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Many lines of reasoning suggest that external galaxies should host planetary systems, but detecting them by methods typically used in our own Galaxy is not possible. An alternative approach is to study the temporal behaviour of X-rays emitted by bright extragalactic X-ray sources, where an orbiting planet would temporarily block the X-rays and cause a brief eclipse. We report on such a potential event in the X-ray binary M51-ULS-1 in the galaxy M51. We examined a range of explanations for the observed X-ray dip, including a variety of transiting objects and enhancements in the density of gas and dust. The latter are ruled out by the absence of changes in X-ray colours, save any with sharp density gradients that cannot be probed with our data. Instead, the data are well fit by a planet transit model in which the eclipser is most likely to be the size of Saturn. We also find that the locations of possible orbits are consistent with the survival of a planet bound to a mass-transfer binary. A brightness dip in the extragalactic X-ray binary M51-ULS-1 can be well fit by a planet transit model in which the eclipser is most likely Saturn-sized. The locations of possible orbits are consistent with the survival of a planet bound to a mass-transfer binary.

This thesis presents an analysis of transiting brown dwarf (BD) systems and examines their effectivenessas tests to substellar evolutionary models. The radius, mass, and age of transiting BDsystems are the parameters most useful in testing these models and in this work, I show how mycollaborators and I have used three facilities to derive these parameters for transiting BD systems.These facilities are: 1) NASA’s Transiting Exoplanet Survey Satellite (TESS) mission, 2) the TillinghastReflector Echelle Spectrograph (TRES), and 3) ESA’s Gaia mission. In this work, I use TESS,TRES, and Gaia in tandem to detect and characterize 10 new transiting BD systems with preciseradius, mass, and age determinations (in most cases). Most of the age determinations in this workcome solely from stellar isochrone models of the host star, but several systems have age constraintsfrom stellar clusters and gyrochronology, which we use to constrain the youth of one transiting BD,TOI-811b, to less than 200 Myr. This is important because it is at these young ages when the radii ofBDs changes most rapidly. In addition, I apply parallax measurements from the Gaia mission’s second data release (GaiaDR2) to improve the radius determinations of 10 transiting BD systems published prior to GaiaDR2 and the launch of the TESS mission. For these 10 previously published systems, new lightcurves from TESS are used, when possible, and the stellar distances, luminosities, and radii areupdated with the parallaxes from Gaia DR2, which improves our constraint on the companionBD’s radius. This work has significantly improved the radius determinations of 7 previously knowntransiting BDs, including CoRoT-15b and AD 3116b, whose radius uncertainties have been improvedfrom 15% to 5-7%, making them much more effective for testing substellar evolutionary models. Using these 20 new and previously known transiting BD systems, I have shown that the substellarevolutionary models ranging from young to old substellar isochrones are generally able toreproduce the observed radius, mass, and age determinations of the known transiting BD population.

I report updates to the substellar mass-radius diagram for 11 transiting brown dwarfs (BDs) and low-mass stars published before the third data release from the Gaia mission (Gaia DR3). I reanalyse these transiting BD systems whose physical parameters were published between 2008 and 2019 and find that when using the parallax measurements from Gaia DR3, 7 BDs show significant differences in their radius estimate or an improvement in the radius uncertainty. This has important implications for how these BDs are used to test substellar evolutionary models in the mass-radius diagram. The remaining 4 BDs show mass-radius estimates that are consistent with their previous pre-Gaia DR3 measurements. The 7 BDs that show significant deviation from the original mass-radius measurements are AD 3116b, CoRoT-3b, CoRoT-15b, EPIC 201702477b, Kepler-39b, KOI-205b, and KOI-415b. Of these, AD 3116b is a known member of the Praesepe cluster at an age of 600 Myr. Additionally, some of the previously smallest known transiting BDs, KOI-205b and KOI-415b, are not as small as once thought, leaving the mass-radius region for the very oldest BDs relatively sparse as a result of this work.

Masses and radii of transiting brown dwarfs can be measured directly in contrast to isolated field brown dwarfs, whose mass and radius inferences are model dependent. Therefore, transiting brown dwarfs are a testbed for the interior and evolutionary models of brown dwarfs and giant exoplanets. We have developed atmospheric and evolutionary models for this emerging population. We show that intense stellar irradiation can cause a large enhancement in the radius of transiting brown dwarfs at all masses, especially if the incident flux exceeds \\(log_{10}(F/cgs)\\ge\\)9 (\\(T_{\\rm eq}\\ge 1450\\) K). Stellar irradiation can significantly alter rates of nuclear burning in irradiated brown dwarfs, making the Deuterium-burning and Hydrogen-burning minimum masses strong functions of incident stellar flux. We show that the D-burning and H-burning minimum masses can decrease by 16% and 13%, respectively, between isolated and strongly irradiated brown dwarfs ( \\(log_{10}(F/cgs)\\ge\\)10 (\\(T_{\\rm eq}\\ge 2570\\) K)). This shows that stellar irradiation has a larger impact on the planet-brown dwarf-star mass boundaries than metallicity or clouds. We show that metal cores or migration affect their evolution to a much lesser extent, whereas low mass highly irradiated old sources can help us test the physics of hot Jupiter radius anomaly. We fit the observed radii of 46 transiting brown dwarfs and show that our irradiated evolutionary models fit their radii better than models that ignore the host star, especially for highly irradiated objects. However, the measured radii of 10 objects are still inconsistent at \\(>3\\sigma\\) level, indicating residual gaps in our irradiated evolutionary model.

