The Kepler Dichotomy among the M Dwarfs: Half of Systems Contain Five or More Coplanar Planets (1410.4192v1)

Abstract: We present a statistical analysis of the Kepler M dwarf planet hosts, with a particular focus on the fractional number of systems hosting multiple transiting planets. We manufacture synthetic planetary systems within a range of planet multiplicity and mutual inclination for comparison to the Kepler yield. We recover the observed number of systems containing between 2 and 5 transiting planets if every M dwarf hosts 6.1+/-1.9 planets with typical mutual inclinations of 2.0 +4.0-2.0 degrees. This range includes the Solar System in its coplanarity and multiplicity. However, similar to studies of Kepler exoplanetary systems around more massive stars, we report that the number of singly-transiting planets found by Kepler is too high to be consistent with a single population of multi-planet systems: a finding that cannot be attributed to selection biases. To account for the excess singleton planetary systems we adopt a mixture model and find that 55 +23-12% of planetary systems are either single or contain multiple planets with large mutual inclinations. Thus, we find that the so-called "Kepler dichotomy" holds for planets orbiting M dwarfs as well as Sun-like stars. Additionally, we compare stellar properties of the hosts to single and multiple transiting planets. For the brightest subset of stars in our sample we find intriguing, yet marginally significant evidence that stars hosting multiply-transiting systems are rotating more quickly, are closer to the midplane of the Milky Way, and are comparatively metal poor. This preliminary finding warrants further investigation.

Overview of the "Kepler Dichotomy among M Dwarfs: Half of Systems Contain Five or More Coplanar Planets"

In the paper "The Kepler Dichotomy among the M dwarfs: Half of Systems Contain Five or More Coplanar Planets," the authors conduct a sophisticated statistical analysis of M dwarfs within the Kepler mission dataset, focusing on the incidence and architecture of their planetary systems. By modeling synthetic planetary systems with varying complexities of planet multiplicity and mutual inclinations, they aim to reconcile the observed multiplicity of transiting planets with theoretical models. This paper provides insights into exoplanet demographics, particularly highlighting the trends and characteristics of systems orbiting the smallest stars, M dwarfs, and confronting the "Kepler dichotomy" that suggests an unexpected frequency of singleton planetary systems.

Key Findings

• Multiplicity and Architecture of M Dwarf Systems: Through statistical modeling, the authors determine that approximately half of the M dwarf systems assessed contain five or more coplanar planets. These systems stand in contrast with others dominated by singular transiting planets, suggesting a dual population model.
• Modeling Insights: Utilizing Monte Carlo simulations, the authors match observed Kepler data, indicating that M dwarfs typically host systems with $6.1 \pm 1.9$ planets and mutual inclinations of around 2.0 degrees. This affirms that a single model cannot account for all the observations, necessitating a mixture model.
• Mixture Model Adoption: The paper introduces a two-mode hypothesis, integrating a mixture model to explain the excess of singly transiting systems. This model posits that 55\% of systems are either single or possess large mutual inclinations, thereby affirming the presence of a "Kepler dichotomy" similarly seen around Sun-like stars.
• Host Star Properties: An examination of stellar characteristics suggests intriguing correlations between planet multiplicity and stellar properties. Hosts of multiply transiting planets tend to be younger, rotate faster, are closer to the galactic midplane, and exhibit lower metallicity. These findings, though marginally significant, propose potential stellar influences on planetary architecture.

Implications

The discoveries outlined in this paper hold significant implications for the understanding of planetary formation and system dynamics around M dwarfs. The differentiated architecture suggests that system formation and evolution might be influenced by initial conditions, such as metallicity and the dynamic environment of stellar hosts. This also raises interesting theoretical questions about circumstellar disk dynamics and planet-disk interactions.

Further, these results contribute to the broader discourse on planet occurrence and architecture around different stellar types, reinforcing the idea that planetary systems are diverse and perhaps subject to varied formation mechanisms. Observations from missions like K2 and future studies could expand on these conclusions, offering rich datasets for continued inquiries into planetary system formation and evolution.

Future Prospects in Exoplanet Research

The findings stimulate further investigation into the mechanisms that drive the diverse planetary system architectures observed, particularly regarding stellar characteristics. Furthermore, they suggest fruitful lines of inquiry regarding stellar multiplicity's impact on planet formation. Upcoming missions, such as the extended Kepler mission (K2) and the Transiting Exoplanet Survey Satellite (TESS), will provide additional data on M dwarfs, enhancing our ability to test these initial findings and explore planetary habitability around the most common stars in our galaxy. The paper demonstrates a path forward for exploring the implications of stellar properties on planet occurrences, paving the way for more comprehensive models of planetary system formation.