# A Comparison of the Composition of Planets in Single- and Multi-Planet Systems Orbiting M dwarfs

**Authors:** Romy Rodríguez Martínez, David V. Martin, B. Scott Gaudi, Joseph G. Schulze, Anusha Pai Asnodkar, Kiersten M. Boley, Sarah Ballard  
**arXiv:** [2307.13034](https://arxiv.org/abs/2307.13034)  
**Date:** 2023-07-24  
**Summary page:** https://www.summarizepaper.com/en/paper/2307.13034v1/

## Key points

- Study investigates and compares composition of M-dwarf planets in "singles" and "multis"
- Analysis focuses on 70 planets (30 singles, 40 multis) across 19 systems
- Single planets have significantly lower densities compared to multis
- Difference in densities cannot be attributed to selection biases
- Multis are denser than singles with a confidence level of 99%
- Rocky multis have lower core/water mass fractions compared to singles
- Multi-planet hosts are more metal-poor compared to single-planet hosts
- Host star metallicity decreases with increasing planet multiplicity
- Modest difference observed in rotation periods between single and multi-planet hosts
- Different physical processes govern formation of single and multi-planet systems in M dwarfs

## Summary

In this study, the authors investigate and compare the composition of M-dwarf planets in two different types of systems: those with only one known planet (referred to as "singles") and those residing in multi-planet systems (referred to as "multis"). They also examine the fundamental properties of the host stars. The analysis is focused on planets with directly measured masses and radii, resulting in a total sample of 70 planets, including 30 singles and 40 multis across 19 systems. The authors begin by comparing the bulk densities of all planets in the sample, which range in size from $0.52 R_{\oplus}$ to $12.8R_\oplus$. They find that single planets have significantly lower densities on average compared to multis. Importantly, this difference cannot be attributed to selection biases. To further investigate this trend, the authors normalize the bulk densities by an Earth model for planets with sizes smaller than $6R_{\oplus}$. The analysis reveals that multis are also denser than singles with a confidence level of 99%. Next, they calculate and compare the core/water mass fractions (CMF/WMF) specifically for low-mass planets ($M_p <10 M_{\oplus}$). They find that rocky multis (with sizes smaller than $1.6 R_{\oplus}$) have lower CMFs compared to singles. Moving on to the properties of host stars, they examine the [Fe/H] metallicity and rotation period for both single-planet hosts and multi-planet hosts using available measurements from literature. Interestingly, they discover that multi-planet hosts are significantly more metal-poor compared to hosts with only one planet. Furthermore, there is a decrease in host star metallicity with increasing planet multiplicity observed. However, there is only a modest difference observed in rotation periods between these two groups. Overall, significant differences in planetary composition and host star metallicity suggest that different physical processes govern the formation of single-planet and multi-planet systems in M dwarfs. This study provides valuable insights into the diversity of exoplanetary systems and their underlying formation mechanisms. The authors acknowledge various sources of support for this research including grants from The Ohio State University and NASA Hubble Fellowship grant awarded by Space Telescope Science Institute among others.