ExoExplorers

Abstract: Future direct imaging missions, including the Roman Space Telescope and the Habitable Worlds Observatory, will enable the detection of exoplanets in reflected light. However, these observations may present unique challenges including planet-detection confusion in multi-planet systems and uncertainties in characterization. I will present two of my PhD projects which are focused on addressing these issues. First, I address the planet-detection confusion problem by augmenting a “deconfusion” algorithm with planetary phase variation and photometric properties to support orbit differentiation in multi-planet systems. This work demonstrates the necessity of combining relative astrometry and photometry to reduce confusion rates. Second, I will present an analysis of high phase angle observations of Uranus from the New Horizons mission to provide a ground-truth for interpreting exo-ice-giant observations. Together these efforts support robust, informed expectations of exoplanet brightness and atmospheric variations, which are critical for informing yield modeling, instrument design, atmospheric modeling, and observation scheduling for future direct imaging missions.

Abstract:In this somewhat disjointed two-part talk, I'll give an overview of the two in-progress halves of my thesis work. In the first half I'll discuss the prospects for and our attempts to detect planetary oblateness, or deviations from spherical symmetry, with current space-based facilities like JWST. Though this measurement is technically feasible, the miniscule size of the signal necessitates careful consideration of modeling choices, especially that of which limb darkening parameterization to use. I will propose a new framework for this: fitting directly in stellar parameters such as effective temperature and metallicity, rather than a potentially non-physical law. In the second half, I will discuss my work on using TESS to discover Trans-Neptunian Objects and how to quantify the impact of unresolved solar system objects on exoplanet transit searches.

Abstract: Earth’s oceans have hosted an abundance of biological innovation, and oceans on Earth-like exoplanets may be similar hotspots for life. Habitability for complex, multicellular life requires more than just liquid water, depending additionally on circulation patterns that transport dissolved oxygen (O2) and nutrients within oceans. However, Earth's own ocean circulation has changed through time due to the slowing of Earth's rotation and the rearrangement of continents. Planetary rotation rate affects atmospheric circulation and (in turn) wind-driven ocean circulation, while continental configuration alters surface and deep ocean circulation patterns. Both rotation rate and continental configuration could vary across Earth-like exoplanets, allowing many potential landscapes of circulation and marine habitability. We use a 3D Earth system model to explore these alternate landscapes. We find that slowing rotation enhances ocean circulation and vertical mixing, leading to more efficient nutrient recycling, greater photosynthetic productivity, and higher O2 production. Stronger circulation oxygenates the deep ocean, creating potentially ‘superhabitable’ marine environments, and atmosphere-ocean interactions may also increase O2 biosignature potential on slower-rotating planets. Additionally, we find that changing continents alone can generate highly variable circulation regimes and biogeochemical outcomes. Importantly, for some configurations, the ocean can be highly deoxygenated even under detectable levels of atmospheric O2. We suggest that detecting Earth-like O2, while it may imply the presence of life, does not guarantee complex habitability.

Abstract: Throughout their lives, short-period (< 100 days) exoplanets are strongly irradiated by their host stars. Especially in the first few hundred megayears, photoionization by the young hot stars' strong XUV fluxes can drive transonic winds that outflow from these planets. The mass loss histories of these planets are essential to understanding the evolution and demographics of these populations; however, mass loss rates are not directly observable. They can only be inferred from models. To that end, we present, Wind-AE an open source 1D, multifrequency (thru to X-ray), multispecies, steady-state Parker Wind photoevaporation relaxation code based on Murray-Clay et al. (2009). The speed and reliability of the tool have allowed used in a number of investigations we will touch on including: exploring the impact of metallicity on mass loss rates; modeling observational limits of escaping H$\alpha$ on WASP-12b; constraining whether sub-Neptunes TOI 776b and c that span the period radius valley are water worlds; investigating the mystery of unusually large X-ray transits in HD189733b; and more.

Abstract: As the precision and stability of our instruments improve, the intrinsic variability of host stars now represents the greatest barrier for the detection of small exoplanets, especially in the case of Earth-twins. Magnetic activity on the surface of stars generates signals in the radial velocities (RVs) that can mimic or hide the periodic signature of a planet. Solar data have proven to be invaluable in developing targeted mitigation techniques for Extreme Precision Radial Velocity (EPRV) surveys, offering both unparalleled cadence and baselines, and the opportunity to directly link spatially resolved solar phenomena to the variability they imprint in the RVs. In this context, data observed with the Birmingham Solar Oscillations Network (BiSON) are an invaluable asset. However, BiSON's data also presents a further unique challenge in its calibration. I thus present work on the complete re-calibration and re-analysis of the 40 years of BiSON Sun-as-a-star radial-velocity data. The inclusion of simultaneous physically-motivated models describing stellar variability will allow for the investigation of stellar activity phenomena across the solar magnetic activity levels over timescales from minutes to years.

