ExoExplorers

Abstract: Brown dwarfs are objects that straddle the mass boundary between stars and planets. With temperatures ranging from ~250-3,000K, brown dwarfs lie in the same range as those of directly imaged exoplanets. Their plentiful and exquisite spectra available make brown dwarfs prime exoplanet analogs for interpreting atmospheric features. Retrievals provide a powerful data-driven technique to delve into questions about the chemistry and atmospheric properties of substellar objects. In this talk, I will discuss lessons learned from spectral retrievals of brown dwarfs using the Brewster retrieval framework. I will present preliminary results from a comparative sample of field sources to the subdwarf SDSS J1416A to determine how their Pressure-Temperature profiles compare to one another to explore what may drive the differences in their spectra. In doing so, I will tell a cautionary tale in trusting retrieval results. Additionally, I will discuss the results of the unusually red L dwarf 2MASS 2224 to explore the nature of clouds and the power of wide spectral coverage.

Abstract: Far out in the uncharted backwaters of the unfashionable end of the western spiral arm of the Galaxy, the ape-descended lifeforms of an utterly insignificant planet asked a question: "Are we alone?" Frank Drake had an answer: Given a handful of statistics about the local universe, one could calculate the number of intelligent civilizations currently residing in the Milky Way. Today, the Drake Equation poses a serious question in astrophysics: how can we better constrain those statistics? In this talk, I will discuss a variety of approaches to studying the detection, formation rates, and biosignatures of exoplanets, and how they point toward a better understanding of sentient life in the Universe.

Abstract: In the field of extreme adaptive optics (exAO), we seek to directly image exoplanets from the ground (often by extinguishing speckles of light due to system or environmental factors that may be brighter than the planet itself.) The Santa Cruz Extreme Adaptive optics Laboratory (SEAL) testbed will emulate the W. M. Keck Observatory, to develop and test novel instrumentation and algorithms concerning wavefront sensing, adaptive optics, and coronagraphy. Classical Lyot coronagraphs, in particular, present a fascinating opportunity for a design study, with a wealth of literature from the 2000s that can now be revisited and verified with high fidelity optical modeling using HCIPy. Similarly, Vortex coronagraphs offer improved light suppression while retaining the ability to image close-in companions. In this talk I present a design study for a Lyot and Vortex coronagraph optimized for the SEAL testbed. I will describe my simulations and optimization both in an idealized case and for a realistic case including wavefront, amplitude, and atmospheric errors. I will discuss my final designs for the coronagraph, plans for its implementation in our testbed, and future iterations of this work.

Abstract: The Kepler mission has provided detailed exoplanet population statistics for a large range of planet sizes close to their host stars. The first half of my talk will focus on how the completeness-corrected giant planet (~5-20 Re; ~0.1-20 Mj) occurrence rate from Kepler compares with that from the Mayor et al. 2011 radial velocity survey. I will also discuss the discovery of a break in the radial velocity giant planet occurrence rate and its implications for giant planets that be detected by direct imaging. The second half of my talk will focus on the Kepler’s short-period, small planet (~1-1.8 Re) population and how it affects estimates of EtaEarth, the frequency of habitable zone Earth-size planets. I will conclude by presenting our ongoing effort with TESS to de-contaminate the close-in small planet population from photoevaporated mini-Neptunes and thus provide more reliable estimates of EtaEarth.

