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Super-puffs are low-density planets of unknown origin and composition. If they form by accreting nebular gas through a circumplanetary disk, one might expect super-puffs to be spinning quickly. Here, we derive upper limits on the rotational oblateness of the super-puff Kepler-51d, based on precise transit observations with the NIRSpec instrument aboard the James Webb Space Telescope. The absence of detectable oblateness-related anomalies in the light curve leads to an upper limit of about $0.15$ on the planet's sky-projected oblateness. Assuming the sky-projected oblateness to be representative of the true oblateness, the rotation period of Kepler-51d is $\gtrsim 40$ hours, or equivalently, its rotation speed is $\lesssim 42\%$ of the breakup speed. Alternatively, if the apparently low density of Kepler-51d is due to an opaque planetary ring, the ring must be oriented within $27\deg$ of face-on and have an inner radius smaller than $1.2$ times the planet's radius. The lack of anomalies exceeding $0.01\%$ in the ingress and egress portions of the light curve also places a constraint on the model of Wang & Dai, in which the planet's apparently low density is due to a dusty outflowing atmosphere., Comment: 10 pages, 4 figures, 1 table. Submitted to ApJL

Constraining planet formation models based on the observed exoplanet population requires generating large samples of synthetic planetary systems, which can be computationally prohibitive. A significant bottleneck is simulating the giant impact phase, during which planetary embryos evolve gravitationally and combine to form planets, which may themselves experience later collisions. To accelerate giant impact simulations, we present a machine learning (ML) approach to predicting collisional outcomes in multiplanet systems. Trained on more than 500,000 $N$-body simulations of three-planet systems, we develop an ML model that can accurately predict which two planets will experience a collision, along with the state of the post-collision planets, from a short integration of the system's initial conditions. Our model greatly improves on non-ML baselines that rely on metrics from dynamics theory, which struggle to accurately predict which pair of planets will experience a collision. By combining with a model for predicting long-term stability, we create an ML-based giant impact emulator, which can predict the outcomes of giant impact simulations with reasonable accuracy and a speedup of up to four orders of magnitude. We expect our model to enable analyses that would not otherwise be computationally feasible. As such, we release our training code, along with an easy-to-use API for our collision outcome model and giant impact emulator., Comment: 15 pages, 7 figures, 1 table. Accepted for publication in ApJ. Easy-to-use API available at https://github.com/dtamayo/spock

HD 110067 is the brightest star known to have six transiting planets. Each adjacent pair of planets has a period ratio that is nearly equal to a ratio of small integers, suggesting the planets are in a chain of mean-motion resonances, but the limited time span of the available data has prevented firm conclusions. Here, we show that the requirement of long-term dynamical stability implies that all six planets are very likely to form a resonant chain. Dynamical simulations of nonresonant systems with initial conditions compatible with the available data almost always suffer an instability within $25$ Myr ($\sim 0.3 \%$ of the system's age). Assuming the system is in resonance, we place upper limits on the planets' eccentricities and lower limits on the masses of the planets that have not yet been measured. We also predict the characteristics of transit timing variations and the values of the three-body libration centers., Comment: 8 pages, 5 figures, 2 tables. Accepted for publication in ApJL

To improve our understanding of orbital instabilities in compact planetary systems, we compare suites of $N$-body simulations against numerical integrations of simplified dynamical models. We show that, surprisingly, dynamical models that account for small sets of resonant interactions between the planets can accurately recover $N$-body instability times. This points toward a simple physical picture in which a handful of three-body resonances, generated by interactions between nearby two-body mean motion resonances, overlap and drive chaotic diffusion, leading to instability. Motivated by this, we show that instability times are well described by a power law relating instability time to planet separations, measured in units of fractional semi-major axis difference divided by the planet-to-star mass ratio to the $1/4$ power, rather than the frequently adopted $1/3$ power implied by measuring separations in units of mutual Hill radii. For idealized systems, the parameters of this power-law relationship depend only on the ratio of the planets' orbital eccentricities to the orbit-crossing value, and we report an empirical fit to enable quick instability time predictions. This relationship predicts that observed systems comprised of three or more sub-Neptune-mass planets must be spaced with period ratios $P \gtrsim 1.35$ and that tightly spaced systems ($P \lesssim 1.5$) must possess very low eccentricities ($e \lesssim 0.05$) to be stable for more than $10^9$ orbits., Comment: 20 pages, 13 figures. Accepted for publication in ApJ

