Kepler Multi-planet Systems Exhibit Unexpected Intra-system Uniformity in Mass and Radius

Millholland, Sarah; Wang, Songhu; Laughlin, Gregory

doi:10.3847/2041-8213/aa9714

Cited by 209 publications

(248 citation statements)

References 32 publications

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“…The figure includes the √ N -errors, marked by error bars, which are smaller than the differences between the two distributions. The distribution for low mass planets shows a significant peak near f ∼ 1/2, consistent with predictions for systems where the self-gravity of planets is subdominant (equation [18]; see also Millholland et al 2017;Weiss et al 2018a). The distribution for high mass planets is nearly flat (within the estimated uncertainties), with slight preferences for the largest and smallest bins (the most unequal mass planets).…”

Section: Comparison To Observationssupporting

confidence: 84%

“…Thousands of extrasolar planets have been discovered over the past two decades. Although the observed collection of planetary systems exhibits a wide range of properties (Borucki et al 2010;Batalha et al 2011), a significant subset of the extrasolar multi-planet systems display an apparent but unexpected degree of regularity: The planets within these systems tend to have nearly equal radii (Weiss et al 2018a,b) and nearly equal masses (Millholland et al 2017;Wang 2017). Taken together, these findings jointly suggest similar mean densities (and perhaps similar chemical compositions).…”

Section: Introductionmentioning

confidence: 99%

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Energy optimization in extrasolar planetary systems: the transition from peas-in-a-pod to runaway growth

Adams

Batygin

Bloch

et al. 2020

Monthly Notices of the Royal Astronomical Society

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Motivated by the trends found in the observed sample of extrasolar planets, this paper determines tidal equilibrium states for forming planetary systemssubject to conservation of angular momentum, constant total mass, and fixed orbital spacing. In the low-mass limit, valid for superearth-class planets with masses of order m p ∼ 10M ⊕ , previous work showed that energy optimization leads to nearly equal mass planets, with circular orbits confined to a plane. The present treatment generalizes previous results by including the self-gravity of the planetary bodies. For systems with sufficiently large total mass m T in planets, the optimized energy state switches over from the case of nearly equal mass planets to a configuration where one planet contains most of the material. This transition occurs for a critical mass threshold of approximately m T > ∼ m C ∼ 40M ⊕ (where the value depends on the semimajor axes of the planetary orbits, the stellar mass, and other system properties). These considerations of energy optimization apply over a wide range of mass scales, from binary stars to planetary systems to the collection of moons orbiting the giant planets in our solar system.

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Section: Comparison To Observationssupporting

confidence: 84%

Section: Introductionmentioning

confidence: 99%

Energy optimization in extrasolar planetary systems: the transition from peas-in-a-pod to runaway growth

Adams

Batygin

Bloch

et al. 2020

Monthly Notices of the Royal Astronomical Society

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“…If the protoplanets of similar masses are forming at the same time, as long as the gas fraction does not vary by many orders of magnitude between the innermost and outermost planet, it is plausible that the planets acquire similar amounts of gas, which would explain why they grow to be the same size. Millholland et al (2017) find that planets in the same system tend to have similar masses, strengthening the evidence that similarly sized planets are remnants of planet formation.…”

Section: In Situ Formationsupporting

confidence: 64%

The California-Kepler Survey. V. Peas in a Pod: Planets in a Kepler Multi-planet System Are Similar in Size and Regularly Spaced^*

et al. 2018

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We have established precise planet radii, semimajor axes, incident stellar fluxes, and stellar masses for 909 planets in 355 multi-planet systems discovered by Kepler. In this sample, we find that planets within a single multi-planet system have correlated sizes: each planet is more likely to be the size of its neighbor than a size drawn at random from the distribution of observed planet sizes. In systems with three or more planets, the planets tend to have a regular spacing: the orbital period ratios of adjacent pairs of planets are correlated. Furthermore, the orbital period ratios are smaller in systems with smaller planets, suggesting that the patterns in planet sizes and spacing are linked through formation and/or subsequent orbital dynamics. Yet, we find that essentially no planets have orbital period ratios smaller than 1.2, regardless of planet size. Using empirical mass-radius relationships, we estimate the mutual Hill separations of planet pairs. We find that 93% of the planet pairs are at least 10 mutual Hill radii apart, and that a spacing of ∼20 mutual Hill radii is most common. We also find that when comparing planet sizes, the outer planet is larger in 65%±0.4% of cases, and the typical ratio of the outer to inner planet size is positively correlated with the temperature difference between the planets. This could be the result of photo-evaporation.