Abstract: This research project seeks to: (1) test the feasibility of determining stellar ages from isotopic measurements; and (2) identify how unexplored stellar abundances correlate with galactic chemical evolution, formation, interior, age, metallicity, activity, and planetary properties for a wide range of host stars. Past isotopic searches have been hindered by limited sensitivity & resolution, strong telluric absorption, and the opacity due to millions of other molecular absorption lines that dominate the observed spectrum of cool stars. Now, however, isotopic abundance analysis is not only possible via high resolution spectroscopy but is also the next logical step for many cool stars. I am measuring the first multi-isotopic (carbon monoxide) abundances in a sample of FGKM stars, to identify possible discrepancies in planetary chemical evolution and accretion models. These isotopic abundance measurements may provide a new means of determining stellar ages and help identify the “missing link” between current Galactic Chemical Evolution models and inconsistent observations. Since most of spectral lines useful in isotopic analysis have low statistical significance and are barely discerned by eye when considered individually, I use a custom list of the strongest lines and create a single line profile for each isotopologue. We create a single, high- S/N line profile by taking the weighted mean, after continuum-normalizing, of each line to create a stacked absorption line. I then create corresponding line profiles for synthetic stellar models corresponding to various enrichments of the targeted isotopologue and compare them to the observed spectra in order to determine final abundances. I will repeat this process for a sample of solar twins, stars in FGK(+M) binaries, stars in known moving groups, and (eventually) any exoplanet host stars that exhibit isotopic signatures. This will provide host star parameters for the currently lacking database as well as the necessary foundations for corollary exoplanet characterization studies and ultimately contribute to the exploration of galactic, stellar, and planetary origins and evolution.

Abstract: When JWST comes online in 2022, it will usher in a golden age of exoatmosphere characterization. Given that available time for exoatmosphere studies will be limited with JWST, it will be impossible for it to survey most exoplanet atmospheres. Thus, it is critically important not only that we have the capability to prioritize the most promising targets for JWST follow-ups, but also multiple instruments available to study the multitude of targets JWST won’t get a chance to survey. These considerations have led to the design of Henrietta, a high-precision, low resolution near-infrared spectrograph for the 1-m Swope Telescope at Las Campanas Observatory. I will talk about why high-precision ground-based spectrophotometry is so challenging in the infrared and how Henrietta’s design choices seek to mitigate these issues. If successful, Henrietta will operate near the photon noise limit and will have ample amounts of telescope time. This will not only provide a consistent stream of targets to JWST, but will also be extremely scientifically productive in its own right - allowing us to begin to place exoplanet atmospheres in a statistical context - and serve as a pathfinder instrument for future ground-based exoatmosphere instruments.

Abstract: Most giant planets are expected to have formed beyond the water ice line where their assembly is most efficient. The presence of giant planets interior to ~3 AU around Sun-like stars indicates that inward orbital migration is likely a common phenomenon. However, the processes by which these gas giants arrived at their present-day locations are poorly constrained because radial velocity and transit surveys have largely avoided young stars. As a result, our knowledge of giant planet statistics is primarily confined to old ages (~1-10 Gyr) after most migration has terminated. One solution to find planets around young stars is to move from optical RVs to the near-infrared (NIR), where jitter is reduced as starspot-to-stellar photosphere contrasts are lower than in the optical. In 2018 we launched a precise RV survey of over one hundred intermediate-age (~20-200 Myr) GK dwarfs with the Habitable-Zone Planet Finder near-infrared spectrograph (HPF) at McDonald Observatory's Hobby Eberly Telescope to determine the timescale and dominant physical mechanism of giant planet migration. The Epoch of Giant Planet Migration survey aims to improve our understanding of how and when giant planets migrate to small separations. In this talk, I will summarize results from the first 14 months of this program. We find that RV scatter is significantly reduced in the NIR compared to the optical, facilitating the search for planets around young, active stars.

Abstract: High-energy particles from M-dwarf superflares -- flares with energy greater than or equal to 1033 erg -- can dramatically impact habitable-zone planets around these cool stars, with possible effects including the excitation of intense aurorae as particles interact with planetary atmospheres. Prior work has demonstrated that Earthlike atmospheres can produce excess emission in M-dwarf spectra, with the star/planet contrast ratio increasing by orders of magnitude in the green 5577-Å auroral line to levels potentially detectable by future surveys. The Evryscopes are gigapixel-scale telescope arrays at Mount Laguna Observatory and Cerro-Tololo Inter-American Observatory; these systems are coupled with the Evryscope Fast Transient Engine (EFTE), which scans Evryscope images in real time for transient phenomena. Together, these systems have the unprecedented ability to identify superflares across the entire sky as they begin, enabling rapid spectroscopic follow-up. With the Evryscopes' all-sky coverage, far more -- and far brighter -- flares are observable than in surveys that focus on individual targets. Using the Goodman spectrograph on the 4.1-m SOAR telescope, we follow the spectroscopic evolution of M-dwarf superflares as they happen, and build a pathfinder survey to constrain upper limits on possible auroral emission from impacted planets. We present our survey here.