There is evidence that exoplanet systems display intra-system uniformity in mass, radius, and orbital spacing (like ``peas in a pod'') when compared with the system-to-system variations of planetary systems. This has been interpreted as the outcome of the early stages of planet formation, indicative of a picture in which planets form at characteristic mass scales with uniform separations. In this paper, we argue instead that intra-system uniformity in planet sizes and orbital spacings likely arose from the dynamical sculpting of initially-overly-packed planetary systems (in other words, the giant impact phase). With a suite of $N$-body simulations, we demonstrate that systems with random initial masses and compact planet spacings naturally develop intra-system uniformity, in quantitative agreement with observations, due to collisions between planets. Our results suggest that the pre-giant impact planet mass distribution is fairly wide and provide evidence for the prevalence of dynamical sculpting in shaping the observed population of exoplanets., Comment: 6 pages, 5 figures. Accepted for publication in MNRAS Letters

Despite the importance of feedback from active galactic nuclei (AGNs) in models of galaxy evolution, observational constraints on the influence of AGN feedback on star formation remain weak. To this end, we have compared the star formation trends of 279 low-redshift AGN galaxies with 558 inactive control galaxies using integral field unit spectroscopy from the SDSS-IV MaNGA survey. With a Gaussian process-based methodology, we reconstruct nonparametric star formation histories in spatially resolved spaxels covering the face of each galaxy. Based on galaxy-wide star formation rates (SFRs) alone, we find no obvious signatures of AGN feedback. However, the AGN galaxies have significantly suppressed central (kiloparsec-scale) SFRs, lying up to a factor of $2$ below those of the control galaxies, providing direct observational evidence of AGN feedback suppressing star formation. The suppression of central SFRs in the AGN galaxies began in the central regions ${\sim} 6$ Gyr ago (redshift $z {\sim} 0.7$), taking place over a few gigayears. A small subset of the AGN galaxies were rapidly driven to quiescence shortly before being observed (in the last $500$ Myr), potentially indicating instances of AGN-driven feedback. More frequently, however, star formation continues in the AGN galaxies, with suppression primarily in the central regions. This is suggestive of a picture in which integrated (Gyr-timescale) AGN feedback can significantly affect central star formation, but may be inefficient in driving galaxy-wide quenching in low-redshift galaxies, instead leaving them in the green valley., Comment: 22 pages, 15 figures. Accepted for publication in ApJ

The Planck list of high-redshift source candidates (the PHz catalogue) contains 2151 peaks in the cosmic infrared background, unresolved by Planck's 5 arcmin beam. Follow-up spectroscopic observations have revealed that some of these objects are $z \approx 2$ protoclusters and strong gravitational lenses, but an unbiased survey has not yet been carried out. To this end, we have used archival Herschel-SPIRE observations to study a uniformly-selected sample of 187 PHz sources. In contrast with follow-up studies that were biased towards bright, compact sources, we find that only one of our PHz sources is a bright gravitationally-lensed galaxy (peak flux $\gtrsim 300$ mJy), indicating that such objects are rarer in the PHz catalogue than previously believed ($< 1$ per cent). The majority of our PHz sources consist of many red, star-forming galaxies, demonstrating that typical PHz sources are candidate protoclusters. However, our new PHz sources are significantly less bright than found in previous studies and differ in colour, suggesting possible differences in redshift and star-formation rate. Nonetheless, 40 of our PHz sources contain $> 3\sigma$ galaxy overdensities, comparable to the fraction of $> 3\sigma$ overdensities found in earlier biased studies. We additionally use a machine-learning approach to identify less extreme (peak flux $\sim 100$ mJy) gravitationally-lensed galaxies among Herschel-SPIRE observations of PHz sources, finding a total of seven candidates in our unbiased sample, and 13 amongst previous biased samples. Our new uniformly-selected catalogues of $> 3\sigma$ candidate protoclusters and strong gravitational lenses provide interesting targets for follow up with higher-resolution facilities, such as ALMA and JWST., Comment: 23 pages, 19 figures, 4 tables. Accepted for publication in MNRAS