Spitzer Reveals Evidence of Molecular Absorption in the Atmosphere of the Hot Neptune LTT 9979b

Abstract

Non-rocky sub-jovian exoplanets in high irradiation environments are rare. LTT 9979b, also known as TESS Object of Interest (TOI) 193.01, is one of the few such planets discovered to date, and the first example of an ultra-hot Neptune. The planet's bulk density indicates that it has a substantial atmosphere, so to investigate its atmospheric composition and shed further light on its origin, we obtained {\it Spitzer} IRAC secondary eclipse observations of LTT 9979b at 3.6 and 4.5 $\mu$m. We combined the {\it Spitzer} observations with a measurement of the secondary eclipse in the {\it TESS} bandpass. The resulting secondary eclipse spectrum strongly prefers a model that includes CO absorption over a blackbody spectrum, incidentally making LTT 9979b the first {\it TESS} exoplanet (and the first ultra-hot Neptune) with evidence of a spectral feature in its atmosphere. We did not find evidence of a thermal inversion, at odds with expectations based on the atmospheres of similarly-irradiated hot Jupiters. We also report a nominal dayside brightness temperature of 2305 $\pm$ 141 K (based on the 3.6 $\mu$m secondary eclipse measurement), and we constrained the planet's orbital eccentricity to $e<0.01$ at the 99.7 \% confidence level. Together with our analysis of LTT 9979b's thermal phase curves reported in a companion paper, our results set the stage for similar investigations of a larger sample of exoplanets discovered in the hot Neptune desert, investigations which are key to uncovering the origin of this population.

arXiv comment12 pages, 5 figures; accepted to ApJ Letters

References in corpus (55)

