As part of our ongoing NTT SoFI survey for variability in young free-floating planets and low mass brown dwarfs, we detect significant variability in the young, free-floating planetary mass object PSO J318.5-22, likely due to rotational modulation of inhomogeneous cloud cover. A member of the 23±3 Myr β Pic moving group, PSO J318.5-22 has T eff = 1160 +30 −40 K and a mass estimate of 8.3±0.5 M Jup for a 23±3 Myr age. PSO J318.5-22 is intermediate in mass between 51 Eri b and β Pic b, the two known exoplanet companions in the β Pic moving group. With variability amplitudes from 7-10% in J S at two separate epochs over 3-5 hour observations, we constrain the rotational period of this object to >5 hours. In K S , we marginally detect a variability trend of up to 3% over a 3 hour observation. This is the first detection of weather on an extrasolar planetary mass object. Among L dwarfs surveyed at high-photometric precision (<3%) this is the highest amplitude variability detection. Given the low surface gravity of this object, the high amplitude preliminarily suggests that such objects may be more variable than their high mass counterparts, although observations of a larger sample is necessary to confirm this. Measuring similar variability for directly imaged planetary companions is possible with instruments such as SPHERE and GPI and will provide important constraints on formation.Measuring variability at multiple wavelengths can help constrain cloud structure.1 Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 095.C-0590

A key hypothesis in the field of exoplanet atmospheres is the trend of atmospheric thermal structure with planetary equilibrium temperature. We explore this trend and report here the first statistical detection of a transition in the near-infrared atmospheric emission between hot and ultra-hot Jupiters. We measure this transition using secondary eclipse observations and interpret this phenomenon as changes in atmospheric properties, and more specifically in terms of transition from non-inverted to inverted thermal profiles. We examine a sample of 78 hot Jupiters with secondary eclipse measurements at 3.6 and 4.5 μm measured with Spitzer Infrared Array Camera. We calculate the planetary brightness temperatures using PHOENIX models to correct for the stellar flux. We measure the deviation of the data from the blackbody, which we define as the difference between the observed 4.5 μm eclipse depth and that expected at this wavelength based on the brightness temperature measured at 3.6 μm. We study how the deviation between 3.6 and 4.5 μm changes with theoretical predictions with equilibrium temperature and incoming stellar irradiation. We reveal a clear transition in the observed emission spectra of the hot Jupiter population at 1660 ± 100 K in the zero albedo, full redistribution equilibrium temperature. We find the hotter exoplanets have even hotter daysides at 4.5 μm compared to 3.6 μm, which manifests as an exponential increase in the emitted power of the planets with stellar insolation. We propose that the measured transition is a result of seeing carbon monoxide in emission due to the formation of temperature inversions in the atmospheres of the hottest planets. These thermal inversions could be caused by the presence of atomic and molecular species with high opacities in the optical and/or the lack of cooling species. Our findings are in remarkable agreement with a new grid of 1D radiative and convective models varying metallicity, carbon to oxygen ratio (C/O), surface gravity, and stellar effective temperature. We find that the population of hot Jupiters statistically disfavors high C/O planets (C/O ≥ 0.85).

The Transiting Exoplanet Survey Satellite (TESS) recently observed 18 transits of the hot Jupiter WASP-4b. The sequence of transits occurred 81.6 ± 11.7 seconds earlier than had been predicted, based on data stretching back to 2007. This is unlikely to be the result of a clock error, because TESS observations of other hot Jupiters (WASP-6b, 18b, and 46b) are compatible with a constant period, ruling out an 81.6-second offset at the 6.4σ level. The 1.3-day orbital period of WASP-4b appears to be decreasing at a rate ofṖ = −12.6 ± 1.2 milliseconds per year. The apparent period change might be caused by tidal orbital decay or apsidal precession, although both interpretations have shortcomings. The gravitational influence of a third body is another possibility, though at present there is minimal evidence for such a body. Further observations are needed to confirm and understand the timing variation.

The emergent spectra of close-in, giant exoplanets ("hot Jupiters") are believed to be distinct from those of young gas giants and brown dwarfs with similar effective temperatures because these objects are primarily heated from above by their host stars rather than internally from the release of energy from their formation 1 . Theoretical models predict a continuum of dayside spectra for hot Jupiters as a function of irradiation level, with the coolest planets having absorption features in their spectra, intermediate-temperature planets having emission features due to thermal inversions, and the hottest planets having blackbody-like spectra due to molecular dissociation and continuum opacity from the H − ion 2-4 . Absorption and emission features have been detected in the spectra of a number of individual hot Jupiters 5,6 , and population-level trends have been observed in photometric measurements [7][8][9][10][11] . However, there has been no unified, population-level study of the thermal emission spectra of hot Jupiters such as has been done for brown dwarfs 12 and transmission spectra of hot Jupiters 13 . Here we show that hot Jupiter secondary eclipse spectra centered around a water absorption band at 1.4 µm follow a common trend in water feature strength with temperature. The observed trend is broadly consistent with the predictions of self-consistent one-dimensional models for how the thermal structures of solar composition planets vary with irradiation level. Nevertheless, the ensemble of planets exhibits significant scatter around the mean trend. The spread can be accounted for if the planets have modest variations in metallicity and/or elemental abundance ratios, which is expected from planet formation models 14-17 . 42 side temperatures in the HST/WFC3+G141 bandpass between 43 1450 − 3100 K and radii between 0.9 − 2.0 Jupiter radii. The 44 full set of 14 spectra are shown in Figure 1. 45 Baxter et al. (2020) 9 presented an analysis of changes in 46 the thermal emission spectra of a subset of planets observed 47 with HST. This study expands on that work by uniformly an-366 brown dwarf models 42 and analytic models 22 ) assuming cloud-367 free, radiative-convective-thermochemical equilibrium atmo-368 spheres. The models' assumption of chemical equilibrium 369 is likely a good approximation for the highly irradiated plan-370 ets that make up the majority of our observed population 43 . 371 A two stream source function technique 44 is employed to 372 solve for the planetary thermal fluxes at each atmospheric 373 level (under the hemispheric mean approximation). We mod-374 eled the stellar flux via a standard two stream approximation 375 (for both direct and diffuse fluxes, under the quadrature ap-376 proximation) assuming cosine incident angle of 0.5, utilizing 377 / the PHOENIX models for the stellar spectra 45 . A Newton-378 Raphson iteration 46 is used to determine the temperature at 379 each model layer which ensures zero net flux divergence. We 380 include absorption cross-sections from 0.1 -100 µm...