Search

Aims. L 98-59 (TIC 307210830, TOI-175) is a nearby M3 dwarf around which TESS revealed three small transiting planets (0.80, 1.35, 1.57 Earth radii) in a compact configuration with orbital periods shorter than 7.5 days. Here we aim to measure the masses of the known transiting planets in this system using precise radial velocity (RV) measurements taken with the HARPS spectrograph. Methods. We considered both trained and untrained Gaussian process regression models of stellar activity, which are modeled simultaneously with the planetary signals. Our RV analysis was then supplemented with dynamical simulations to provide strong constraints on the planets' orbital eccentricities by requiring long-term stability. Results. We measure the planet masses of the two outermost planets to be 2.42 ± 0.35 and 2.31 ± 0.46 Earth masses, which confirms the bulk terrestrial composition of the former and eludes to a significant radius fraction in an extended gaseous envelope for the latter. We are able to place an upper limit on the mass of the smallest, innermost planet of <1.01 Earth masses with 95% confidence. Our RV plus dynamical stability analysis places strong constraints on the orbital eccentricities and reveals that each planet's orbit likely has e < 0.1. Conclusions. L 98-59 is likely a compact system of two rocky planets plus a third outer planet with a lower bulk density possibly indicative of the planet having retained a modest atmosphere. The system offers a unique laboratory for studies of planet formation, dynamical stability, and comparative atmospheric planetology as the two outer planets are attractive targets for atmospheric characterization through transmission spectroscopy. Continued RV monitoring will help refine the characterization of the innermost planet and potentially reveal additional planets in the system at wider separations. [ABSTRACT FROM AUTHOR]

Short-period super-Earths and Neptunes are now known to be very frequent around solar-type stars. Improving our understanding of these mysterious planets requires the detection of a significant sample of objects suitable for detailed characterization. Searching for the transits of the low-mass planets detected by Doppler surveys is a straightforward way to achieve this goal. Indeed, Doppler surveys target the most nearby main-sequence stars, they regularly detect close-in low-mass planets with significant transit probability, and their radial velocity data constrain strongly the ephemeris of possible transits. In this context, we initiated in 2010 an ambitious Spitzer multi-Cycle transit search project that targeted 25 low-mass planets detected by radial velocity, focusing mainly on the shortestperiod planets detected by the HARPS spectrograph. We report here null results for 19 targets of the project. For 16 planets out of 19, a transiting configuration is strongly disfavored or firmly rejected by our data for most planetary compositions. We derive a posterior probability of 83% that none of the probed 19 planets transits (for a prior probability of 22%), which still leaves a significant probability of 17% that at least one of them does transit. Globally, our Spitzer project revealed or confirmed transits for three of its 25 targeted planets, and discarded or disfavored the transiting nature of 20 of them. Our light curves demonstrate for Warm Spitzer excellent photometric precisions: for 14 targets out of 19, we were able to reach standard deviations that were better than 50 ppm per 30 min intervals. Combined with its Earth-trailing orbit, which makes it capable of pointing any star in the sky and to monitor it continuously for days, this work confirms Spitzer as an optimal instrument to detect sub-mmag-deep transits on the bright nearby stars targeted by Doppler surveys. [ABSTRACT FROM AUTHOR]

High-precision eclipse spectrophotometry of transiting terrestrial exoplanets represents a promising path for the first atmospheric characterizations of habitable worlds and the search for life outside our solar system. The detection of terrestrial planets transiting nearby late-type M-dwarfs could make this approach applicable within the next decade, with soon-to-come general facilities. In this context, we previously identified GJ 1214 as a high-priority target for a transit search, as the transit probability of a habitable planet orbiting this nearby M4.5 dwarf would be significantly enhanced by the transiting nature of GJ 1214 b, the super-Earth already known to orbit the star. Based on this observation, we have set up an ambitious high-precision photometric monitoring of GJ 1214 with the Spitzer Space Telescope to probe the inner part of its habitable zone in search of a transiting planet as small as Mars. We present here the results of this transit search. Unfortunately, we did not detect any other transiting planets. Assuming that GJ 1214 hosts a habitable planet larger than Mars that has an orbital period smaller than 20.9 days, our global analysis of the whole Spitzer dataset leads to an a posteriori no-transit probability of ~98%. Our analysis allows us to significantly improve the characterization of GJ 1214 b, to measure its occultation depth to be 70 ± 35 ppm at 4.5 μm, and to constrain it to be smaller than 205 ppm (3σ upper limit) at 3.6 μm. In agreement with the many transmission measurements published so far for GJ 1214 b, these emission measurements are consistent with both a metal-rich and a cloudy hydrogen-rich atmosphere. [ABSTRACT FROM AUTHOR]