We present the first measurement of the sky-projected orbital obliquity of a benchmark transiting brown dwarf host, HIP 33609, as a part of the Orbital Architectures of Transiting Massive Exoplanets And Low-mass stars (OATMEAL) survey. HIP 33609 b is a highly eccentric, 68 \\(M_{\\rm J}\\) brown dwarf orbiting a 10,300 K, A-type star with an orbital period of 39 days. Its host star is a known member of the 150 Myr old MELANGE-6 moving group, making it an excellent laboratory for testing sub-stellar evolutionary models. Using in-transit spectra collected by the Planet Finder Spectrograph (PFS) on the Magellan II Clay 6.5 m telescope, we measured a sky-projected orbital obliquity of \\(|\\lambda|= 12.7 \\pm 1.3\\){\\deg}. The mass of the brown dwarf is most consistent with a stellar-like fragmentation formation history followed by a period of migration. Given the high eccentricity (\\(e=0.557\\)) but low orbital obliquity of the brown dwarf, we claim that coplanar high eccentricity tidal migration seems to be the most plausible pathway, however, it remains difficult to conclusively rule out other migration mechanisms. The low orbital obliquity for HIP 33609 is consistent with previous measurements of high mass-ratio companions, and bears a striking resemblance to the obliquity distribution of transiting warm Jupiters. We suggest brown dwarfs may follow a dynamically quiescent migration pathway, consistent with them forming in isolated conditions.

We present new analysis of the CWW 89 system as part of the Orbital Architectures of Transiting Massive Exoplanets And Low-mass stars (OATMEAL) survey. The CWW 89 system is a member of the 2.8 Gyr old Ruprecht 147 (NGC 6774) cluster and features two stars, CWW 89A (EPIC 219388192) and CWW 89B, with the primary hosting a transiting brown dwarf. We use in-transit, highly precise radial velocity measurements with the Keck Planet Finder (KPF) to characterize the Rossiter-McLaughlin (RM) effect and measure the projected spin-orbit obliquity \\(|\\lambda|=1.4\\pm2.5^\\circ\\) and the full 3D spin-orbit obliquity of the brown dwarf to be \\(\\psi=15.1^{+15.0^\\circ}_{-10.9}\\). This value of \\(\\lambda\\) implies that the brown dwarf's orbit is prograde and well-aligned with the equator of the host star, continuing the trend of transiting brown dwarfs showing a preference for alignment (\\(\\lambda \\approx 0^\\circ\\)) regardless of the stellar effective temperature. We find that this contrast with the transiting giant planet population, whose spin-orbit alignments depend on host \\(T_{\\rm eff}\\), shows an increasingly clear distinction in the formation and orbital migration mechanisms between transiting giant planets and transiting brown dwarfs like CWW 89Ab. For this system in particular, we find it plausible that the brown dwarf may have undergone coplanar high-eccentricity migration influence by CWW 89B.

We present the discoveries of a brown dwarf and a low mass star from the Kepler and K2 missions. The newly discovered brown dwarf is EPIC 212036875b and the low mass star is KOI-607b. EPIC 212036875b has a mass of \\( M_{b}=52.3\\pm 1.9M_J\\), a radius of \\( R_{b}=0.874\\pm 0.017R_J\\), and orbits its host star in \\( P=5.169885 \\pm 0.000027\\) days. Its host star is a late F-type star with \\( M_\\star=1.288\\pm 0.065M_\\odot\\), \\( R_\\star=1.498\\pm 0.025R_\\odot\\), and \\( T_{\\rm eff}=6238 \\pm 60\\)K. KOI-607b has a mass of \\( M_{b}=95.1\\pm 3.4M_J\\), a radius of \\( R_{b}=1.089\\pm 0.089R_J\\), and an orbital period of \\( P=5.89399148 \\pm 0.00000060\\) days. The primary star in the KOI-607 system is a G dwarf with \\( M_\\star=0.993\\pm 0.052M_\\odot\\), \\( R_\\star=0.915\\pm 0.031R_\\odot\\), and \\( T_{\\rm eff} = 5418\\pm 87\\)K. We also revisit a brown dwarf, CWW 89Ab, that was previously published by Nowak et al. 2017 (under the designation EPIC 219388192b). CWW 89Ab is one of two known transiting brown dwarfs associated with a star cluster, which illustrates the need for more brown dwarfs with accurate masses and radii and reliable age determinations to test theoretical models. We find that the newly discovered brown dwarf, EPIC 212036875b, falls in the middle of the so-called \"brown dwarf desert\", indicating that EPIC 212036875b is either a particularly rare object, or the brown dwarf desert may not be so dry after all.