Abstract: No Solar System analog planet to super-Earths exists, a class of exoplanets with masses 2-10x Earth’s mass which can retain a hydrogen atmosphere. Super-Earth atmospheres can have different compositions from nitrogen and oxygen dominated atmosphere of Earth. The James Webb Space Telescope (JWST) will offer unprecedented insight into the atmospheric composition of potentially habitable super-Earths through transmission and emission spectroscopy. I will present work on the investigation of NH3 (ammonia, a potential biosignature) detectability on super-Earths with an H2-dominated atmosphere using the Mid-Infrared Instrument (MIRI) and the Near InfraRed Spectrograph (NIRSpec) on the upcoming JWST mission. We use a radiative transfer code, petitRADTRANS, to generate synthetic spectra of optimal targets for observations given their proximity to Earth (<50 pc), radii (1.7-3.36 Earth radii), and equilibrium temperature (< 450 K). I will review the constraints of the MIRI LRS Instrument (flux ratio contrast of host star and planet ~ 10^-4), and discuss optimal targets for this instrument. For NIRSpec, I explore how varying cloud conditions, mean molecular weights (MMWs), and NH3 mixing ratios affects spectral features. Finally, I will discuss the use of PandExo to simulate mock observations with JWST and the detection significance findings for ammonia features with transmission spectroscopy.

Abstract: The Nancy Grace Roman Space Telescope (Roman) will perform its Galactic Exoplanet Survey when it launches in the mid-2020's. With this first space-based microlensing survey, Roman will be sensitive to planets with orbital separations from roughly 1 AU to those unbound from any host star with masses as low as ten percent that of Earth's. The Roman Galactic Exoplanet Survey will be similar in scale to the Kepler mission, and will produce statistics on exoplanet demographics vital in improving planet formation models that are otherwise inaccessible. In this talk, I will give a brief overview of the Roman Galactic Exoplanet Survey and how it will use microlensing to detect these planets. I will highlight some of the unique insights Roman will give us, including its ability to detect Earth-analog systems and what it can teach us about the presence of free-floating planets in our Galaxy.

Abstract: The First Near-Infrared Transmission Spectrum of HIP 41378 f, a Low-Mass Temperate Jovian World in a Multi-Planet System Abstract: We present a near-infrared transmission spectrum of the long period (P=542 days), temperate (T_eq=294 K) giant planet HIP 41378 f obtained with the Wide-Field Camera 3 (WFC3) instrument aboard the Hubble Space Telescope (HST). With a measured mass of ~12 M_earth and a radius of ~9R_earth, HIP 41378 f has an extremely low bulk density (0.09 g/cm^3). We measure the transit depth with a typical precision of 84 ppm in 30 spectrophotometric channels with uniformly-sized widths of 0.018 microns. Within this level of precision, the spectrum shows no evidence of absorption from gaseous molecular features between 1.1-1.7 microns. Comparing the observed transmission spectrum to a suite of 1D radiative-convective-thermochemical-equilibrium forward models, we rule out clear, low-metallicity atmospheres and find that the data prefer high-metallicity atmospheres or models with an additional opacity source such as high-altitude hazes and/or circumplanetary rings. We explore the ringed scenario for this planet further by jointly fitting the K2 and HST light curves to constrain the properties of putative rings. We also assess the possibility of distinguishing between hazy, ringed, and high-metallicity scenarios at longer wavelengths with JWST. HIP 41378 f provides a rare opportunity to probe the atmospheric composition of a cool giant planet spanning the gap between the Solar System giants, directly imaged planets, and the highly-irradiated hot Jupiters traditionally studied via transit spectroscopy.

