Over a hundred rocky planets orbiting Sun-like stars in very short orbital periods ( 1 day) have been discovered by the Kepler mission. These planets, known as ultra-short-period (USP) planets, are unlikely to have formed locally, or have attained their current orbits when their birth protoplanetary disks were still present. Instead, they must have migrated in later in life. Here we propose that these planets reach their current orbits by high-eccentricity migration. In a scaled-down version of the dynamics that may have been experienced by their high mass analog, the hot Jupiters, these planets reach high eccentricities via chaotic secular interactions with their companion planets and then undergo orbital circularization due to dissipation of tides raised on the planet. This proposal is motivated by the following observations: planetary systems observed by Kepler often contain several super-Earths with non-negligible eccentricities and inclinations, and possibly extending beyond ∼ AU distances; while only a small fraction of USP planets have known transiting companions, and none closely spaced, we argue that most of them should have companions at periods of ∼ 10 − 50 days. The outer sibling planets, through secular chaos, can remove angular momentum from the inner most planet, originally at periods of ∼ 5 − 10 days. When the latter reaches an eccentricity higher than 0.8, it is tidally captured by the central star and becomes an USP planet. This scenario naturally explains the observation that most USP planets have significantly more distant transiting companions compared to their counterparts at slightly longer periods (1 − 3 days), a feature un-accounted for in other proposed scenarios. Our model also predicts that USP planets should have: (i) spin-orbit angles, and inclinations relative to outer planets, in the range of ∼ 10 • − 50 • ; (ii) several outer planetary companions extending to beyond ∼ 1 AU distances, both of which may be tested by TESS and its follow-up observations. Subject headings: planets and satellites: dynamical evolution and stability

The hundreds of multiple planetary systems discovered by the Kepler mission are typically observed to reside in close-in ( 0.5 AU), low-eccentricity, and low-inclination orbits. We run N-body experiments to study the effect that unstable outer ( 1 AU) giant planets, whose end orbital configurations resemble those in the Radial Velocity population, have on these close-in multiple super-Earth systems. Our experiments show that the giant planets greatly reduce the multiplicity of the inner super-Earths and the surviving population can have large eccentricities (e 0.3) and inclinations (i 20• ) at levels that anti-correlate with multiplicity. Consequently, this model predicts the existence of a population of dynamically hot single-transiting planets with typical eccentricities and inclinations of ∼ 0.1 − 0.5 and ∼ 10• − 40• . We show that these results can explain the following observations: (i) the recent eccentricity measurements of Kepler super-Earths from transit durations; (ii) the tentative observation that single-transiting systems have a wider distribution of stellar obliquity angles compared to the multiple-transiting systems; (iii) the architecture of some eccentric super-Earths discovered by Radial Velocity surveys such as HD 125612c. Future observations from TESS will reveal many more dynamically hot single transiting planets, for which follow up Radial Velocity studies will be able to test our models and see whether they have outer giant planets. Subject headings: planets and satellites: dynamical evolution and stability

Recent observations of the ultra-hot Jupiter WASP-76b have revealed a diversity of atmospheric species. Here we present new high-resolution transit spectroscopy of WASP-76b with GRACES at the Gemini North Observatory, serving as a baseline for the Large and Long Program “Exploring the Diversity of Exoplanet Atmospheres at High Spectral Resolution” (Exoplanets with Gemini Spectroscopy, or ExoGemS for short). With a broad spectral range of 400–1050 nm, these observations allow us to search for a suite of atomic species. We recover absorption features due to neutral sodium (Na i), and report a new detection of the ionized calcium (Ca ii) triplet at ∼850 nm in the atmosphere of WASP-76b, complementing a previous detection of the Ca ii H and K lines. The triplet has line depths of 0.295% ± 0.034% at ∼849.2 nm, 0.574% ± 0.041% at ∼854.2 nm, and 0.454% ± 0.024% at ∼866.2 nm, corresponding to effective radii close to (but within) the planet’s Roche radius. These measured line depths are significantly larger than those predicted by model LTE and NLTE spectra obtained on the basis of a pressure–temperature profile computed assuming radiative equilibrium. The discrepancy suggests that the layers probed by our observations are either significantly hotter than predicted by radiative equilibrium and/or in a hydrodynamic state. Our results shed light on the exotic atmosphere of this ultra-hot world, and will inform future analyses from the ExoGemS survey.