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Hunt for Planet Nine: Atmosphere, Spectra, Evolution, and Detectability

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Hunt for Planet Nine: Atmosphere, Spectra, Evolution, and Detectability Digital Collections @ Dordt Faculty Work Comprehensive List 6-2016 Hunt for Planet Nine: Atmosphere, Spectra, Evolution, and Detectability Jonathan J. Fortney University of California, Santa Cruz Mark S. Marley NASA Ames Research Center Gregory Laughlin University of California, Santa Cruz Nadine Nettelmann University of Rostock Caroline V. Morley University of California, Santa Cruz See next page for additional authors Follow this and additional works at: https://digitalcollections.dordt.edu/faculty_work Part of the Astrophysics and Astronomy Commons Recommended Citation Fortney, J. J., Marley, M. S., Laughlin, G., Nettelmann, N., Morley, C. V., Lupu, R. E., Visscher, C., Jeremic, P., Khadder, W. G., & Hargrave, M. (2016). Hunt for Planet Nine: Atmosphere, Spectra, Evolution, and Detectability. Astrophysical Journal Letters, 824 (2) https://doi.org/10.3847/2041-8205/824/2/L25 This Article is brought to you for free and open access by Digital Collections @ Dordt. It has been accepted for inclusion in Faculty Work Comprehensive List by an authorized administrator of Digital Collections @ Dordt. For more information, please contact [email protected]. Hunt for Planet Nine: Atmosphere, Spectra, Evolution, and Detectability Abstract We investigate the physical characteristics of the solar system's proposed Planet Nine using modeling tools with a heritage of studying Uranus and Neptune. For a range of plausible masses and interior structures, we find upper limits on the intrinsic , from ~35 to 50 K for masses of 5–20 M ⊕, and we also explore lower values. Possible planetary radii could readily span from 2.7 to 6 R ⊕, depending on the mass fraction of any H/He envelope. Given its cold atmospheric temperatures, the planet encounters significant methane condensation, which dramatically alters the atmosphere away from simple Neptune-like expectations. We find that the atmosphere is strongly depleted in molecular absorption at visible wavelengths, suggesting a Rayleigh scattering atmosphere with a high geometric albedo approaching 0.75. We highlight two diagnostics for the atmosphere's temperature structure: (1) the value of the methane mixing ratio above the methane cloud and (2) the wavelength at which cloud scattering can be seen, which yields the cloud-top pressure. Surface reflection may be seen if the atmosphere is thin. Due to collision-induced opacity of H2 in the infrared, the planet would be extremely blue instead of red in the shortest wavelength WISE colors if methane is depleted and would, in some cases, exist on the verge of detectability by WISE. For a range of models, thermal fluxes from ~3 to 5 μm are ~20 orders of magnitude larger than blackbody expectations. We report a search of the AllWISE Source Catalog for Planet Nine, but find no detection. Keywords Kuiper belt, general, planets and satellites, atmospheres, detection, physical evolution Disciplines Astrophysics and Astronomy Comments Copyright © Astrophysical Journal Letters Authors Jonathan J. Fortney, Mark S. Marley, Gregory Laughlin, Nadine Nettelmann, Caroline V. Morley, Roxana E. Lupu, Channon Visscher, Pavle Jeremic, Wade G. Khadder, and Mason Hargrave This article is available at Digital Collections @ Dordt: https://digitalcollections.dordt.edu/faculty_work/567 DRAFT VERSION JUNE 7, 2016 Preprint typeset using LATEX style emulateapj v. 5/2/11 THE HUNT FOR PLANET NINE: ATMOSPHERE, SPECTRA, EVOLUTION, AND DETECTABILITY JONATHAN J. FORTNEY1, MARK S. MARLEY2,GREGORY LAUGHLIN1,NADINE NETTELMANN3,CAROLINE V. MORLEY1,ROXANA E. LUPU2,4 ,CHANNON VISSCHER5,PAVLE JEREMIC1,WADE G. KHADDER1, MASON HARGRAVE1 Draft version June 7, 2016 ABSTRACT We investigate the physical characteristics of the Solar System’s proposed