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.

Abstract: The Gemini Planet Imager (GPI) is a high contrast imaging instrument designed to directly detect and characterize young, Jupiter-mass exoplanets. After six years of operation at Gemini South in Chile, the instrument is being upgraded and relocated to Gemini North in Hawaii as GPI 2.0. GPI helped establish that Jovian-mass planets have a higher occurrence rate at smaller separations (~1-10 AU), and their formation pathways are still not completely understood. These questions motivate several sub-system upgrades to obtain deeper contrasts particularly at small inner working angles. One of GPI 2.0’s upgrades will be on its adaptive optics system, by replacing the current Shack-Hartmann wavefront sensor (WFS) with a pyramid WFS and a custom EMCCD. Electron multiplying CCDs (EMCCDs) are detectors capable of counting single photon events at high speed and high sensitivity. The upgraded ultra low-noise wavefront sensor is expected to give the adaptive optics (AO) system the capability to achieve high Strehl ratios on stars two magnitudes fainter than the current limit. GPI 2.0 is expected to go on-sky in late 2025. Here I will present on GPI 2.0’s science goals, its adaptive optics upgrades and the latest timeline for operations and current status.

Abstract: JWST will produce transit spectra of several habitable zone rocky planets orbiting M-dwarfs in the coming years. To provide context for interpreting observations, I use a 3D climate model combined with a radiative transfer model to generate synthetic transit spectra for a synchronously rotating rocky planet over a large parameter space of possible climates. Since it will be difficult to constrain a planet’s surface conditions empirically, I systematically vary the planet’s land cover and atmosphere mass in order to characterize the climate uncertainties associated with these parameters. These variations result in a large range of possible climate states featuring significant differences in surface temperature and humidity. I will show that planets in different climate regimes can have similar transit spectra, which means that it will likely be difficult to measure a given planet’s liquid water inventory or the size of its temperate region using transit spectroscopy. Land cover and atmosphere mass are therefore important sources of climate uncertainty to account for when interpreting JWST spectra.

Abstract: The Transiting Exoplanet Survey Satellite mission (TESS) enables the discovery of exoplanets around tens of millions of stars by regularly recording its entire field of view in Full Frame Images (FFIs). However, current TESS planet searches require significant manual inspection efforts to identify planets among transit-like detections, which limits their scope to small subsets of this stellar sample. I will present an ongoing search for transiting exoplanets around all ~20 million stars brighter than T = 13.5 mag that have been observed in TESS FFIs, made possible by the development of a near-fully automated vetting pipeline to efficiently distinguish planets from false positives. This search has uncovered ~2700 TESS Objects of Interest (TOIs), most of which are giant, close-in exoplanets around faint stars not explored by other searches. I will highlight some particularly exciting discoveries, including rare types of exoplanets and intriguing targets for atmospheric characterization. The automated vetting pipeline developed for this project, as well as the new candidates discovered in this ongoing search, will allow TESS to significantly improve the statistical power of demographic studies in the future.

Abstract: The TRAPPIST-1 planets have become prime targets for studying the habitability of planets around M-dwarf stars. Modeling geochemical cycles in these planets can provide insight on their evolution and their potential for habitability. Through planet formation and long-term tectonic evolution, there is an exchange of volatiles between the interior and the atmosphere of rocky planets. We model the combined deep water and carbonate-silicate cycles to trace the production of different gas species including hydrogen, water, carbon dioxide, and carbon monoxide. We also aim to study how exoplanet interior structure and material properties, like oxygen fugacity, influence the atmosphere of these planets. These outgassing models can help us understand the evolution of the atmospheric composition and its effect on planetary climate. We present the results from our models, which include the atmospheric abundances as well as the surface and mantle temperatures of exoplanets TRAPPIST-1 d, e and f.

Abstract: One of the most pressing challenges in the new era of Extreme Precision Radial Velocity (EPRV) instruments is to disentangle signals induced by stellar activity from planetary signals. With small exception, planetary signals are distinct from activity signals in that they have a constant frequency, phase, and amplitude. Meanwhile, activity signals may come and go, growing and decaying over a characteristic lifetime, and returning again with possible phase and frequency shifts. Here, we present an approach exploiting this feature: we decompose the RV signal on a basis of apodized sinusoidal functions. Bayesian methods are precise and interpretable but computationally expensive. Periodograms are fast and provide statistics, but are prone to aliasing because they search one signal at a time. Here, we introduce the L1 Apodized Periodogram, L1A or Lia (pronounced like the name, “Leah”). This new software uses an L1 minimization approach, allowing to search for several signals at the same time with a moderate computational cost, to identify and characterize both the periodicity and decay lifetime of signals in a dataset. With a new way to look into our RV data sets, we can gain new insights and better understand the astrophysical origin of signals.

Abstract: A persistent question in exoplanet characterization is whether exoplanetary systems form from similar compositional building blocks to our own. Polluted white dwarf stars offer a unique way to address this question as they provide measurements of the bulk compositions of exoplanetary material. These stars show evidence of recent accretion of rocky bodies in the form of metal lines in their spectra, which tell us about the relative elemental abundances of the accreted material. In this talk I will share a statistical analysis of the rocks polluting white dwarfs and compare their compositions to Solar System rocks, such as chondrites, bulk Earth, and crust. In this study, I find that the majority of the extrasolar rocks are consistent with the composition of chondrites, a result that is supported by the compositions of stars in the solar neighborhood.