Siblings or Only Child: M Dwarf Planets

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

Authors: Romy Rodríguez Martínez et al.

First-author institution: Ohio State

Status: The Astronomical Journal, Volume 166, Issue 4, preprint on arXiv

The most common type of star in the universe is the M Dwarf, making up ~70% of all stars. And because they are so common, and because we know exoplanets are common, it is only natural that we have found many exoplanets orbiting M Dwarf stars. In fact, Astrobites covered a seminal paper on the occurrence rate of planets around M Dwarfs and their compositions, read it here and here. But with all these planets, we can take our knowledge one step further and begin to ask deeper questions. For example, in M Dwarf star planetary systems, how do single planet systems (only child) and multi-planet systems (sibilings) compare? 

A new study seeks to answer this question, focusing on 3 key parameters: planet bulk density, planet core mass fraction, and host star metallicity. In particular, teh authors wish to investigate whether siblings and only child systems are two outcomes of the same formation process, or if they truly form differently, ie are they from the same population or are they two distinct populations of planets.

Bulk Density

First, bulk density, or the average density of the planet as a whole. We know that our planet Earth is made up of many different materials (rocks, water, gases, etc) and each one of these has its own density. But for exoplanets, we cannot explore the details of different materials and so instead we measure bulk density by simply taking the total mass of the planet and dividing by the total volume (assuming the planet is a sphere). The authors compute bulk density for a sample of planets around M Dwarfs, both those single planet systems and all the planets in multi-planet systems. Next they apply a statistical test (the Kolmogorov–Smirnov test) to determine if the two sets of planets are truly distinct populations or are consistent with one population, see Figure 1 top panel. The result is an overwhelming determination that these are two different populations. However, the authors caveat that this result may be biased. Many of the single planet systems in the sample are giant planets, which naturally have lower density than smaller planets because they have higher gas fractions. Removing the gas giants from the sample and re-running the test, they find that actually the siblings and only child planets are consistent with coming from the same population, see Figure 1 bottom panel.

Two plots showing the cumulative distribution function vs planet density for two K-S tests. Top (all planets included) the blue and red lines diverge, indicating two populations. Bottom (no giant planets included), the red and blue lines overlap, indicating one population.
Figure 1 (Figure 3 in paper): Top: The results of the statistical test when including all planets to determine if the two populations are distinct. The gap between the single and multi’s suggests they are indeed two populations. Bottom: the same as top but for the sample that excludes giant planets. Here the finding of two populations is less statistically significant.

Core Mass

Next, planet core mass. The mass of a planet is generally meant to include everything that makes up the planet. However, the core mass is just that, the mass of the core of the planet alone. Core masses are valuable pieces of information because it is thought that the core, which is the first to form, can determine how big the planet eventually grows to be. Bigger cores are better at gravitationally attracting material, including gas, to grow the planet. While we cannot directly measure the core mass of a planet, we can use models that are tuned to Earth’s parameters in order to estimate planet core mass based on a few things we can measure, like mass and radius. Now taking only the planets that are likely to be rocky and again splitting by single vs multi planet system, the authors find that planets in single planet systems have, on average, larger core masses than those in multi-planet systems. They further test if core mass correlates with orbital period but find none. 


Lastly, the authors explore the host star, particularly its metallicity, or the percentage of the star’s composition that is made up of “metals” (astronomers define “metal” as anything heavier than helium!). Host star metallicity is thought to correlate with the kinds of planets and number of planets in the system, the thinking being that since planets form out of the same disk of material as the host star, if there is more heavy materials in that disk (which would appear as higher metallicity in the host star) then there is more opportunity to make more and bigger planets. The authors here find that host stars of single planets are more metal rich than those hosts of multi-planet systems. This is counter-intuitive but the authors hypothesize this could be because more metal rich stars might produce more and bigger planets, which may gravitationally interact in the early days of the system and fling out all but one planet; while on the other hand, metal poor stars cannot build big planets and instead build small planets that are dynamically “quiet”. 

In all, the authors find that single and multi planet M Dwarf systems are likely two distinct populations. This could have large implications for how we understand the formation and evolution of planetary systems. 

Edited by Mark Popinchalk

Featured image credit: Rodríguez Martínez et al. 2023

About Jack Lubin

Jack received his PhD in astrophysics from UC Irvine and is now a postdoc at UCLA. His research focuses on exoplanet detection and characterization, primarily using the Radial Velocity method. He enjoys communicating science and encourages everyone to be an observer of the world around them.

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