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Published November 1, 2020 | Accepted Version + Published
Journal Article Open

Spitzer Reveals Evidence of Molecular Absorption in the Atmosphere of the Hot Neptune LTT 9779b


Non-rocky sub-Jovian exoplanets in high-irradiation environments are rare. LTT 9779b, also known as Transiting Exoplanet Survey Satellite (TESS) object of interest (TOI) 193.01, is one of the few such planets discovered to date, and the first example of an ultrahot 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 Spitzer InfraRed Array Camera secondary eclipse observations of LTT 9779b at 3.6 and 4.5 μm. We combined the Spitzer observations with a measurement of the secondary eclipse in the TESS bandpass. The resulting secondary eclipse spectrum strongly prefers a model that includes CO absorption over a blackbody spectrum, incidentally making LTT 9779b the first TESS exoplanet (and the first ultrahot 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 ± 141 K (based on the 3.6 μm 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 9779b'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 that are key to uncovering the origin of this population.

Additional Information

© 2020 The American Astronomical Society. Received 2020 July 8; revised 2020 September 7; accepted 2020 September 21; published 2020 October 26. We are grateful to the referee for feedback that has improved the clarity of the Letter and has prompted us to perform additional tests to further verify our results. We thank James Owen for his thoughts on photoevaporation's impact on exoplanet atmospheres. D.D. acknowledges support from NASA through Caltech/JPL grant RSA-1006130 and through the TESS Guest Investigator Program Grant 80NSSC19K1727. I.J.M.C. acknowledges support from the NSF through grant AST-1824644, and from NASA through Caltech/JPL grant RSA-1610091. T.D. acknowledges support from MIT's Kavli Institute as a Kavli postdoctoral fellow. M.R.D acknowledges the support of CONICYT/PFCHA-Doctorado Nacional 21140646, Chile. J.N.W. thanks the Heising-Simons Foundation for support. C.D.D. acknowledges support from the Hellman Faculty Fund, the Alfred P. Sloan Foundation, and the David and Lucile Packard Foundation. J.S.J. acknowledges support through FONDECYT grant 1201371, and partial support from CONICYT project Basal AFB-170002. This work is based [in part] on observations made with the Spitzer Space Telescope, which was operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. It is thanks to Spitzer's unique mid-IR capabilities combined with the overlap between its final year of operations and most of TESS's primary mission, that we have been able to obtain and present in this Letter the first glimpse into a hot Neptune's atmosphere. Funding for the TESS mission is provided by NASA's Science Mission directorate. We acknowledge the use of public TESS Alert data from pipelines at the TESS Science Office and at the TESS Science Processing Operations Center. This research has made use of the Exoplanet Follow-up Observation Program website, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center for the production of the SPOC data products. Software: allesfitter (Günther & Daylan 2019), ellc (Maxted 2016), dynesty (Speagle 2020), emcee (Foreman-Mackey et al. 2013), batman (Kreidberg 2015), matplotlib (Hunter 2007), numpy (van der Walt et al. 2011), scipy (Virtanen et al. 2020). Facilities: Spitzer - Spitzer Space Telescope satellite, TESS. -

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Published - Dragomir_2020_ApJL_903_L6.pdf

Accepted Version - 2010.12744.pdf


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August 22, 2023
October 20, 2023