Over the past two decades, astronomers have discovered that the Milky Way is teeming with exoplanets, planets orbiting stars other than the Sun. Although thousands of exoplanets have been confirmed, ...only a small fraction of them have well-measured masses and radii, two properties which, when combined, give us an estimate of their density, which in turn provides a constraint on their composition. Determining the composition of exoplanets is crucial to identify habitable, Earth-like planets and life in the coming decades, which is one of the main pillars of the field of exoplanets. In addition, the composition of a planet provides valuable insights into its formation and interior dynamics. Small planets between roughly 1.5-4 Earth radii (R⊕) exhibit a vast diversity of physical properties, orbits, and compositions not seen in the Solar System. This fact, combined with the observational challenges of obtaining precise mass and radius measurements of such planets, makes the determination of their composition difficult. In my thesis, I helped shed light on the population of low-mass planets by characterizing these systems in detail using observations from ground- and space-based telescopes. Specifically, I determined their fundamental physical properties and the photospheric chemical abundances of their host stars to better constrain their composition. In this dissertation, I present various observational studies that broadly explore topics at the intersection of planet composition, demographics, and habitability. In the first part of my thesis, I present the discovery and characterization of two substellar objects orbiting hot stars found with the Kilodegree Extremely Little Telescope (KELT) collaboration. For the remainder of my studies, I focused on the composition of small planets. I first investigated the uncertainties in the fundamental properties of transiting exoplanets as a function of purely observable parameters, and I determined a useful relationship between a planet’s interior and its surface gravity. I used the results of that work to study an intriguing class of highly dense planets, called super-Mercuries, by characterizing a previously discovered super-Mercury candidate. In the latter part of my thesis, I zoomed out of individual systems to examine the demographics and composition of M-dwarf exoplanetary systems. In particular, I investigated the differences in the properties and composition of planets in systems with only one planet (“singles”) versus those in multiple planets (“multis”) using archival data from the literature. In that study, I found evidence for a dichotomy in the formation pathways between these two types of planets/systems. These studies broaden our understanding of the composition and demographics of M-dwarf planetary systems, and more generally, they help shed light on the formation and evolution of low-mass planets in our Galaxy.
ABSTRACT
We explore the prospects for Twinkle to determine the atmospheric composition of the nearby terrestrial-like planet LTT 1445 Ab, including the possibility of detecting the potential ...biosignature ammonia (NH3). At a distance of 6.9 pc, this system is the second closest known transiting system and will be observed through transmission spectroscopy with the upcoming Twinkle mission. Although LTT 1445 Ab has been suggested to be a candidate for a Hycean world, constraints on the interior composition based on its mass and radius suggests that the planet lacks a substantial water layer, and thus the proposed Hycean scenario is disfavoured. We use PETITRADTRANS and a Twinkle simulator to simulate transmission spectra for the more likely scenario of a cold Haber world for which NH3 is considered to be a biosignature. We study the detectability under different scenarios: varying hydrogen fraction, concentration of ammonia, and cloud coverage. We find that ammonia can be detected at an ∼3σ level for optimal (non-cloudy) conditions with 25 transits and a volume mixing ration of 4.0 ppm of NH3. We provide examples of retrieval analysis to constrain potential NH3 and H2O in the atmosphere. Our study illustrates the potential of Twinkle to characterize atmospheres of potentially habitable exoplanets.
