Browsing by Subject "Exoplanets"

Now showing 1 - 6 of 6

Discovering new solar systems : Jupiter analogs and the quest to find another Earth
(2013-08) Robertson, Paul Montgomery; Dodson-Robinson, Sarah E.; Endl, Michael
Exoplanets are now known to be ubiquitous throughout the Galaxy. From the Kepler survey, we expect nearly every main-sequence star to form planetary systems during its formation phase. However, the detection limits of Kepler are confined to planets with short orbital periods, comparable to those in the inner solar system. Thanks to the long observational time baseline of the McDonald Observatory Radial Velocity (RV) Survey, we can identify gas giant planets in the outer regions of extrasolar planetary systems. The statistics of such planets are not well known, and are important for understanding the physics behind planet formation and migration. In this dissertation, I detail the discovery of five giant exoplanets on long-period orbits–so-called “Jupiter analogs.” For two systems of giant planets discovered through our survey, pairs of planets follow closely-packed orbits, creating the possibility for dynamical instability. I therefore examine the orbital resonances that allow these planets to avoid gravitational disruption. Because we see an abundance of small, potentially habitable exoplanets in the Kepler data set, current and upcoming exoplanet surveys concentrate on finding Earth-mass planets orbiting stars near enough to facilitate detailed follow-up observations. Particularly attractive targets are cool, low-mass “M dwarf” stars. Their low masses (and thus higher RV amplitudes from exoplanets) and close-in habitable zones allow for relatively quick detection of low-mass planets in the habitable zone. However, the RV signals of such planets will be obscured by stellar magnetic activity, which is poorly understood for M stars. In an effort to improve the planet detection capabilities of our M dwarf planet survey, I have conducted a detailed investigation of the magnetic behavior of our target stars. I show that, while stellar activity does not appear to systematically influence RV measurements above a precision level of ∼ 5 m/s, activity cycles can occasionally produce RV signals in excess of 10 m/s. Additionally, I show that long-term, solar-type stellar activity cycles are common amongst our M dwarf targets, although they are significantly less frequent than for FGK stars. In the case of GJ 328, I have discovered a magnetic activity cycle that appears in the RV data, causing the giant planet around the star to appear to be on a more circular orbit than indicated by the activity-corrected data. Such corrections are essential for the discovery of Earthlike exoplanets.
Revealing star and planet formation with stellar multiplicity
(2023-08-07) Sullivan, Kendall; Kraus, Adam L.; Herczeg, Gregory; Offner, Stella; Morley, Caroline; Hawkins, Keith
Studies of star and planet formation work to understand the processes that produced the Solar System and the many other systems now known to host exoplanets. Understanding star and planet formation requires measurement of accurate stellar properties at all evolutionary stages of stellar and planetary systems. These stellar properties include age, mass, effective temperature (T [subscript eff]), stellar radius, and stellar multiplicity. Binary stars and higher-order multiples comprise about half of the population of main-sequence solar-type stars, and stellar multiplicity impacts the observed properties of stars across their lifetimes. Because exoplanet and stellar demographics are typically inferred from stellar properties, incorrect stellar characterization because of binaries feeds into biases and errors in stellar populations and exoplanet demographics. In this dissertation, I explored the impact of binary stars in the two scientific contexts of young stellar associations and binary stars that host exoplanets. In my studies of young stellar associations, I developed a simulation suite to perform synthetic spectroscopic surveys. I implemented mass-dependent binary properties to explore the origins of apparent mass-dependent age gradients previously observed in star-forming regions. My subsequent work added starspots to the simulation. I found that although binary stars can explain mass-dependent age gradients, starspots become the dominant contributor to the gradient in populations with Gaia distances. I also explored the nature of the relationship between accretion and circumstellar disks in young stars and found that the inner disks of binaries and single stars are probably similar, and that the inner rim of the dust disk is related to the accretion rate as a result of mass transfer through the disk. These studies demonstrated the importance of considering binary stars when attempting to measure ages or understand star formation histories in young stellar associations. In my studies of main sequence binary star exoplanet hosts, I developed an algorithm to accurately characterize the individual components of binary stars that are unresolved in most observations. As an initial step, I tested this code with an archival sample of M stars. Then, I performed a spectroscopic survey of binary stars from the Kepler sample using the Hobby-Eberly Telescope, and carried out two targeted studies of subsamples from the survey. The first study explored binary stars that supposedly host rocky Earth-analog planets and found that most of them are actually gaseous planets, which has implications for exoplanet demographics and attempts to measure the frequency of Earth analogs. The second study explored the radius distribution of small exoplanets and found that the gap in the radius distribution separating rocky and gaseous exoplanets in single systems was not present in binary stars. This result suggested that the location of the gap may be binary-separation-dependent and therefore “blurred out” by a range of stellar separations in the sample. This series of papers has demonstrated the power of using binary stars that host planets as a laboratory for controlled experiments in planet formation and evolution, because the binary properties leave a record of the planet-forming environment. The work presented in this dissertation has shown the ability of binary stars to influence observations of young stars and exoplanet hosts, and has demonstrated the potential of binary stars to provide a direct link between formation environment and exoplanet properties for the first time.
