-α transits and helium 1083 nm transits, the latter in which we present a novel method to measure the magnetic fields of exoplanets undergoing atmospheric escape.

March 1, 2024
noon - 1 p.m.
3814 Geology

Presented By:

• Greg Gilbert - Department of Physics and Astronomy, UCLA

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NASA's Kepler mission identified over 4000 extrasolar planets that transit (cross in front of) their host stars. This sample has revealed detailed features in the demographics of planet sizes and orbital spacings. However, knowledge of their orbital shapes --- a key tracer of planetary formation and evolution --- remains far more limited. We present measurements of eccentricities for 2108 Kepler planets, 92% of which are smaller than Neptune. We find that for all planet sizes, the eccentricity distribution has its mode at e=0 and falls monotonically toward zero at e=1. As planet size increases, the mean population eccentricity rises from ~0.05 for small planets to ~0.28 for planets larger than 3.7 Earth-radii. The overall planet occurrence rate and planet-metallicity correlation also change abruptly at this size. Taken together, these patterns indicate distinct formation channels for planets above and below 3.7 Earth-radii. We also find size dependent associations between eccentricity, host star metallicity, and orbital period. While the smaller planets generally have low eccentricities, there is a noteworthy exception: eccentricities are elevated over a narrow range of sizes, 1.7--2.2 Earth radii. This is consistent with another key demographic feature, the ``radius valley'' a narrow band of low planet occurrence that separates rocky ``super-Earths'' from gas-rich ``sub-Neptunes.'' The eccentricity peak in the radius valley may be the result of mergers of super-Earths that build larger planets or the result of giant impacts on sub-Neptunes that strip their gas envelopes and decrease their size. Planets in single- and multi-transiting systems exhibit the same size-eccentricity relationship, except that the singles 2.5 times more eccentric, suggesting they are drawn from the same parent population.