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Register a new userDynamical instabilities in systems of multiple short-period planets are likely driven by secular chaos: a case study of Kepler-102
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We investigated the dynamical stability of high-multiplicity Kepler and K2 planetary systems. Our numerical simulations find instabilities in $sim20%$ of the cases on a wide range of timescales (up to $5times10^9$ orbits) and over an unexpectedly wide range of initial dynamical spacings. To identify the triggers of long-term instability in multi-planet systems, we investigated in detail the five-planet Kepler-102 system. Despite having several near-resonant period ratios, we find that mean motion resonances are unlikely to directly cause instability for plausible planet masses in this system. Instead, we find strong evidence that slow inward transfer of angular momentum deficit (AMD) via secular chaos excites the eccentricity of the innermost planet, Kepler-102 b, eventually leading to planet-planet collisions in $sim80%$ of Kepler-102 simulations. Kepler-102 b likely has a mass $>sim0.1M_{oplus}$, hence a bulk density exceeding about half Earths, in order to avoid dynamical instability. To investigate the role of secular chaos in our wider set of simulations, we characterize each planetary systems AMD evolution with a spectral fraction calculated from the power spectrum of short integrations ($sim5times10^6$ orbits). We find that small spectral fractions ($lesssim0.01$) are strongly associated with dynamical stability on long timescales ($5times10^9$ orbits) and that the median time to instability decreases with increasing spectral fraction. Our results support the hypothesis that secular chaos is the driver of instabilities in many non-resonant multi-planet systems, and also demonstrate that the spectral analysis method is an efficient numerical tool to diagnose long term (in)stability of multi-planet systems from short simulations.
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We analyze data from the Quarter 1-17 Data Release 24 (Q1--Q17 DR24) planet candidate catalog from NASAs Kepler mission, specifically comparing systems with single transiting planets to systems with multiple transiting planets, and identify a distinct population of exoplanets with a necessarily distinct system architecture. Such an architecture likely indicates a different branch in their evolutionary past relative to the typical Kepler system. The key feature of these planetary systems is an isolated, Earth-sized planet with a roughly one-day orbital period. We estimate that at least 24 of the 144 systems we examined (>~17%) are members of this population. Accounting for detection efficiency, such planetary systems occur with a frequency similar to the hot Jupiters.
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We present a study of the relative sizes of planets within the multiple candidate systems discovered with the $Kepler$ mission. We have compared the size of each planet to the size of every other planet within a given planetary system after correcting the sample for detection and geometric biases. We find that for planet-pairs for which one or both objects is approximately Neptune-sized or larger, the larger planet is most often the planet with the longer period. No such size--location correlation is seen for pairs of planets when both planets are smaller than Neptune. Specifically, if at least one planet in a planet-pair has a radius of $gtrsim 3R_oplus$, $68pm 6%$ of the planet pairs have the inner planet smaller than the outer planet, while no preferred sequential ordering of the planets is observed if both planets in a pair are smaller than $lesssim3 R_oplus$.
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