The carbon-deficient evolution of TRAPPIST-1c
(2023-12) Teixeira, Katie; Morley, Caroline V.
Transiting planets orbiting M dwarfs provide the best opportunity to study the atmospheres of rocky planets with current facilities. As JWST enters its second year of science operations, an important initial endeavor is to determine whether these rocky planets have atmospheres at all. M dwarf host stars are thought to pose a major threat to planetary atmospheres due to their high magnetic activity over several billion-year timescales, and might completely strip atmospheres. Several Cycle 1 and 2 GO and GTO programs are testing this hypothesis, observing a series of rocky planets to determine whether they retained their atmospheres. A key case-study is TRAPPIST-1c, which receives almost the same bolometric flux as Venus. We might, therefore, expect TRAPPIST-1c to possess a thick, CO₂-dominated atmosphere. Instead, Zieba et al. (2023) show that TRAPPIST-1c has little to no CO₂ in its atmosphere. To interpret these results, we run coupled time-dependent simulations of planetary outgassing and atmospheric escape to model the evolution of TRAPPIST-1c’s atmosphere. We find that the stellar wind stripping that is expected to occur on TRAPPIST-1c over its lifetime can only remove up to ~ 16 bar of CO₂, less than the modern CO₂ inventory of either Earth or Venus. Therefore, we infer that TRAPPIST-1c either formed volatile-poor, as compared to Earth and Venus, or lost a substantial amount of CO₂ during an early phase of hydrodynamic hydrogen escape. Finally, we scale our results for the other TRAPPIST-1 planets, finding that the more distant TRAPPIST-1 planets may readily retain atmospheres.
The demographics and circumplanetary disk properties of wide planetary mass companions : a comprehensive study of Spitzer/IRAC archival data
(2021-08-16) Martinez, Raquel Angelina; Kraus, Adam L.; Bowler, Brendan; Evans, Neal; Jaffe, Daniel; Kratter, Kaitlin
Over the past decade, a growing population of planetary-mass companions (<20 M [subscript Jup]; PMCs) orbiting young stars have been discovered. They are located at wide separations (>100 au) from their young host stars, challenging models of star and planet formation. It is unclear whether these systems represent the low-mass extreme of stellar binary formation, or the high-mass and wide-orbit extreme of planet formation theories, as various formation pathways inadequately explain their physical and orbital aspects. Determining which scenario best reproduces the observed characteristics of PMCs will come once a statistically robust sample of directly imaged planets are found and studied. PMC systems also provide an opportunity to witness planet assembly, thus characterizing the spectral energy distributions of PMCs will help with future interpretation of exoplanet observations. The extensive Spitzer/IRAC data set of nearby young populations has great potential to be mined for wide companions to stars. For my thesis, I developed an automated pipeline to find faint PMCs via point spread function (PSF) subtraction in existing Spitzer/IRAC images. I identified candidates for further study and pursued follow-up observations of candidate companion systems as I endeavored to leverage the wealth of Spitzer images to find undiscovered companions. I discovered two wide-orbit substellar companion systems and described characterization efforts of them. I measured the mid-infrared photometry of 16 wide-orbit companions, compared them to brown dwarfs in star-forming regions and the field, and determined the global disk frequency of young (<15 Myr) wide companions with low masses to be high (56%+/-12%). I determined that my PSF-subtraction infrastructure is sensitive to ~2 M [subscript Jup] companions at [rho]>300 au. I also expanded my search for companions to the Taurus star-forming region, constraining the frequency of 0.5--30 M [subscript Jup] companions on semi-major axes 50--5000 au to <3.7% at a 95% confidence level. My thesis has set the stage to reveal the demographics of wide-orbit PMCs from which better constraints on the models of extreme binary star and planet formation will emerge, ultimately enhancing our understanding of where these systems come from, how they evolve, and where they fit into the paradigm of star and planet formation.