Context. Mapping distant worlds is the next frontier for exoplanet infrared (IR) photometry studies. Ultimately, constraining spatial and temporal properties of an exoplanet atmosphere (e.g., its temperature) will provide further insight into its physics. For tidally-locked hot Jupiters that transit and are eclipsed by their host star, the first steps are now possible. Aims. Our aim is to constrain an exoplanet's (1) shape, (2) brightness distribution (BD) and (3) system parameters from its phase curve and eclipse measurements. In particular, we rely on the secondary-eclipse scanning which is obtained while an exoplanet is gradually masked by its host star. Methods. We use archived Spitzer/IRAC 8-μm data of HD189733 (six transits, eight secondary eclipses, and a phase curve) in a global Markov chain Monte Carlo (MCMC) procedure for mitigating systematics. We also include HD189733's out-of-transit radial velocity (RV) measurements to assess their incidence on the inferences obtained solely from the photometry. Results. We find a 6σ deviation from the expected occultation of a uniformly-bright disk. This deviation emerges mainly from a large-scale hot spot in HD 189733b's atmosphere, not from HD 189733b's shape. We indicate that the correlation of the exoplanet orbital eccentricity, e, and BD ("uniform time o set") does also depend on the stellar density, ⩽?, and the exoplanet impact parameter, b ("e-b-ρ⋆-BD correlation"). For HD 189733b, we find that relaxing the eccentricity constraint and using more complex BDs lead to lower stellar/planetary densities and a more localized and latitudinally-shifted hot spot. We, therefore, show that the light curve of an exoplanet does not constrain uniquely its brightness peak localization. Finally, we obtain an improved constraint on the upper limit of HD 189733b's orbital eccentricity, e ⩽ 0:011 (95% confidence), when including HD189733's RV measurements. Conclusions. Reanalysis of archived HD 189733's data constrains HD 189733b's shape and BD at 8 μm. Our study provides new insights into the analysis of exoplanet light curves and a proper framework for future eclipse-scanning observations. In particular, observations of the same exoplanet at di erent wavelengths could improve the constraints on HD 189733's system parameters while ultimately yielding a large-scale time-dependent 3D map of HD189733b's atmosphere. Finally, we discuss the perspective of extending our method to observations in the visible (e.g., Kepler data), in particular to better understand exoplanet albedos. [ABSTRACT FROM AUTHOR]

We report on new transit photometry for the super-Earth 55 Cnc e obtained with Warm Spitzer/IRAC at 4.5 μm. An individual analysis of these new data leads to a planet radius of 2.21+0.15 -0.16 R☉, which agrees well with the values previously derived from the MOST and Spitzer transit discovery data. A global analysis of both Spitzer transit time-series improves the precision on the radius of the planet at 4.5 μm to 2.20 ± 0.12 R☉. We also performed an independent analysis of the MOST data, paying particular attention to the influence of the systematic effects of instrumental origin on the derived parameters and errors by including them in a global model instead of performing a preliminary detrending-filtering processing. We deduce an optical planet radius of 2.04 ± 0.15 R☉ from this reanalysis of MOST data, which is consistent with the previous MOST result and with our Spitzer infrared radius. Assuming the achromaticity of the transit depth, we performed a global analysis combining Spitzer and MOST data that results in a planet radius of 2.17±0.10 R☉ (13 820 ± 620 km). These results point to 55 Cnc e having a gaseous envelope overlying a rocky nucleus, in agreement with previous works. A plausible composition for the envelope is water which would be in super-critical form given the equilibrium temperature of the planet. [ABSTRACT FROM AUTHOR]