by Sakhee Bhure
Authors: Romy Rodríguez Martínez, David V. Martin, B. Scott Gaudi, Joseph G. Schulze, Anusha Pai Asnodkar, Kiersten M Boley, Sarah Ballard
First Author’s Institution: Department of Astronomy, The Ohio State University, 140 W. 18th Avenue, Columbus OH 43210, USA
Status: Published in The Astronomical Journal [open access]
Planet populations are generally divided into single planets and planets in multi-planet systems, and are common for Sun-like stars as well as M dwarfs. Unlike our Sun, M dwarfs have lower surface temperatures (<4000 K). They also have shorter-period habitable zones, which could host an abundant number of small planets. They are substantially active and challenging to disentangle stellar activity from transit signals, but, where possible, facilitate easier transit detection (in terms of the transit depths of smaller planets’ transits) and planet characterization, and are thus highly sought after for understanding planet formation and habitability.
Today’s paper tested whether and how planets orbiting M dwarfs inherit properties from their host stars and protoplanetary disks that determine their evolution into single-planet or multi-planet systems: either host stars of single planets and multis possess distinct properties from the outset, or host stars and disks are largely indistinguishable but their disks evolve and diverge into single-planet or multi-planet systems.
Sample Selection and Methodology
Publicly available exoplanet data hosted by the NASA Exoplanet Archive was carefully filtered by ensuring that only planets or stars with well-measured values and non-zero uncertainties were chosen to curate the sample of 30 singles and 40 multis in 49 systems. And since the study wanted to focus on the relationship between small planets and their host stars, giant planets (Rp > 6 R⊕) were excluded and the analyses repeated. To determine the statistical significance of the findings, the Kolmogorov-Smirnov (K-S) and Anderson-Darling (A-D) tests were conducted for comparison.
Planetary Bulk Density
Bulk density (total mass/total volume), is a first-order approximation of planet composition (the Method section in this astrobite provides a good explanation). In the full sample, single planets are found to be less dense than multis (Figure 1). The calculated p-values from the K-S and A-D tests are very small, indicating that the densities of singles and multis belong to different (underlying) populations.
The bulk densities were then normalized by an Earth model, i.e. the bulk density of the planet divided by the density an Earth-like planet (rocky with ~0.32 core mass fraction). This (Figure 2) shows a statistically significant distinction between the two populations!
Core Mass Fraction (CMF) and Water Mass Fraction (WMF)
Bulk densities, however, do not necessarily inform detailed compositions, especially the fractional distribution of a core and an envelope of rock, liquid, or volatiles.
Planets from the full sample with masses less than 10 M⊕ and densities greater than pure silicate were then modeled with a pure liquid iron core and rock (magnesium silicate) mantle. Likewise, planets with densities lower than silicate were modeled as ‘water worlds’, solving for the water mass fraction
From this analysis, the authors found that the core mass fractions of the likely rocky, low-mass planets in the sample (Mp < 10 M⊕), were on average considerably higher for singles than for multis (Figure 3), and uncorrelated with orbital period, planet mass, or distance from the host star within individual planetary systems.
Stellar Properties
The authors also investigated the properties of each host star to see how they affected their planetary systems. Stellar rotation periods and Fe/H metallicities were studied for all stars in the literature having those measurements with reported uncertainties and hosting single or multi-planet systems.
The authors found that single planet hosts rotate slightly faster than multi-planet hosts but only marginally.
But what about stellar metallicity?
Stellar abundances serve as a proxy for planet compositions. Variations in the extent to which host star compositions are reflected in their planets provides clues about planet formation and evolution processes. Larger planets orbiting metal-rich stars have been observed in the case of hot Jupiters around Sun-like stars. Similarly, single, Saturn-like planets around metal-rich M dwarfs are seen among the larger, single planets in the sample considered, in agreement with the core-accretion theory of planet formation.
Summary and Results
The authors compared planets and host stars in single-planets and multi-planet systems around M dwarfs looking for variations across four properties: bulk density, core/water mass fraction, stellar rotation period, and stellar metallicity. The statistically robust observations made in this work indicate that host stars with multiple planets are metal-poor compared to host stars of singles; within the host stars with multi-planet systems, metallicity decreases with an increasing number of planets; and planets in multi-planet systems tend to be denser than singles.
From this, it can be inferred that it is probable that distinct formation mechanisms create single planet systems and multi-planet systems. Metal-rich stars might favor the formation of giant planets, with metal-poor M dwarfs forming smaller planets in more compact configurations.
Open Questions and Future Work
It is less likely that singles and multis share a common origin given the differences in their host stars, however, extending these analyses to a larger sample of M dwarf planets (validating and confirming candidates) and Sun-like stars, and the metallicity studies to other elemental abundances would provide further insights into planet formation. Further studies would also help answer other questions, such as why the eccentricities of singles around M dwarfs in this sample are higher than those of multis! Such an analysis for M dwarfs has not been conducted previously, but with more and more planets found every day, as well as an increasing number of planet atmospheres and compositions explored, thoroughly constraining planet forming mechanisms may soon be on the horizon!
Astrobite edited by Amaya Sinha
Featured Image Credit: Rodríguez Martínez et al. 2023
