ExoExplorers

Abstract: The population of giant planets on wide orbits around low-mass M dwarf stars is poorly understood. However, the discovery and characterization of these planets is key to understanding the architectures and evolution of M dwarf planetary systems and places their frequent and potentially habitable inner planets in context. While current ground-based imaging struggles to probe below a Jupiter mass at large separations, the unprecedented sensitivity of JWST NIRCam coronagraphic imaging provides direct access to planets significantly less massive than Jupiter beyond 10 AU around the closest, youngest M dwarfs. In this talk, I will introduce the key aspects of exoplanet direct imaging and present the survey design, observations, and preliminary results of JWST GTO Program 1184, a NIRCam coronagraphic imaging survey of very nearby, young low-mass stars.

Abstract: This dissertation models the composition and architecture of planetary systems formed via pebble accretion. The modeling is achieved using a combination of the pebble coagulation model “pebble-predictor” (Drazkowska et al., 2017) and accretion efficiency recipes (Ormel & Liu 2017) to consistently develop the pebble properties and protoplanet formation rates based on disk conditions. The composition of protoplanets is modeled by relating the disk properties to changes across the water ice line and assuming the local composition determines the pebble composition. In this way, the composition of pebbles, their accretion efficiency, and therefore protoplanet composition, are consistently modeled from disk properties. Model outputs are systems of protoplanets with a consistently determined mass, bulk composition, and orbital distance at the protoplanetary disk stage when gas fully dissipates. The dissertation will further explore variations in stellar mass, ice line evolution, and seed mass distributions to explore trends in the occurrence rates of different types and bulk compositions of planets. In this research, I assume a stage of gravitational n-body interactions follows pebble accretion. N-body simulations of this sort can be computationally expensive. Fortunately, simulations encompassing a range of starting system architectures already exist in the genesis models (Mulders et al. 2018). The results of the genesis models are recorded and publicly available in the form of interaction histories between protoplanets, or “collision trees”. This research will compare model outputs to inputs of genesis to determine what conditions, if any, can be followed through the late stages of planet formation and inspect final planetary system bulk composition and architecture.

Abstract: Transiting planet systems offer the best opportunity to measure the masses and radii of a large sample of planets and their host stars. However, relative photometry and radial velocity measurements alone only constrain the density of the host star. Thus, there is a one-parameter degeneracy in the mass and radius of the host star, and by extension the planet. Several theoretical, semi-empirical, and nearly empirical methods have been used to break this degeneracy and independently measure the mass and radius of the host star and planets(s). We focus our analysis on modelling KELT-15b, a fairly typical hot Jupiter, using each of these methods implemented in EXOFASTv2. As we approach an era of few percent precisions on some of these properties, it is critical to assess whether these different methods are providing accuracies that are of the same order, or better than, the stated statistical precisions. We investigate the differences in the planet parameter estimates inferred when using the Torres empirical relations, YY isochrones, MIST isochrones, and a nearly-direct empirical measurement of the radius of the host star using its spectral energy distribution, effective temperature, and Gaia parallax.

Abstract: Planets orbiting the same star tend to display a striking degree of uniformity in their size, mass, and orbital spacing, exhibiting a “peas-in-a-pod” phenomenon that serves to place invaluable constraints on the formation of multiple-planet systems. In this talk, I shall discuss a pair of statistical analyses that probe the relationship between mean motion resonance (MMR) and the emergence of these peas-in-a-pod architectures. Recent demonstrations of planetary mass uniformity have largely been limited to systems that exhibit strong transit-timing variations (TTVs), and are thus near MMR. Accordingly, I shall present in the first half of this talk a novel demonstration of mass uniformity for a sample of planetary systems entirely devoid of TTVs, suggesting that peas-in-a-pod architectures indeed persist for non-resonant systems as well. While this result may seem to imply that the emergence of peas-in-a-pod architectures occurs agnostically with regard to resonance, the question still remains if the degree of the associated planetary uniformity differs between near-resonant and non-resonant configurations. I shall thus present in the second half of this talk a direct comparison of size uniformity between the two modes, finding that near-resonant planetary configurations display enhanced size uniformity compared to their non-resonant counterparts, both across entire systems and within the same planetary system. These results are broadly consistent with a variety of formation paradigms for multiple-planet systems, though further investigation is necessary to ascertain whether the respective evolutionary channels for non-resonant and near-resonant configurations comprise a singular process or are themselves wholly distinct.

Abstract: Studying the properties of planets across a wide range of ages will help provide insights into the processes that shape their formation and evolution. While all-sky surveys have discovered dozens of young planets (<1 Gyr), their atmospheres remain largely unknown. In this study, we explore the transmission spectrum of K2-33b, the youngest (10 Myr), transiting exoplanet discovered. Using multi-wavelength data obtained from K2, MEarth, HST, and Spitzer, we found that the optical transit depths were nearly 2 times deeper than the near-infrared depths. This difference holds across multiple data sets taken over two years, ruling out issues of data analysis and unconstrained systematics. While surface inhomogeneities on the young star could have contributed to the observed difference, the observed stellar spectra ruled out the required spot coverage fractions. Instead, we found that a tholin haze with carbon monoxide as the dominant carbon carrier provided a better fit to the transmission spectrum. A companion study found that a circumplanetary dust ring can also explain the transit depth difference. Further observations are needed to separate the two scenarios, confirm the presence of CO, and map out the role of spots on the transmission spectrum.

Abstract: The super-Earth population spans a wide range of bulk densities, indicating a diversity in interior conditions beyond that seen in the solar system. In particular, a growing population of low-density super-Earths may be explained by volatile-rich interior compositions. Among these, lava worlds, with dayside temperatures high enough to evaporate their surfaces, provide a unique opportunity to probe the diverse surface and interior compositions of super-Earths. In this talk, I will discuss the atmospheric observability of low-density lava worlds, whose bulk densities are consistent with volatile-rich interior compositions. Using self-consistent 1D atmospheric models, I explore the atmospheric structures and thermal emission spectra of these planets across a range of mixed rock vapor/volatile compositions. Spectral features due to both volatile and rock vapor species are present in the infrared thermal emission spectra, though the strength of such features - and whether they appear as emission or absorption features - depends on the dayside temperature and atmospheric composition. In order to assess the observability of such features with JWST, I simulate JWST thermal emission observations and perform synthetic atmospheric retrievals for three promising targets. Detecting volatiles in the atmospheres of these evaporating exoplanets would provide new evidence that volatile-rich interiors exist among the super-Earth population.