Masses and radii of transiting brown dwarfs can be measured directly in contrast to isolated field brown dwarfs, whose mass and radius inferences are model dependent. Therefore, transiting brown dwarfs are a testbed for the interior and evolutionary models of brown dwarfs and giant exoplanets. We have developed atmospheric and evolutionary models for this emerging population. We show that intense stellar irradiation can cause a large enhancement in the radius of transiting brown dwarfs at all masses, especially if the incident flux exceeds \\(log_{10}(F/cgs)\\ge\\)9 (\\(T_{\\rm eq}\\ge 1450\\) K). Stellar irradiation can significantly alter rates of nuclear burning in irradiated brown dwarfs, making the Deuterium-burning and Hydrogen-burning minimum masses strong functions of incident stellar flux. We show that the D-burning and H-burning minimum masses can decrease by 16% and 13%, respectively, between isolated and strongly irradiated brown dwarfs ( \\(log_{10}(F/cgs)\\ge\\)10 (\\(T_{\\rm eq}\\ge 2570\\) K)). This shows that stellar irradiation has a larger impact on the planet-brown dwarf-star mass boundaries than metallicity or clouds. We show that metal cores or migration affect their evolution to a much lesser extent, whereas low mass highly irradiated old sources can help us test the physics of hot Jupiter radius anomaly. We fit the observed radii of 46 transiting brown dwarfs and show that our irradiated evolutionary models fit their radii better than models that ignore the host star, especially for highly irradiated objects. However, the measured radii of 10 objects are still inconsistent at \\(>3\\sigma\\) level, indicating residual gaps in our irradiated evolutionary model.

We introduce the OATMEAL survey, an effort to measure the obliquities of stars with transiting brown dwarf companions. We observed a transit of the close-in (\\(P_{\\rm orb} = 1.74 \\,\\) days) brown dwarf GPX-1 b using the Keck Planet Finder (KPF) spectrograph to measure the sky-projected angle between its orbital axis and the spin axis of its early F-type host star (\\(\\lambda\\)). We measured \\(\\lambda = 6.88 \\pm 1.72 ^\\circ\\) (with additional unquantified systematic uncertainty), suggesting an orbit that is prograde and well aligned with the stellar equator. Hot Jupiters around early F stars are frequently found to have highly misaligned orbits, with polar and retrograde orbits being commonplace. It has been theorized that these misalignments stem from dynamical interactions, such as von Zeipel-Kozai-Lidov cycles, and are retained over long timescales due to weak tidal dissipation in stars with radiative envelopes. By comparing GPX-1 to similar systems under the frameworks of different tidal evolution theories, we argued that the rate of tidal dissipation is too slow to have re-aligned the system. This suggests that GPX-1 may have arrived at its close-in orbit via coplanar high-eccentricity migration or migration through an aligned protoplanetary disk. Our result for GPX-1 is one of few measurements of the obliquity of a star with a transiting brown dwarf. By enlarging the number of such measurements and comparing them with hot Jupiter systems, we will more clearly discern the differences between the mechanisms that dictate the formation and evolution of both classes of objects.

The angle between stellar spin axes and planetary orbits -- stellar obliquity -- probes the dynamics of planetary migration and evolution. The obliquities of giant planets have been extensively studied because they are the most easily measured. Smaller planets, while more difficult to measure, have the advantage of better reflecting the dynamics of planetary systems because they trigger negligible back-reactions onto the host star. This paper introduces a new observational campaign called the Small, Low-mass Oblique Planets Experiment (SLOPE) survey with the Keck Planet Finder (KPF) spectrograph, and presents four new obliquity measurements. The SLOPE survey focuses on planets smaller than Saturn across a variety of system architectures. The sky-projected obliquities of the four planets measured -- TOI-1386b, TOI-480b, TOI-4596b, and TOI-1823b -- are all consistent with spin-orbit alignment. We validate the planetary nature of TOI-4596b with a significant obliquity detection. Including these measurements, we conducted a statistical analysis of the obliquities of sub-Saturn size planets in different planetary system architectures. Compared to other architectures, those in compact multi-planet systems reside in orbits that appear preferentially aligned with the stellar equator with 6 sigma confidence.