Multiple-transiting Systems Research Articles

Evidence for Primordial Alignment: Insights from Stellar Obliquity Measurements for Compact Sub-Saturn Systems

Despite decades of effort, the mechanisms by which the spin axis of a star and the orbital axes of its planets become misaligned remain elusive. In particular, it is of great interest whether the large spin–orbit misalignments observed are driven primarily by high-eccentricity migration—expected to have occurred for short-period, isolated planets—or reflect a more universal process that operates across systems with a variety of present-day architectures. Compact multiplanet systems offer a unique opportunity to differentiate between these competing hypotheses, as their tightly packed configurations preclude violent dynamical histories, including high-eccentricity migration, allowing them to trace the primordial disk plane. In this context, we report measurements of the sky-projected stellar obliquity (λ) via the Rossiter–McLaughlin effect for two sub-Saturns in multiple-transiting systems: TOI-5126 b (λ = 1 ± 48°) and TOI-5398 b ( λ=−8.1−6.3+5.3° ). Both are spin–orbit aligned, joining a fast-growing group of just three other compact sub-Saturn systems, all of which exhibit spin–orbit alignment. In aggregate with archival data, our results strongly suggest that sub-Saturn systems are primordially aligned and become misaligned largely in the postdisk phase, as appears to be the case increasingly for other exoplanet populations.

Evidence for a Nondichotomous Solution to the Kepler Dichotomy: Mutual Inclinations of Kepler Planetary Systems from Transit Duration Variations

Abstract Early analyses of exoplanet statistics from the Kepler mission revealed that a model population of multiplanet systems with low mutual inclinations (∼1°–2°) adequately describes the multiple-transiting systems but underpredicts the number of single-transiting systems. This so-called “Kepler dichotomy” signals the existence of a subpopulation of multiplanet systems possessing larger mutual inclinations. However, the details of these inclinations remain uncertain. In this work, we derive constraints on the intrinsic mutual inclination distribution by statistically exploiting transit duration variations (TDVs) of the Kepler planet population. When planetary orbits are mutually inclined, planet–planet interactions cause orbital precession, which can lead to detectable long-term changes in transit durations. These TDV signals are inclination sensitive and have been detected for roughly two dozen Kepler planets. We compare the properties of the Kepler-observed TDV detections to TDV detections of simulated planetary systems constructed from two population models with differing assumptions about the mutual inclination distribution. We find strong evidence for a continuous distribution of relatively low mutual inclinations that is well characterized by a power-law relationship between the median mutual inclination ( μ ˜ i , n ) and the intrinsic multiplicity (n): μ ˜ i , n = μ ˜ i , 5 ( n / 5 ) α , where μ ˜ i , 5 = 1.10 − 0.11 + 0.15 and α = − 1.73 − 0.08 + 0.09 . These results suggest that late-stage planet assembly and possibly stellar oblateness are the dominant physical origins for the excitation of Kepler planet mutual inclinations.

The Orbital Eccentricity of Small Planet Systems

Abstract We determine the orbital eccentricities of individual small Kepler planets, through a combination of asteroseismology and transit light-curve analysis. We are able to constrain the eccentricities of 51 systems with a single transiting planet, which supplement our previous measurements of 66 planets in multi-planet systems. Through a Bayesian hierarchical analysis, we find evidence that systems with only one detected transiting planet have a different eccentricity distribution than systems with multiple detected transiting planets. The eccentricity distribution of the single-transiting systems is well described by the positive half of a zero-mean Gaussian distribution with a dispersion σ e = 0.32 ± 0.06, while the multiple-transit systems are consistent with . A mixture model suggests a fraction of of single-transiting systems have a moderate eccentricity, represented by a Rayleigh distribution that peaks at . This finding may reflect differences in the formation pathways of systems with different numbers of transiting planets. We investigate the possibility that eccentricities are self-excited in closely packed planetary systems, as well as the influence of long-period giant companion planets. We find that both mechanisms can qualitatively explain the observations. We do not find any evidence for a correlation between eccentricity and stellar metallicity, as has been seen for giant planets. Neither do we find any evidence that orbital eccentricity is linked to the detection of a companion star. Along with this paper, we make available all of the parameters and uncertainties in the eccentricity distributions, as well as the properties of individual systems, for use in future studies.