Abstract: Solar and extra-solar terrestrial planet formation: The bleak prospects for habitability around the smallest stars Abstract: While the solar system's geologic and observational accessibility makes it an unparalleled laboratory in which to study planet formation, exoplanet science has revealed a diverse continuum of evolutionary pathways followed by other systems. Emboldened by these advancements, recent investigations have reevaluated and modernized standard models of planet formation originally based on classic solar system studies. Building from this solar system analogy, contemporary work on exoplanet formation has found that the generic terrestrial planet growth regime is highly sensitive to several key processes. Namely, these include the presence of giant planets, the radial pebble flux, and the formation location of planetary cores. Further invigorating this field, pioneering exoplanet survey missions like Kepler and TESS have spurred a prolific output of multifaceted investigations into the formation of newly detected worlds. Through these advancements, a paradigm shift has occurred in exoplanet science, wherein low-mass stars are increasingly viewed as a foundational pillar of the search for potentially habitable worlds in the solar neighborhood. However, the processes that led to the formation of this rapidly accumulating sample of systems are still poorly understood. Moreover, it is unclear whether tenuous primordial atmospheres around these Earth-analogs could have survived the intense epoch of heightened stellar activity that is typical for low-mass stars. I will summarize our understanding of rocky planet formation and volatile delivery in the solar system, and how these ideas extend to the low-mass regime. I will then present results from new simulations of in-situ planet formation across the M-dwarf mass spectrum. From these calculations, we derive leftover debris populations of small bodies that might source delayed volatile delivery. We then follow the evolution of this debris with high-resolution models of real systems of habitable zone planets around low-mass stars such as TRAPPIST-1.

Abstract: The last couple of years has seen a significant increase in detections of evaporating exoplanets, owing mainly to the discovery of the metastable helium as a probe for atmospheric escape. This process is thought to be an important factor to explain features in the exoplanet population, such as the hot-Neptune desert and the radius valley. While part of exoplanet community, in general, enjoys a swath of open-source codes that help them plan and interpret observations, the same cannot be said about those who study atmospheric escape. At least, not until recently. We developed a new open-source code, named p-winds, with the objective of supplying the community with an easy to use, well-documented tool designed for observations of evaporating exoplanets. In this talk, I will discuss the motivation, implementation, and use cases for p-winds. I will also briefly discuss some recent results that benefitted from this code, and future plans in sight.

Abstract: The effect of stellar multiplicity on planet formation remains an open question. Investigations carried out using high-resolution imaging and constraints from RV planet searches have indicated that planet formation can be disrupted by close binaries while being relatively unaffected by wide companions. The magnitude and distance-limited nature of those tools have left unexplored companion parameter space in our best planet sample, the Kepler survey. The Early Data Release 3 (EDR3) from the Gaia Mission includes RV measurements of over 7 million targets that can be used to probe this parameter space. Many of these stars are members of unresolved multiple star systems and the effects of these orbits are generally seen as a source of contamination in the Gaia RV catalog. We will demonstrate that the published RV error estimates can provide evidence for the existence of an unresolved stellar companion for Kepler (and other) planet hosts and place constraints on their orbital parameters.

Abstract: Direct imaging of exoplanets is a promising route to finding and characterizing exoplanets in the thermal infrared. Currently the technique is most sensitive to massive, young planets on wide orbits. Innovative observing techniques are necessary to probe smaller angles from host stars, or search for older or lower-mass planets. The Large Binocular Telescope (LBT) is in a unique position to push these frontiers in preparation for the era of 30-m-class extremely large telescopes. When light is combined coherently in a "Fizeau" mode between the LBT's twin 8.4-m sub-telescopes, the facility effectively becomes a masked 22.7-m telescope. I will present an observation of the nearby star Altair in this mode, which represents the first LBT Fizeau dataset with a degree of automated phase control. These data constrain the existence of companions of 1.3 M⊙ down to an inner angle of ≈0.15", closer than any previously published direct imaging of Altair. I outline ways forward, in terms of software and physical upgrades, and post-processing with longer integration times.

