Tentative Evidence for Water Vapor in the Atmosphere of the Neptune-sized Exoplanet HD 106315c

Creators: Kreidberg, Laura^{1, 2}; Mollière, Paul¹; Crossfield, Ian J. M.³; Thorngren, Daniel P.⁴; Kawashima, Yui⁵; Morley, Caroline V.⁶; Benneke, Björn⁴; Mikal-Evans, Thomas⁷; Berardo, David⁷; Kosiarek, Molly R.^{8, 9}; Gorjian, Varoujan¹⁰; Ciardi, David R.^{11, 12}; Christiansen, Jessie L.^{11, 12}; Dragomir, Diana¹³; Dressing, Courtney D.¹⁴; Fortney, Jonathan J.⁸; Fulton, Benjamin J.^{15, 12}; Greene, Thomas P.¹⁶; Hardegree-Ullman, Kevin K.^{11, 12}; Howard, Andrew W.¹²; Howell, Steve B.¹⁶; Isaacson, Howard^{14, 17}; Krick, Jessica E.^{11, 12}; Livingston, John H.¹⁸; Lothringer, Joshua D.¹⁹; Morales, Farisa Y.¹⁰; Petigura, Erik A.²⁰; Rodriguez, Joseph E.²; Schlieder, Joshua E.²¹; Weiss, Lauren M.²²

1. Max Planck Institute for Astronomy
2. Harvard-Smithsonian Center for Astrophysics
3. University of Kansas
4. University of Montreal
5. RIKEN
6. The University of Texas at Austin
7. Massachusetts Institute of Technology
8. University of California, Santa Cruz
9. NSF Graduate Research Fellow
10. Jet Propulsion Lab
11. Infrared Processing and Analysis Center
12. California Institute of Technology
13. University of New Mexico
14. University of California, Berkeley
15. NASA Exoplanet Science Institute
16. Ames Research Center
17. University of Southern Queensland
18. University of Tokyo
19. Johns Hopkins University
20. University of California, Los Angeles
21. Goddard Space Flight Center
22. University of Hawaii at Manoa

Abstract

We present a transmission spectrum for the Neptune-sized exoplanet HD 106315c from optical to infrared wavelengths based on transit observations from the Hubble Space Telescope/Wide Field Camera 3, K2, and Spitzer. The spectrum shows tentative evidence for a water absorption feature in the 1.1–1.7 μm wavelength range with a small amplitude of 30 ppm (corresponding to just 0.8 ± 0.04 atmospheric scale heights). Based on an atmospheric retrieval analysis, the presence of water vapor is tentatively favored with a Bayes factor of 1.7–2.6 (depending on prior assumptions). The spectrum is most consistent with either an enhanced metallicity or high-altitude condensates, or both. Cloud-free solar composition atmospheres are ruled out at >5σ confidence. We compare the spectrum to grids of cloudy and hazy forward models and find that the spectrum is fit well by models with moderate cloud lofting or haze formation efficiency over a wide range of metallicities (1–100× solar). We combine the constraints on the envelope composition with an interior structure model and estimate that the core mass fraction is ≳0.3. With a bulk composition reminiscent of that of Neptune and an orbital distance of 0.15 au, HD 106315c hints that planets may form out of broadly similar material and arrive at vastly different orbits later in their evolution.

Copyright and License

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

Support for HST program GO-15333 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. 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. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. Some of the data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST). Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NNX13AC07G and by other grants and contracts. This research made use of matplotlib, a Python library for the publication of quality graphics (Hunter 2007). This research made use of SciPy (Virtanen et al. 2020). This research made use of NumPy (van der Walt et al. 2011). P.M. acknowledges support from the European Research Council under the European Union's Horizon 2020 research and innovation program under grant agreement No. 832428. Y.K. acknowledges support from the European Unions Horizon 2020 Research and Innovation Programme under Grant Agreement 776403. M.R.K. is supported by the NSF Graduate Research Fellowship, grant No. DGE 1339067. L.K. acknowledges M.R. Line for illuminating discussions.

Software References

petitRADTRANS (Mollière et al. 2019), PyMultiNest (Buchner et al. 2014), batman (Kreidberg 2015), dynesty (Speagle 2020).

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