Abstract: Mid-IR wavelengths are of particular interest to exoplanet science due to the fact they access the strongly-absorbing and well-understood CO ro-vibronic mode at ~4.6 microns. However, a significant source of uncertainty at mid-IR wavelengths is the thermal background present in ground-based observations. This background comes from blackbody radiation in the atmosphere and telescope and is therefore dependent on instrument design and atmospheric conditions. When performing imaging observations, this background manifests as a slowly varying inhomogeneous signal throughout the image, underlying our data. Using an M-band direct imaging observing sequence on NIRC2, we evaluate the thermal background of the Keck II telescope. The usage of 10-m class ground-based telescopes such as Keck are necessary since we typically require high angular resolution to image close-in exoplanets. Photometry at mid-IR can greatly constrain atmospheric models but existing data is usually scarce or has significant error bars due to the difficulty of subtracting the background. This work largely aims to improve photometric error in mid-IR data by reducing systematics. We present results that improve thermal background subtraction by factoring in the systematic background component of the de-rotator. We typically are able to image exoplanets ~12.5 magnitudes fainter than the host star at M-band, and by factoring in the thermal contribution of the de-rotator, we improve our contrast and sensitivity by a factor of ~2-4. Future on-sky and engineering observations using NIRC2 will test new strategies that minimize the telescope contribution to the thermal background and we will further evaluate the thermal background environment of the telescope. Beyond NIRC2, these results also have important implications for the impact of telescope design and how to treat systematics when performing data reduction and analysis.

Abstract: A rich diversity of extrasolar planetary systems has been discovered in recent decades, with many displaying unexpected architectures that challenge theories of planet formation informed by the Solar System. In particular, many giant planets have high orbital eccentricities, and many others orbit extremely close to their host stars. A proposed solution to both of these puzzles is high-eccentricity migration, in which an initially distant “cold” Jupiter is excited to high eccentricities, allowing for tidal interactions during close passages that drag the planet onto a close-in “hot” orbit. Secular (long-term) perturbations from a third body of planetary or stellar nature are a potential source of the eccentricity excitations, a phenomenon known as the Eccentric Kozai-Lidov (EKL) mechanism. I will discuss novel insights into the three-body problem that provide a new analytical understanding of the eccentricity evolution of planets subject to EKL from a distant perturber. Then, I will discuss recent work to characterize the effect of EKL from stellar companions on the giant planet population. We perform a population synthesis study of cold giant planets in stellar binaries, including the additional effects of tides, general relativity, and stellar evolution. The eccentricity distribution of the cold Jupiters is calculated, considering that planet-planet scattering may generate modest eccentricities on ~Myr timescales before EKL shapes the distribution on ~Myr-Gyr timescales. We find that the simulated eccentricity distribution is statistically consistent with the observed sample, suggesting that the EKL mechanism in stellar binaries may play an important role in driving the eccentricities of cold Jupiters and contributing to the formation of hot Jupiters.

Abstract:The disintegrating ultra-short period rocky exoplanet K2-22b periodically emits dusty clouds in a dynamically chaotic process resulting in a variable transit depth from 0-1.3%. The effluents that sublimate off the surface and condense out in space are probably representative of the formerly interior layers convectively transported to the molten surface. Transmission spectroscopy of these transiting clouds reveal spectral fingerprints of the interior composition of this rocky world. We used JWST's Mid-Infrared Instrument (MIRI) as a low-resolution slitless spectrograph to observe four predicted transit windows for K2-22b. For each observation, we extracted a transmission spectrum over the spectral range of 4.3-11.8 μm. We detect one transit at high significance and two at low significance. We find that the data 1) disfavor featureless, iron-dominated core material, 2) are consistent with some form of magnesium silicate minerals, likely from mantle material, and 3) show a distinct and unexpected feature at ∼5 μm. The unexpected feature, also seen weakly in the low-significance transits, is consistent with some gas features, possibly NO and/or CO2. These findings warrant further study to improve the constraints on the composition of this disintegrating rocky world.

Abstract: Multi-planet system architectures are powerful tools to constrain the evolutionary pathways of observed exoplanets. Therefore, understanding the predictive and descriptive power of empirical models of system architectures is critical to probing system formation histories. In this work, we analyzed 52 TESS multi-planet systems previously studied using DYNAMITE (Dietrich & Apai, 2020), who used empirical models based on Kepler planets to predict additional planets in each system. We used additional TESS data to search for these predicted planets, and thereby evaluated the predictive power of the underlying empirical models. Specifically, we studied whether a period ratio method or clustered period model more accurately predicted additional planets. We found that neither model is highly predictive, highlighting the need for additional data and nuanced models to describe the full exoplanet population.

Abstract: In just a few short years, JWST has already revolutionized our understanding of exoplanetary atmospheres. However, cutting edge characterization is still being done with ground-based facilities. Space-based, low resolution spectroscopy (such as with JWST) and ground-based, high resolution spectroscopy are the two main avenues with which we can study transiting exoplanet atmospheres. In this talk, I will discuss the differing capabilities of these two methods for measuring and interpreting exoplanet atmospheric spectra, and I will highlight ways in which ground-based spectroscopy in particular is transforming our understanding of planet formation and climate. Additionally, high and low resolution spectroscopy have complementary strengths and weaknesses, and the combination of the two can provide a more powerful probe of a planet's atmosphere than can be achieved by either method individually. I will describe how combining the two methods can result in more precise inferences and more comprehensive pictures of a planet's atmosphere.