Abstract: Super-Mercuries are a class of exoplanets with radii less than ~1.5 Re, high bulk densities and relatively large core-mass fractions (CMFs). The study of super-Mercuries will shed light on the composition of low-mass, terrestrial exoplanets as well as on the mechanisms that lead to the formation of iron-rich planets. However, only a few exoplanets have been confirmed as super-Mercuries, in part because of the challenges of obtaining the precise stellar and planetary parameters required to confirm them. I present a reanalysis of the K2-106 system, which contains an ultra-short period, super-Mercury candidate with a density from the literature of 13.1 (+5.4 -3.6) g/cc, approximately twice the density of Earth. We globally model extant photometry and radial velocity of the system and derive a planetary mass and radius that leads to a considerably lower density than previously reported. We derive the host star’s Fe, Mg and Si abundances and combine them with planet interior models to infer the CMF and interior composition of K2-106b. Using a statistical framework, we compared the planet’s CMF as expected from the planet’s density and the CMF as expected from the host star. Our analysis suggests that, although K2-106b has a high density and CMF, it is statistically unlikely to be a super-Mercury.

Abstract: Small debris, both in and out of our Solar System, provides a window into planet formation. Within our Solar System, we have the Kuiper Belt, icy and dusty debris beyond Neptune, as well as the asteroid belt and Oort Cloud. In 2015, the New Horizons mission provided the first up-close view of a Kuiper Belt Object when it visited Pluto. Pluto’s surface was revealed to be geologically complex, with volatile ices that are mobile on seasonal and longer timescales. Using New Horizons data, we investigated the distributions and movements of ices on Pluto’s surface. Detailed studies of solar system objects, like this work on Pluto’s surface geology, are complementary to our investigations into other planetary systems that harbor debris disks, sometimes referred to as “Exo-Kuiper Belts”. Both provide insight into the processes of planet formation. High-contrast imaging has been key in providing new information about this extrasolar debris, such as measurements of disk extent and morphological asymmetry — information that is not available from infrared excesses alone. However, we are currently limited in our ability to discern composition of debris disks, relying on color measurements and other coarse methods. As characterization capabilities continue to grow, we can expect further discoveries in these complementary fields, especially as new observatories like JWST and the ELTs provide even more highly detailed observations of both solar system objects and debris disks. I will also briefly discuss another small piece of our “solar system” of astronomy—science writing, and pedagogical best practices for integrating writing into physics and astronomy curricula.

Abstract: ESPRESSO, as the first high-resolution spectrograph of the 2020s, has brought a significant increase in line precision, as shown with the re-observation of WASP-76b (Tabernero et al. 2020) and WASP-121b (Borsa et al. 2021), and allows us to move beyond the sole observation of the sodium doublet with a plethora of resolved spectral lines - probing different altitudes in the atmosphere. Using the MERC code (Seidel et al. 2020a), a retrieval tool to determine winds in exoplanet atmospheres, the resolved lines can then be used to retrieve wind patterns directly. Compared to the analysis of the line shape on HARPS data only (e.g. Seidel et al. 2020a for HD189733b), the analysis of the line shape of ESPRESSO data permits to retrieve wind patterns in the upper atmosphere, but additionally also gives us unprecedented observational insights into the lower atmosphere from the line wings (Seidel et al. 2021). In this talk I will provide the community with a guide what we can, and can't. derive about atmospheric winds in exoplanet atmospheres with the current data quality and where our current limits lie as we move to smaller and cooler planets (Seidel et al. 2022). I will also provide a short introduction of ESPRESSO and other ESO instruments that might be useful for the cohort in future observing proposals.