Abstract
We measure abundances of 12 elements (Na, Mg, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni) in a sample of 86 metal-poor (−2 ≲ Fe/H ≲ −1) subgiant stars in the solar neighborhood. Abundances are ...derived from high-resolution spectra taken with the Potsdam Echelle Polarimetric and Spectroscopic Instrument on the Large Binocular Telescope, modeled using iSpec and MOOG. By carefully quantifying the impact of photon-noise (<0.05 dex for all elements), we robustly measure the
intrinsic
scatter of abundance ratios. At fixed Fe/H, the rms intrinsic scatter in X/Fe ranges from 0.04 (Cr) to 0.16 dex (Na), with a median of 0.08 dex. Scatter in X/Mg is similar, and accounting for
α
/Fe only reduces the overall scatter moderately. We consider several possible origins of the intrinsic scatter with particular attention to fluctuations in the relative enrichment by core-collapse supernovae (CCSN) and Type Ia supernovae and stochastic sampling of the CCSN progenitor mass distribution. The stochastic sampling scenario provides a good quantitative explanation of our data if the effective number of CCSN contributing to the enrichment of a typical sample star is
N
∼ 50. At the median metallicity of our sample, this interpretation implies that the CCSN ejecta are mixed over a gas mass ∼6 × 10
4
M
⊙
before forming stars. The scatter of elemental abundance ratios is a powerful diagnostic test for simulations of star formation, feedback, and gas mixing in the early phases of the Galaxy.
We investigate and compare the composition of M-dwarf planets in systems with only one known planet (``singles") to those residing in multi-planet systems (``multis") and the fundamental properties ...of their host stars. We restrict our analysis to planets with directly measured masses and radii, which comprise a total of 70 planets: 30 singles and 40 multis in 19 systems. We compare the bulk densities for the full sample, which includes planets ranging in size from \(0.52 R_{\oplus}\) to \(12.8R_\oplus\), and find that single planets have significantly lower densities on average than multis, which we cannot attribute to selection biases. We compare the bulk densities normalized by an Earth model for planets with \(R_{p} < 6R_{\oplus}\), and find that multis are also denser with 99\% confidence. We calculate and compare the core/water mass fractions (CMF/WMF) of low-mass planets (\(M_p <10 M_{\oplus}\)), and find that the likely rocky multis (with \(R_p <1.6 R_{\oplus}\)) have lower CMFs than singles. We also compare the Fe/H metallicity and rotation period of all single versus multi-planet host stars with such measurements in the literature and find that multi-planet hosts are significantly more metal-poor than those hosting a single planet. Moreover, we find that host star metallicity decreases with increasing planet multiplicity. In contrast, we find only a modest difference in the rotation period. The significant differences in planetary composition and metallicity of the host stars point to different physical processes governing the formation of single- and multi-planet systems in M dwarfs.
We present the discoveries of KELT-25 b (TIC 65412605, TOI-626.01) and KELT-26 b (TIC 160708862, TOI-1337.01), two transiting companions orbiting relatively bright, early A stars. The transit signals ...were initially detected by the KELT survey and subsequently confirmed by Transiting Exoplanet Survey Satellite (TESS) photometry. KELT-25 b is on a 4.40 day orbit around the V=9.66 star CD-24 5016(=-+T8280eff180440K,Må=-+2.180.110.12Me), while KELT-26 b is on a 3.34 day orbit around the V=9.95 star HD 134004 (Teff=-+8640240500K,Må=-+1.930.160.14Me), which is likely an Am star. We have confirmed the substellar nature of both companions through detailed characterization of each system using ground-based and TESS photometry, radial velocity measurements, Doppler tomography, and high-resolution imaging. For KELT-25, we determine a companion radius of RP=-+1.640.0430.039RJ and a 3σupper limit on the companion’s mass of64MJ. For KELT-26 b, we infer a planetary mass and radius of MP=-+1.410.510.43MJ and RP=-+1.940.0580.060RJ. From Doppler tomographic observations, we find KELT-26 b to reside in a highly misaligned orbit. This conclusion is weakly corroborated by a subtle asymmetry in the transit light curve from the TESS data. KELT-25 b appears to be in a well-aligned, prograde orbit, and the system is likely a member of the cluster Theia 449.
