Four Sub-Saturns with Dissimilar Densities: Windows into Planetary Cores and Envelopes

Petigura, Erik A.; Sinukoff, Evan; Lopez, Eric; Crossfield, Ian J. M.; Howard, Andrew W.; Brewer, John M.; Fulton, Benjamin J.; Isaacson, Howard; Ciardi, David R.; Howell, Steve B.; Everett, Mark E.; Horch, Elliott; Hirsch, Lea A.; Weiss, Lauren M.; Schlieder, Joshua E.

doi:10.3847/1538-3881/aa5ea5

Cited by 107 publications

(145 citation statements)

References 86 publications

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“…Our new values are consistent with Petigura et al (2017), but with smaller formal uncertainties. This stems mainly from the improved stellar radius (see Table 3) and from the fact that, in the sub-Saturn size range, radius alone is a good proxy for envelope fraction (Lopez & Fortney 2014).…”

Section: Core/envelope Structuresupporting

confidence: 86%

Dynamics and Formation of the Near-resonant K2-24 System: Insights from Transit-timing Variations and Radial Velocities

Petigura

Benneke

Batygin

et al. 2018

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While planets between the size of Uranus and Saturn are absent within the solar system, the star K2-24 hosts two such planets, K2-24b and c, with radii equal to 5.4 Å R and 7.5 Å R , respectively. The two planets have orbital periods of 20.9days and 42.4days, residing only 1% outside the nominal 2:1 mean-motion resonance. In this work, we present results from a coordinated observing campaign to measure planet masses and eccentricities that combines radial velocity measurements from Keck/HIRES and transit-timing measurements from K2 and Spitzer. K2-24b and c have low, but nonzero, eccentricities of~ẽ e 0.08 1 2. The low observed eccentricities provide clues to the formation and dynamical evolution of K2-24b and K2-24c, suggesting that they could be the result of stochastic gravitational interactions with a turbulent protoplanetary disk, among other mechanisms. K2-24b and c are M , respectively; K2-24c is 20% less massive than K2-24b, despite being 40% larger. Their large sizes and low masses imply large envelope fractions, which we estimate at -+ 26 3 3 % and -+ 52 3 5 %. In particular, K2-24c's large envelope presents an intriguing challenge to the standard model of core-nucleated accretion that predicts the onset of runaway accretion when f env ≈50%.

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Section: Core/envelope Structuresupporting

confidence: 86%

Dynamics and Formation of the Near-resonant K2-24 System: Insights from Transit-timing Variations and Radial Velocities

Petigura

Benneke

Batygin

et al. 2018

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“…However, abrupt strengthening of the metallicity correlation above 4R Å and the roughly order of magnitude decrease in the frequency of sub-Saturns compared to subNeptunes suggests the formation pathways of sub-Saturns and sub-Neptunes may be quite different. Around 20 sub-Saturns have well-measured masses from either RVs or TTVs (see Petigura et al 2017b for a recent compilation). For sub-Saturns, we observe an order of magnitude scatter in the observed masses at a given size, indicating a diversity in core and envelope masses.…”

Section: Fractions Of M M 1%mentioning

confidence: 99%

“…While some sub-Saturns have ≈5M Å cores, similar to the super-Earths and sub-Neptunes, many have cores of ≈50M Å . In addition, the most massive sub-Saturns tend to be found around the most metal-rich hosts (Petigura et al 2017b).…”

Section: Fractions Of M M 1%mentioning

confidence: 99%

The California-Kepler Survey. IV. Metal-rich Stars Host a Greater Diversity of Planets