Abstract: Planet formation models predict that below a certain protoplanetary disk metallicity, the surface density of solid material is too low to form planets. Observationally, previous works have indicated that short-period planets preferentially form around stars with solar and super solar metallicities. Given these findings, it is challenging to form planets within metal-poor environments. Due to the target selection process of previous surveys, there is little constraint on planet occurrence rates below [Fe/H] ~ -0.5, which is still higher than the predicted metallicity at which planet formation cannot occur. Expanding upon previous works, we construct a large sample of ~100,000 metal-poor stars with spectroscopically-derived stellar parameters observed by TESS. With this sample, we constrain planet occurrence rates within the metal-poor regime (-1.0 ≤ [Fe/H] ≤ -0.4) placing the most stringent upper limits on planet occurrence rates around metal-poor stars.

Abstract: Exoplanet systems are of interest not only for their potential for habitability, but also in the opportunity they provide for the study of comparative heliophysics. In applying solar- and heliophysics-based modeling tools to exoplanet systems, we can expand our understanding of the influence of stellar behavior on planetary environments and processes such as atmospheric loss. I will discuss my work employing a surface flux transport (SFT) model to examine the dynamics of magnetic flux on the surfaces of cool stars like the Sun and exoplanet host stars of interest. This flux transport modeling approach has been used to examine stellar coronal X-ray emission and other asterospheric properties as a function of host star magnetic activity. I will provide an overview of current efforts to extend this modeling framework to investigate host star EUV emission and stellar wind parameters for a range of exoplanet host stars. Since stellar EUV emission is both a) a key driver of atmospheric loss processes and b) difficult to observe due to interstellar medium extinction, these simulations of energetic coronal emission could fill gaps in our understanding of exoplanet atmospheric evolution caused by this dearth of observational evidence.

Abstract: Following the launch of JWST, in addition to a selection of ongoing commissioning activities, we now stand just months away from the beginning of scientific observations. Dispersed throughout these initial data will be a selection of Director’s Discretionary Early Release Science (ERS) observations which were designed under a primary goal of rapidly informing the community on JWST’s performance and capabilities. With respect to the study of exoplanets, only two ERS programs exist: “The Transit Community Early Release Science Program” (PI: N. Batalha, ERS-1366), and “High Contrast Imaging of Exoplanets and Exoplanetary Systems” (PI: S. Hinkley, ERS-1386). In totality, these two programs will set the tone for JWST exoplanet observations throughout its lifetime. In this talk I will provide an up-to-date overview of both of these programs, the timelines for their data releases, and a description of their technical and scientific goals. Additionally, I will discuss existing and ongoing preparatory work towards the production of a range of science enabling products. These products (e.g. data reduction pipelines, analyses of best practices) are an integral part of the ERS programs and will support JWST investigations throughout Cycle 2 and beyond.

Abstract: Dynamical evolution within planetary systems can cause planets to be engulfed by their host stars. Following engulfment, the stellar photosphere abundance pattern will reflect accretion of rocky material from planetary cores by exhibiting refractory enhancements in order of condensation temperature $T_c$. Multi-star systems are excellent environments to search for such abundance trends because stellar companions share the same natal gas cloud and primordial chemical composition to within $\sim$0.05 dex. Thus, refractory differences above $\sim$0.05 dex that trend with $T_c$ between companions are a signpost of engulfment. Abundance measurements have occasionally yielded such engulfment signatures, but few observations targeted systems with known planets. To address this gap, we carried out a survey of 36 multi-star systems where one star is a known planet host with the Keck High Resolution Echelle Spectrometer. None of the 36 systems observed exhibit abundance patterns strongly indicative of engulfment events, which could be explained by our modeling efforts that show observable refractory enrichments from 10 $M_{\oplus}$ engulfment events are depleted on timescales of $\sim$1 Gyr for solar-like stars.