Surveys have revealed many multi-planet systems containing super-Earths and Neptunes in orbits of a few days to a few months. There is debate whether in situ assembly or inward migration is the dominant mechanism of the formation of such planetary systems. Simulations suggest that migration creates tightly packed systems with planets whose orbital periods may be expressed as ratios of small integers (resonances), often in a many-planet series (chain). In the hundreds of multi-planet systems of sub-Neptunes, more planet pairs are observed near resonances than would generally be expected, but no individual system has hitherto been identified that must have been formed by migration. Proximity to resonance enables the detection of planets perturbing each other. Here we report transit timing variations of the four planets in the Kepler-223 system, model these variations as resonant-angle librations, and compute the long-term stability of the resonant chain. The architecture of Kepler-223 is too finely tuned to have been formed by scattering, and our numerical simulations demonstrate that its properties are natural outcomes of the migration hypothesis. Similar systems could be destabilized by any of several mechanisms, contributing to the observed orbital-period distribution, where many planets are not in resonances. Planetesimal interactions in particular are thought to be responsible for establishing the current orbits of the four giant planets in the Solar System by disrupting a theoretical initial resonant chain similar to that observed in Kepler-223.

AU Microscopii (AU Mic) is the second closest pre-main-sequence star, at a distance of 9.79 parsecs and with an age of 22 million years 1 . AU Mic possesses a relatively rare 2 and spatially resolved 3 edge-on debris disk extending from about 35 to 210 astronomical units from the star 4 , and with clumps exhibiting non-Keplerian motion 5-7 . Detection of newly formed planets around such a star is challenged by the presence of spots, plage, flares and other manifestations of magnetic 'activity' on the star 8,9 . Here we report observations of a planet transiting AU Mic. The transiting planet, AU Mic b, has an orbital period of 8.46 days, an orbital distance of 0.07 astronomical units, a radius of 0.4 Jupiter radii, and a mass of less than 0.18 Jupiter masses at 3σ confidence. Our observations of a planet co-existing with a debris disk offer the opportunity to test the predictions of current models of planet formation and evolution.

We present the discovery of Kepler-453 b, a 6.2 R Å planet in a low-eccentricity, 240.5 day orbit about an eclipsing binary. The binary itself consists of a 0.94 and 0.195 M ☉ pair of stars with an orbital period of 27.32 days. The plane of the planetʼs orbit is rapidly precessing, and its inclination only becomes sufficiently aligned with the primary star in the latter portion of the Kepler data. Thus three transits are present in the second half of the light curve, but none of the three conjunctions that occurred during the first half of the light curve produced observable transits. The precession period is ∼103 years, and during that cycle, transits are visible only ∼8.9% of the time. This has the important implication that for every system like Kepler-453 that we detect, there are ∼11.5 circumbinary systems that exist but are not currently exhibiting transits. The planetʼs mass is too small to noticeably perturb the binary, and consequently its mass is not measurable with these data; however, our photodynamical model places a 1σ upper limit of M 16 Å . With a period 8.8 times that of the binary, the planet is well outside the dynamical instability zone. It does, however, lie within the habitable zone of the binary, making it the third of 10 Kepler circumbinary planets to do so.

Of the nine confirmed transiting circumbinary planet systems, only Kepler-47 is known to contain more than one planet. Kepler-47 b (the "inner planet") has an orbital period of 49.5 days and a radius of about 3 R ⊕ . Kepler-47 c (the "outer planet") has an orbital period of 303.2 days and a radius of about 4.7 R ⊕ . Here we report the discovery of a third planet, Kepler-47 d (the "middle planet"), which has an orbital period of 187.4 days and a radius of about 7 R ⊕ . The presence of the middle planet allows us to place much better constraints on the masses of all three planets, where the 1σ ranges are less than 26 M ⊕ , between 7 − 43 M ⊕ , and between 2 − 5 M ⊕ for the inner, middle, and outer planets, respectively. The middle and outer planets have low bulk densities,

Characterizing the dependence of the orbital architectures and formation environments on the eccentricity distribution of planets is vital for understanding planet formation. In this work, we perform statistical eccentricity studies of transiting exoplanets using transit durations measured via Kepler combined with precise and accurate stellar radii from the California-Kepler Survey and Gaia. Compared to previous works that characterized the eccentricity distribution from transit durations, our analysis benefits from both high precision stellar radii (∼3%) and a large sample of ∼1000 planets. We observe that that systems with only a single observed transiting planet have a higher mean eccentricity (ē ∼ 0.21) than systems with multiple transiting planets (ē ∼ 0.05), in agreement with previous studies. We confirm the preference for high and low eccentricity subpopulations among the singly transiting systems. Finally, we show suggestive new evidence that high e planets in the Kepler sample are preferentially found around high metallicity ([Fe/H] > 0) stars. We conclude by discussing the implications on planetary formation theories.

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%.

The vast majority of well studied giant-planet systems, including the solar system, are nearly coplanar, which implies dissipation within a primordial gas disk. However, intrinsic instability may lead to planet–planet scattering, which often produces non-coplanar, eccentric orbits. Planet scattering theories have been developed to explain observed high-eccentricity systems and also hot Jupiters; thus far their predictions for mutual inclination (I) have barely been tested. Here we characterize a highly mutually inclined ( °), moderately eccentric ( ) giant planet system: Kepler-108. This system consists of two approximately Saturn-mass planets with periods of approximately 49 and 190 days around a star with a wide (∼300 au) binary companion in an orbital configuration inconsistent with a purely disk migration origin.

The two most common techniques for measuring planetary masses-the radial velocity (RV) and the transit timing variations (TTVs) techniques-have been observed to yield systematically different masses for planets of similar radii. Following Steffen (2016), we consider the effects of the observational biases of the two methods as a possible cause for this difference. We find that at short orbital periods (P < 11 day), the two methods produce statistically similar results, whereas at long periods (P > 11 day) the RV masses are systematically higher than the TTV ones. We suggest that this is consistent with an RV detection-sensitivity bias for longer periods. On the other hand, we do find an apparently significant difference between the short and the long-period planets, obtained by both observing techniques-the mass-radius relationship parameterized as a power law has a steeper index at short periods than at long periods. We also point out another anticipated observational bias between the two techniques-multiple planet systems with derived RV masses have substantially larger period ratios than the systems with TTV mass derivation.