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.

12 pages, 5 figures; accepted to ApJ Letters

References in corpus (18)

• The NumPy array: a structure for efficient numerical computation
• 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
• Prospects for the Characterization and Confirmation of Transiting Exoplanets via the Rossiter-McLaughlin Effect
• 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
• A consistent retrieval analysis of 10 Hot Jupiters observed in transmission
• Long Term Evolution of Close Planets Including the Effects of Secular Interactions
• Spitzer Observations Confirm and Rescue the Habitable-Zone Super-Earth K2-18b for Future Characterization
• Evidence for a Dayside Thermal Inversion and High Metallicity for the Hot Jupiter WASP-18b
• New Avenues for Thermal Inversions in hot Jupiters
• 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
• A transition between the hot and the ultra-hot Jupiter atmospheres
• $TESS$ Phase Curve of the Hot Jupiter WASP-19b
• Phase Curves of Hot Neptune LTT 9779b Suggest a High-Metallicity Atmosphere

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• 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
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