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标题：K2-19b and c are in a 3:2 Commensurability but out of Resonance: A Challenge to Planet Assembly by Convergent Migration
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作者：Erik A.Petigura ; John Livingston ; Konstantin Batygin 等
期刊名称：The Astronomical journal
印刷版ISSN：0004-6256
电子版ISSN：1538-3881
出版年度：2020
卷号：159
期号：1
页码：15-29
DOI：10.3847/1538-3881/ab5220
语种：English
出版社：American Institute of Physics
摘要：K2-19b and c were among the first planets discovered by NASA's K2 mission and together stand in stark contrast with the physical and orbital properties of the solar system planets.The planets are between the size of Uranus and Saturn at 7.0 ± 0.2 ${R}_{\oplus }$ and 4.1 ± 0.2 ${R}_{\oplus }$, respectively, and reside a mere 0.1% outside the nominal 3:2 mean-motion resonance.They represent a different outcome of the planet formation process than the solar system, as well as the vast majority of known exoplanets.We measured the physical and orbital properties of these planets using photometry from K2, Spitzer, and ground-based telescopes, along with radial velocities from Keck/HIRES.Through a joint photodynamical model, we found that the planets have moderate eccentricities of e ≈ 0.20 and well-aligned apsides Δpiv ≈ 0°.The planets occupy a strictly nonresonant configuration: the resonant angles circulate rather than librate.This defies the predictions of standard formation pathways that invoke convergent or divergent migration, both of which predict Δpiv ≈ 180° and eccentricities of a few percent or less.We measured masses of Mp,b = 32.4 ± 1.7 ${M}_{\oplus }$ and Mp,c = 10.8 ± 0.6 ${M}_{\oplus }$.Our measurements, with 5% fractional uncertainties, are among the most precise of any sub-Jovian exoplanet.Mass and size reflect a planet's core/envelope structure.Despite having a relatively massive core of ${M}_{\mathrm{core}}$ ≈ 15 ${M}_{\oplus }$, K2-19b is envelope-rich, with an envelope mass fraction of roughly 50%.This planet poses a challenge to standard models of core-nucleated accretion, which predict that cores gsim10 ${M}_{\oplus }$ will quickly accrete gas and trigger runaway accretion when the envelope mass exceeds that of the core.
关键词：Exoplanet astronomy;Exoplanet dynamics;Exoplanet formation;Exoplanet evolution;Extrasolar gas giants;Exoplanet structure;Radial velocity