• SciPy 1.0--Fundamental Algorithms for Scientific Computing in Python
• emcee: The MCMC Hammer
• The NumPy array: a structure for efficient numerical computation
• The Infrared Array Camera (IRAC) for the Spitzer Space Telescope
• MultiNest: an efficient and robust Bayesian inference tool for cosmology and particle physics
• The Spitzer Space Telescope Mission
• Analytic Lightcurves for Planetary Transit Searches
• dynesty: A Dynamic Nested Sampling Package for Estimating Bayesian Posteriors and Evidences
• X-ray spectral modelling of the AGN obscuring region in the CDFS: Bayesian model selection and catalogue
• batman: BAsic Transit Model cAlculatioN in Python
• Kepler Presearch Data Conditioning II - A Bayesian Approach to Systematic Error Correction
• A Unified Theory for the Atmospheres of the Hot and Very Hot Jupiters: Two Classes of Irradiated Atmospheres
• The Occurrence and Mass Distribution of Close-in Super-Earths, Neptunes, and Jupiters
• petitRADTRANS: a Python radiative transfer package for exoplanet characterization and retrieval
• The effect of red noise on planetary transit detection
• On the radiative equilibrium of irradiated planetary atmospheres
• The California Kepler Survey VII. Precise Planet Radii Leveraging Gaia DR2 Reveal the Stellar Mass Dependence of the Planet Radius Gap
• A Possible Bifurcation in Atmospheres of Strongly Irradiated Stars and Planets
• 3D mixing in hot Jupiter atmospheres. I. application to the day/night cold trap in HD 209458b
• Dearth of short-period Neptunian exoplanets - a desert in period-mass and period-radius planes
• Prospects for the Characterization and Confirmation of Transiting Exoplanets via the Rossiter-McLaughlin Effect
• Transitions in the cloud composition of hot Jupiters
• H- Opacity and Water Dissociation in the Dayside Atmosphere of the Very Hot Gas Giant WASP-18 b
• A map of the large day-night temperature gradient of a super-Earth exoplanet
• Compositional diversity in the atmospheres of hot Neptunes, with application to GJ 436b
• Spitzer Secondary Eclipses of the Dense, Modestly-irradiated, Giant Exoplanet HAT-P-20b Using Pixel-Level Decorrelation
• An ultrahot gas-giant exoplanet with a stratosphere
• Extremely Irradiated Hot Jupiters: Non-Oxide Inversions, H- Opacity, and Thermal Dissociation of Molecules
• Photoevaporation and High-Eccentricity Migration Created the Sub-Jovian Desert
• Model atmospheres of irradiated exoplanets: The influence of stellar parameters, metallicity, and the C/O ratio
• A short-period censor of sub-Jupiter mass exoplanets with low density
• Global Climate and Atmospheric Composition of the Ultra-Hot Jupiter WASP-103b from HST and Spitzer Phase Curve Observations
• Detection of a transit of the super-Earth 55 Cnc e with Warm Spitzer
• A consistent retrieval analysis of 10 Hot Jupiters observed in transmission
• Long Term Evolution of Close Planets Including the Effects of Secular Interactions
• Transit and Eclipse Analyses of Exoplanet HD 149026b Using BLISS Mapping
• ELLC - a fast, flexible light curve model for detached eclipsing binary stars and transiting exoplanets
• Detection of Thermal Emission from a Super-Earth
• Spitzer Observations Confirm and Rescue the Habitable-Zone Super-Earth K2-18b for Future Characterization
• Studying the atmosphere of the exoplanet HAT-P-7b via secondary eclipse measurements with EPOXI, Spitzer and Kepler
• TESS full orbital phase curve of the WASP-18b system
• Evidence for a Dayside Thermal Inversion and High Metallicity for the Hot Jupiter WASP-18b
• Statistical Characterization of Hot Jupiter Atmospheres using Spitzer's Secondary Eclipses
• New Avenues for Thermal Inversions in hot Jupiters
• Warm Spitzer Photometry of the Transiting Exoplanets CoRoT-1 and CoRoT-2 at Secondary Eclipse
• An emission spectrum for WASP-121b measured across the 0.8-1.1 micron wavelength range using the Hubble Space Telescope
• An Ultra-Hot Neptune in the Neptune desert
• Tidally-driven Roche-Lobe Overflow of Hot Jupiters with MESA
• Carbon Dioxide in Exoplanetary Atmospheres: Rarely Dominant Compared to Carbon Monoxide and Water in Hot, Hydrogen-dominated Atmospheres
• A transition between the hot and the ultra-hot Jupiter atmospheres
• WASP-127b transmission spectrum]{Abundance measurements of H${}_2$O and carbon-bearing species in the atmosphere of WASP-127b confirm its super-solar metallicity
• $TESS$ Phase Curve of the Hot Jupiter WASP-19b
• Phase Curves of Hot Neptune LTT 9779b Suggest a High-Metallicity Atmosphere
• Investigating Trends in Atmospheric Compositions of Cool Gas Giant Planets Using Spitzer Secondary Eclipses
• Characterization of Exoplanets: Secondary Eclipses

Cited by in corpus (14)

• Awesome SOSS: Transmission Spectroscopy of WASP-96b with NIRISS/SOSS
• TKS X: Confirmation of TOI-1444b and a Comparative Analysis of the Ultra-short-period Planets with Hot Neptunes
• Phase Curves of Hot Neptune LTT 9779b Suggest a High-Metallicity Atmosphere
• The Hot Neptune WASP-166 b with ESPRESSO II: Confirmation of atmospheric sodium
• Sub-Jovian desert of exoplanets at its boundaries. Parameter dependence along the main sequence
• A dense mini-Neptune orbiting the bright young star HD 18599
• A closer look at LTT 9779b: The ESPRESSO endeavour to pierce the atmospheric veil
• A High Geometric Albedo for LTT9779b Points Towards a Metal-rich Atmosphere and Silicate Clouds
• HST Transmission Spectra of the Hot-Neptune HD 219666 b: Detection of Water and the Challenge of Constraining Both Water and Methane
• Converting the sub-Jovian desert of exoplanets to a savanna with TESS, PLATO and Ariel
• No TiO detected in the hot Neptune-desert planet LTT-9779 b in reflected light at high spectral resolution
• Atmospheric characterization of hot Jupiters using hierarchical models of Spitzer observations
• HST/WFC3 Constraints on the Abundances of OH and FeH in the Atmosphere of the Ultra-Hot Neptune LTT-9779 b
• TOI-333b: A Neptune Desert planet around a F7V star