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“…The agreement of the observed and simulated period ratios but disagreement of the Hill spacing implies that our tightly spaced simulated planets have lower masses than those assumed for the observed planets. In Section 4.2, we will show that our small period ratio simulated planets do have lower masses at a given radius and that there is tentative evidence that observed planets may exhibit the same trend (Weiss & Marcy 2014;Millholland et al 2017). Therefore the disagreement may lie in the assumed mass-radius relationship for observed planets ( Fig.…”

Section: A Diversity Of Orbits From a Continuum Of Formation Conditionsmentioning

confidence: 79%

“…the smallest period ratios for adjacent planets), and we will explore whether this failure is a fundamental limitation of in situ formation or can be addressed with a more flexible range of initial conditions. Finally, we will compare our simulations to more recent observed trends of super-Earth diversity: intrinsic scatter in super-Earths' mass-radius relation (Wolfgang et al 2016) and similarity in size among planets in the same system (Millholland et al 2017;Weiss et al 2018).…”

Section: Introductionmentioning

confidence: 94%

Forming Diverse Super-Earth Systems In Situ

MacDonald¹,

Dawson²,

Morrison

et al. 2020

ApJ

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Super-Earths and mini-Neptunes exhibit great diversity in their compositional and orbital properties. Their bulk densities span a large range, from those dense enough to be purely rocky to those needing a substantial contribution from volatiles to their volumes. Their orbital configurations range from compact, circular multi-transiting systems like Kepler-11 to systems like our Solar System's terrestrial planets with wider spacings and modest but significant eccentricities and mutual inclinations. Here we investigate whether a continuum of formation conditions resulting from variation in the amount of solids available in the inner disk can account for the diversity of orbital and compositional properties observed for super Earths, including the apparent dichotomy between single transiting and multiple transiting system. We simulate in situ formation of super-Earths via giant impacts and compare to the observed Kepler sample. We find that intrinsic variations among disks in the amount of solids available for in situ formation can account for the orbital and compositional diversity observed among Kepler's transiting planets. Our simulations can account for the planets' distributions of orbital period ratios, transit duration ratios, and transit multiplicity; higher eccentricities for single than multi transiting planets; smaller eccentricities for larger planets; scatter in the mass-radius relation, including lower densities for planets with masses measured with TTVs than RVs; and similarity in planets' sizes and spacings within each system. Our findings support the theory that variation among super-Earth and mini-Neptune properties is primarily locked in by different in situ formation conditions, rather than arising stochastically through subsequent evolution.

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Kepler Multi-planet Systems Exhibit Unexpected Intra-system Uniformity in Mass and Radius

Cited by 209 publications

References 32 publications

Energy optimization in extrasolar planetary systems: the transition from peas-in-a-pod to runaway growth

Energy optimization in extrasolar planetary systems: the transition from peas-in-a-pod to runaway growth

The California-Kepler Survey. V. Peas in a Pod: Planets in a Kepler Multi-planet System Are Similar in Size and Regularly Spaced^*

Forming Diverse Super-Earth Systems In Situ

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Kepler Multi-planet Systems Exhibit Unexpected Intra-system Uniformity in Mass and Radius

Cited by 209 publications

References 32 publications

Energy optimization in extrasolar planetary systems: the transition from peas-in-a-pod to runaway growth

Energy optimization in extrasolar planetary systems: the transition from peas-in-a-pod to runaway growth

The California-Kepler Survey. V. Peas in a Pod: Planets in a Kepler Multi-planet System Are Similar in Size and Regularly Spaced*

Forming Diverse Super-Earth Systems In Situ

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Product

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The California-Kepler Survey. V. Peas in a Pod: Planets in a Kepler Multi-planet System Are Similar in Size and Regularly Spaced^*