Well-characterised M-dwarfs are rare, particularly with respect to effective temperature. In this letter we re-analyse two benchmark M-dwarfs in eclipsing binaries from Kepler/K2: KIC 1571511AB and ...HD 24465AB. Both have temperatures reported to be hotter or colder by approximately 1000 K in comparison with both models and the majority of the literature. By modelling the secondary eclipses with both the original data and new data from TESS we derive significantly different temperatures which are not outliers. Removing this discrepancy allows these M-dwarfs to be truly benchmarks. Our work also provides relief to stellar modellers. We encourage more measurements of M-dwarf effective temperatures with robust methods.
M-dwarfs are the most abundant stars in the galaxy and popular targets for exoplanet searches. However, their intrinsic faintness and complex spectra inhibit precise characterisation. We only know of ...dozens of M-dwarfs with fundamental parameters of mass, radius and effective temperature characterised to better than a few per cent. Eclipsing binaries remain the most robust means of stellar characterisation. Here we present two targets from the Eclipsing Binary Low Mass (EBLM) survey that were observed with K2: EBLM J0055-00 and EBLM J2217-04. Combined with HARPS and CORALIE spectroscopy, we measure M-dwarf masses with precisions better than 5%, radii better than 3% and effective temperatures on order 1%. However, our fits require invoking a model to derive parameters for the primary star. By investigating three popular models, we determine that the model uncertainty is of similar magnitude to the statistical uncertainty in the model fits. Therefore, whilst these can be considered benchmark M-dwarfs, we caution the community to consider model uncertainty when pushing the limits of precise stellar characterisation.
We present a reanalysis of the K2-106 transiting planetary system, with a focus on the composition of K2-106b, an ultra-short period, super-Mercury candidate. We globally model existing photometric ...and radial velocity data and derive a planetary mass and radius for K2-106b of \(M_{p} = 8.53\pm1.02~M_{\oplus}\) and \(R_{p} = 1.71^{+0.069}_{-0.057}~R_{\oplus}\), which leads to a density of \(\rho_{p} = 9.4^{+1.6}_{-1.5}\) \(\rm g~cm^{-3}\), a significantly lower value than previously reported in the literature. We use planet interior models that assume a two-layer planet comprised of a liquid, pure Fe core and iron-free, \(\rm MgSiO_{3}\) mantle, and we determine the range of core mass fractions that are consistent with the observed mass and radius. We use existing high-resolution spectra of the host star to derive Fe/Mg/Si abundances (Fe/H\(=-0.03 \pm 0.01\), Mg/H\(= 0.04 \pm 0.02\), Si/H\(=0.03 \pm 0.06\)) to infer the composition of K2-106b. We find that although K2-106b has a high density and core mass fraction (\(44^{+12}_{-15}\%\)) compared to the Earth (\(33\%\)), its composition is consistent with what is expected assuming that it reflects the relative refractory abundances of its host star. K2-106b is therefore unlikely to be a super-Mercury, as has been suggested in previous literature.
We present moderate resolution near-infrared spectra in \(H, J\) and \(K\) band of M dwarf hosts to candidate transiting exoplanets discovered by NASA's K2 mission. We employ known empirical ...relationships between spectral features and physical stellar properties to measure the effective temperature, radius, metallicity, and luminosity of our sample. Out of an initial sample of 56 late-type stars in K2, we identify 35 objects as M dwarfs. For that sub-sample, we derive temperatures ranging from \(2,870\) to \(4,187\) K, radii of \(0.09-0.83\) \(R_{\odot}\), luminosities of \(-2.67<log L/L_{\odot}<-0.67\) and Fe/H metallicities between \(-0.49\) and \(0.83\) dex. We then employ the stellar properties derived from spectra, in tandem with the K2 lightcurves, to characterize their planets. We report 33 exoplanet candidates with orbital periods ranging from 0.19 to 21.16 days, and median radii and equilibrium temperatures of 2.3 \(R_{\oplus}\) and 986 K, respectively. Using planet mass-radius relationships from the literature, we identify 7 exoplanets as potentially rocky, although we conclude that probably none reside in the habitable zone of their parent stars.
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