A Six-planet System around the Star HD 34445

Abstract We present a new precision radial velocity (RV) data set that reveals a multi-planet system orbiting the G0V star HD 34445. Our 18-year span consists of 333 precision RV observations, 56 of which were previously published and 277 of which are new data from the Keck Observatory, Magellan at Las Campanas Observatory, and the Automated Planet Finder at Lick Observatory. These data indicate the presence of six planet candidates in Keplerian motion about the host star with periods of 1057, 215, 118, 49, 677, and 5700 days, and minimum masses of 0.63, 0.17, 0.1, 0.05, 0.12, and 0.38 M J, respectively. The HD 34445 planetary system, with its high degree of multiplicity, its long orbital periods, and its induced stellar RV half-amplitudes in the range 2 m s−1 ≲ K ≲ 5 m s−1 is fundamentally unlike either our own solar system (in which only Jupiter and Saturn induce significant reflex velocities for the Sun), or the Kepler multiple-transiting systems (which tend to have much more compact orbital configurations).

Dynamically Hot Super-Earths from Outer Giant Planet Scattering

Abstract The hundreds of multiple planetary systems discovered by the Kepler mission are typically observed to reside in close-in ( AU), low-eccentricity, low-inclination orbits. We run N-body experiments to study the effect that unstable outer ( AU) giant planets, whose end orbital configurations resemble those in the Radial Velocity population, have on these close-in multiple super-Earth systems. Our experiments show that the giant planets greatly reduce the multiplicity of the inner super-Earths, and the surviving population can have large eccentricities ( ) and inclinations ( ) at levels that anti-correlate with multiplicity. Consequently, this model predicts the existence of a population of dynamically hot single-transiting planets with typical eccentricities and inclinations of ∼0.1–0.5 and ∼10°–40°. We show that these results can explain the following observations: (i) the recent eccentricity measurements of Kepler super-Earths from transit durations; (ii) the tentative observation that single-transiting systems have a wider distribution of stellar obliquity angles compared to the multiple-transiting systems; (iii) the architecture of some eccentric super-Earths discovered by Radial Velocity surveys such as HD 125612c. Future observations from TESS will reveal many more dynamically hot single transiting planets, for which follow up radial velocity studies will be able to test our models and see whether they have outer giant planets.

Urban Transit System Microbial Communities Differ by Surface Type and Interaction with Humans and the Environment.

Public transit systems are ideal for studying the urban microbiome and interindividual community transfer. In this study, we used 16S amplicon and shotgun metagenomic sequencing to profile microbial communities on multiple transit surfaces across train lines and stations in the Boston metropolitan transit system. The greatest determinant of microbial community structure was the transit surface type. In contrast, little variation was observed between geographically distinct train lines and stations serving different demographics. All surfaces were dominated by human skin and oral commensals such as Propionibacterium, Corynebacterium, Staphylococcus, and Streptococcus. The detected taxa not associated with humans included generalists from alphaproteobacteria, which were especially abundant on outdoor touchscreens. Shotgun metagenomics further identified viral and eukaryotic microbes, including Propionibacterium phage and Malassezia globosa. Functional profiling showed that Propionibacterium acnes pathways such as propionate production and porphyrin synthesis were enriched on train holding surfaces (holds), while electron transport chain components for aerobic respiration were enriched on touchscreens and seats. Lastly, the transit environment was not found to be a reservoir of antimicrobial resistance and virulence genes. Our results suggest that microbial communities on transit surfaces are maintained from a metapopulation of human skin commensals and environmental generalists, with enrichments corresponding to local interactions with the human body and environmental exposures. IMPORTANCE Mass transit environments, specifically, urban subways, are distinct microbial environments with high occupant densities, diversities, and turnovers, and they are thus especially relevant to public health. Despite this, only three culture-independent subway studies have been performed, all since 2013 and all with widely differing designs and conclusions. In this study, we profiled the Boston subway system, which provides 238 million trips per year overseen by the Massachusetts Bay Transportation Authority (MBTA). This yielded the first high-precision microbial survey of a variety of surfaces, ridership environments, and microbiological functions (including tests for potential pathogenicity) in a mass transit environment. Characterizing microbial profiles for multiple transit systems will become increasingly important for biosurveillance of antibiotic resistance genes or pathogens, which can be early indicators for outbreak or sanitation events. Understanding how human contact, materials, and the environment affect microbial profiles may eventually allow us to rationally design public spaces to sustain our health in the presence of microbial reservoirs. Author Video: An author video summary of this article is available.