The epoch of giant planet migration : searching for young planets within the stellar noise
(2024-08) Tran, Nhat Quang Hoang ; Bowler, Brendan P.; William D. Cochran; Adam L. Kraus; Keith Hawkins; Caroline Morley; Suvrath Mahadevan
The origin of gas giants interior to the water ice line remains undetermined. Giant planets are expected to form beyond this boundary—about 2.5 AU for a Sun-like star—and occasionally migrate inward. Detecting and characterizing giant planets at young ages, when inward orbital migration mechanisms are expected to still be operating, provides clues about when and how giant planets arrive at small separations. However, finding young planets is difficult as young, rapidly rotating stars have large starspots that produce strong variations in time series observations, which can mask real signals from planets or masquerade as reflex motion. As a result, our knowledge of giant planet statistics is largely confined to ages when most migration has already terminated (∼1–10 Gyr). In this dissertation, I address questions on the detection, prevalence, and migration history of young giant planets in a multi-pronged approach with a case study of the TOI-1670 multi-planet system, a measurement of the most precise giant planet occurrence rate at young ages to date, and an evaluation of Gaussian process stellar activity models using radial velocities (RVs) paired with light curves. In the characterization of TOI-1670, I confirm the planetary nature of two transiting planets. This system adds to a rare but growing population of short-period giant planets on circular orbits with smaller, inner companions which together suggest that several inward migration mechanisms, including non-disruptive processes, are at play. I also present the Epoch of Giant Planet Migration program, a 4-year precision RV survey targeting 104 intermediate age (20–200 Myr) Sun-like stars with the near-infrared Habitable–zone Planet Finder spectrograph on the Hobby-Eberly Telescope. As part of this survey, I characterize the new ≈130 Myr hot Jupiter candidate HS Psc b and show that the occurrence rate of giant planets interior to the water ice line at young ages is similar to values at older ages. These results are consistent with giant planets migrating to small separations more rapidly than several hundred Myr, and excludes a decreasing occurrence rate over time. Finally, I demonstrate that the application of commonly adopted stellar activity mitigation frameworks that employ Gaussian process regression models can significantly impact the interpretation of planetary properties inferred from RVs. I introduce a novel, physically motivated Gaussian process framework designed to jointly model stellar activity signals in both RVs and photometry that performs better at predicting starspot-driven stellar activity signals compared to other commonly used approaches. Altogether, this dissertation showcases the strength of near-infrared precision RVs and Gaussian process stellar activity models in detecting, characterizing, and determining the migration histories of young giant planets.
The Epoch of Giant Planet Migration planet search program : near-infrared radial velocity jitter of young Sun-like stars
(2022-07-01) Tran, Nhat Quang Hoang; Bowler, Brendan P.
I present early results from the Epoch of Giant Planet Migration program, a precise RV survey of over one hundred intermediate-age (~20–200 Myr) G and K dwarfs with the Habitable–Zone Planet Finder spectrograph (HPF) at McDonald Observatory's Hobby-Eberly Telescope (HET). The goals of this program are to determine the timescale and dominant physical mechanism of giant planet migration interior to the water ice line of Sun-like stars. Here, I summarize results from the first 14 months of this program, with a focus on my custom RV pipeline for HPF, a measurement of the intrinsic near-infrared RV activity of young Solar analogs, and modeling the underlying population-level distribution of stellar jitter. I demonstrate on-sky stability at the sub-2 m s⁻¹ level for the K2 standard HD 3765 using a least-squares matching method to extract precise RVs. Based on a subsample of 29 stars with at least three RV measurements from my program, I find a median RMS level of 34 m s⁻¹ . This is nearly a factor of 2 lower than the median RMS level in the optical of 60 m s⁻¹ for a comparison sample with similar ages and spectral types as my targets. The observed near-infrared jitter measurements for this subsample are well reproduced with a log-normal parent distribution with μ = 4.15 and σ = 1.02. Finally, by compiling RMS values from previous planet search programs, I show that near-infrared jitter for G and K dwarfs generally decays with age in a similar fashion to optical wavelengths, albeit with a shallower slope and lower overall values for ages [less than or approximately equal to] 1 Gyr.