Petigura

Marcy

Winn

et al. 2018

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Probing the connection between a star's metallicity and the presence and properties of any associated planets offers an observational link between conditions during the epoch of planet formation and mature planetary systems. We explore this connection by analyzing the metallicities of Kepler target stars and the subset of stars found to host transiting planets. After correcting for survey incompleteness, we measure planet occurrence: the number of planets per 100 stars with a given metallicity M. Planet occurrence correlates with metallicity for some, but not all, planet sizes and orbital periods. For warm super-Earths having P=10-100days and R P =1.0-1.7R Å , planet occurrence is nearly constant over metallicities spanning −0.4 to +0.4dex. We find 20 warm super-Earths per 100 stars, regardless of metallicity. In contrast, the occurrence of warm sub-Neptunes (R P = 1.7-4.0 R Å ) doubles over that same metallicity interval, from 20 to 40 planets per 100 stars. We model the distribution of planets as, where β characterizes the strength of any metallicity correlation. This correlation steepens with decreasing orbital period and increasing planet size. + -+ . High metallicities in protoplanetary disks may increase the mass of the largest rocky cores or the speed at which they are assembled, enhancing the production of planets larger than 1.7R Å . The association between high metallicity and short-period planets may reflect disk density profiles that facilitate the inward migration of solids or higher rates of planet-planet scattering.

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“…The resulting grid of planet masses, radii, envelope fractions, insolation fluxes, and ages has been used to infer the compositions of a multitude of planets (e.g., Wolfgang & Lopez 2015). The studies most germane to our analysis of K2-55b are those of Petigura et al (2016Petigura et al ( , 2017, who employed the models of Lopez & Fortney (2014) to analyze a set of sub-Saturns. As defined by Petigura et al (2016Petigura et al ( , 2017 and a correspondingly large density range of 0.09-2.40 g cm −3 .…”

Section: The Composition Of K2-55bmentioning

confidence: 99%

“…The studies most germane to our analysis of K2-55b are those of Petigura et al (2016Petigura et al ( , 2017, who employed the models of Lopez & Fortney (2014) to analyze a set of sub-Saturns. As defined by Petigura et al (2016Petigura et al ( , 2017 and a correspondingly large density range of 0.09-2.40 g cm −3 . The observed masses and radii of the planets in their sample could be explained by envelope fractions of 7%-60% H/He by mass.…”

Section: The Composition Of K2-55bmentioning

confidence: 99%

Characterizing K2 Candidate Planetary Systems Orbiting Low-mass Stars. III. A High Mass and Low Envelope Fraction for the Warm Neptune K2-55b*

Dressing

Sinukoff

Fulton

et al. 2018

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K2-55b is a Neptune-sized planet orbiting a K7 dwarf with a radius of 107 K. Having characterized the host star using near-infrared spectra obtained at IRTF/SpeX, we observed a transit of K2-55b with Spitzer/Infrared Array Camera (IRAC) and confirmed the accuracy of the original K2 ephemeris for future follow-up transit observations. Performing a joint fit to the Spitzer/IRAC and K2 photometry, we found a planet radius of , indicating that K2-55b has a bulk density of -+ 2.8 0.6 0.8 g cm −3 and can be modeled as a rocky planet capped by a modest H/He envelope (M envelope = 12 ± 3% M p ). K2-55b is denser than most similarly sized planets, raising the question of whether the high planetary bulk density of K2-55b could be attributed to the high metallicity of K2-55. The absence of a substantial volatile envelope despite the high mass of K2-55b poses a challenge to current theories of gas giant formation. We posit that K2-55b may have escaped runaway accretion by migration, late formation, or inefficient core accretion, or that K2-55b was stripped of its envelope by a late giant impact.

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Four Sub-Saturns with Dissimilar Densities: Windows into Planetary Cores and Envelopes

Cited by 107 publications

References 86 publications

Dynamics and Formation of the Near-resonant K2-24 System: Insights from Transit-timing Variations and Radial Velocities

Dynamics and Formation of the Near-resonant K2-24 System: Insights from Transit-timing Variations and Radial Velocities

The California-Kepler Survey. IV. Metal-rich Stars Host a Greater Diversity of Planets

Characterizing K2 Candidate Planetary Systems Orbiting Low-mass Stars. III. A High Mass and Low Envelope Fraction for the Warm Neptune K2-55b*

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