OBLIQUITIES OFKEPLERSTARS: COMPARISON OF SINGLE- AND MULTIPLE-TRANSIT SYSTEMS

The stellar obliquity of a transiting planetary system can be constrained by combining measurements of the star's rotation period, radius, and projected rotational velocity. Here we present a hierarchical Bayesian technique for recovering the obliquity distribution of a population of transiting planetary systems, and apply it to a sample of 70 Kepler Objects of Interest. With ~95% confidence we find that the obliquities of stars with only a single detected transiting planet are systematically larger than those with multiple detected transiting planets. This suggests that a substantial fraction of Kepler's single-transiting systems represent dynamically hotter, less orderly systems than the "pancake-flat" multiple-transiting systems.

Leveraging the General Transit Feed Specification for Efficient Transit Analysis

Since 2007, the transit industry has benefited from a widely adopted data standard called the General Transit Feed Specification (GTFS), which has enabled the development of numerous traveler information tools (i.e., transit trip planners). The purpose of this project is to demonstrate the potential for GTFS feeds as a data source for transit analyses, such as those found in the Transit Capacity and Quality of Service Manual. There are three primary project tasks: an analysis of GTFS field usage by different agencies; an analysis of a single agency's operations at the stop, route, and system levels; and a batch analysis and comparison of 50 large transit agencies in North America. Compared with manually transcribing schedules from transit websites or parsing printed schedules, the use of scripts and database queries suggests that the GTFS is a highly efficient data source and proves the importance of broadly accepted data standards. The methodology documented in this paper and the open source scripts (made available online) will be useful for any analyst or researcher who has tasks related to the analysis of single or multiple transit systems at the stop, route, or system level.

Comparing the Performance of Bus Routes after Adjusting for the Environment Using Data Envelopment Analysis

Public transit managers strive to attain multiple goals with tightly constrained resources. Ratio analysis has evolved into a powerful tool for dealing with these goals and constraints. Ratio analysis provides analytical methods for comparing the performance of multiple agencies, as well as the performance of subunits within a particular agency, in order to identify opportunities for improvement. One ratio analysis procedure that has become increasingly popular is data envelopment analysis (DEA). DEA yields a single, comprehensive measure of performance, the ratio of the aggregated, weighted outputs to aggregated, weighted inputs. This paper makes two contributions to the practice of transit performance evaluation using DEA. First, instead of using DEA to compare the performance of multiple transit systems, it uses DEA to compare the performance of multiple bus routes of one transit system. Second, it introduces a new procedure for adjusting the raw DEA scores that modifies these scores to account for the environmental influences that are beyond the control of the transit agency.

Integrated National Transit Database Analysis System

The Integrated National Transit Database Analysis System (INTDAS) is a user-friendly software designed to facilitate the retrieval and analysis of data from the National Transit Database (NTD). INTDAS is an integrated database system that ( a) combines individual NTD data files from multiple years into a single, standardized database and ( b) provides customized tools for easy data retrieval, visualization, analysis, and transfer. The system is useful for both the practitioners and researchers who use NTD data to improve transit performance and services. The INTDAS database combines the 1984 to 2000 NTD data for all transit systems reported to the NTD program. INTDAS is equipped with highly customized functions that allow the user to quickly retrieve data of multiple transit systems of multiple years for trend analyses, peer comparisons, and data inquiries. A customized query editor allows the user to identify transit systems that meet a certain profile. An automated Florida peer selection process is also implemented to allow the user to identify comparable transit systems in peer analyses. Retrieved data can be displayed in tables, forms, or charts. The user may also set up formulas to derive new variables from the original NTD variables. All output from INTDAS can be saved to files in various formats and automatically sent to a third party as an e-mail file attachment. Although the system was developed for Florida, its functions are applicable to general users.