Planet Nine using modeling tools with a heritage in studying Uranus and Neptune. For a range of plausible masses and interior structures, we find upper limits on the intrinsic Teff, from ∼35-50 K for masses of 5-20 M⊕, and we also explore lower Teff values. Possible planetary radii could readily span from 2.7 to 6 R⊕ depending on the mass fraction of any H/He envelope. Given its cold atmospheric temperatures, the planet encounters significant methane condensation, which dramatically alters the atmosphere away from simple Neptune-like expectations. We find the atmosphere is strongly depleted in molecular absorption at visible wavelengths, suggesting a Rayleigh scattering atmosphere with a high geometric albedo approaching 0.75. We highlight two diagnostics for the atmosphere’s temperature structure, the first being the value of the methane mixing ratio above the methane cloud. The second is the wavelength at which cloud scattering can be seen, which yields the cloud-toppressure. Surface reflection may be seen if the atmosphere is thin. Due to collision-induced opacity of H2 in the infrared, the planet would be extremely blue (instead of red) in the shortest wavelength WISE colors if methane is depleted, and would, in some cases, exist on the vergeof detectability by WISE. For a range of models, thermal fluxes from ∼ 3 − 5 µm are ∼ 20 orders of magnitude larger than blackbody expectations. We report a search of the AllWISE Source Catalog for Planet Nine, but find no detection. 1. INTRODUCTION could serve as a proxy for the physical and atmospheric prop- Recently, Batygin & Brown (2016) have found that the or- erties of the as-yet largely unstudied worlds that are among bits of long period Kuiper Belt Objects cluster in their argu- the most common outcomes of the planetary formation pro- ments of perihelion and also in physical space in the solar sys- cess. tem. They suggest that dynamical perturbations arising from In this Letter we first investigate the cooling history of the Planet Nine, over a wide range of possible masses from 5– a relatively small giant planet of perhaps 10 M⊕, on an eccen- tric orbit with a semi-major axis distance of perhaps a ∼ 700 50 M⊕. Here we assume a planet that has at least some AU, can explain this clustering. H/He atmosphere, in analogy with Uranus and Neptune as The formation, evolution, and detectability of this well as the thousands of sub-Neptune exoplanets that suggest possible “Planet Nine” has been the target of intense a non-negligible H/He envelope (Lopez & Fortney 2014). We study (de la Fuente Marcos& de la Fuente Marcos 2016; draw connections between the cooling history of Planet Nine Malhotra et al. 2016; Li & Adams 2016; Linder & Mordasini and our incomplete understanding of the thermal evolution 2016; Cowan et al. 2016; Fienga et al. 2016; Ginzburg et al. of Uranus and Neptune. With fluxes from the planet’s in- 2016; Brown&Batygin 2016; Kenyon&Bromley terior and from the Sun then reasonably well-bounded, we 2016; Bromley & Kenyon 2016; Mustill et al. 2016; can begin to estimate the atmospheric temperature structure. Holman&Payne 2016). To date, studies that have in- We assess the condensation of methane, atmospheric opac- cluded discussion of the atmosphere, emission, or cooling ity sources, and the spectrum of the planet from optical to thermal infrared wavelengths, focusing on novel and at times history of the planet have focused on Planet Nine as a − blackbody. This is certainly a valid starting point, especially counter-intuitive behavior at very low temperature (∼ 30 40 since nothing is yet known about this possible object. Yet a K). Finally, we apply these findings to a revised assessment arXiv:1604.07424v2 [astro-ph.EP] 6 Jun 2016 straightforward, but more detailed look at the planet yieldsa of the detectability of Planet Nine and report on a new search better understanding of the planet’s atmosphere, evolution, of the WISE catalog. and spectra that cannot come out of simple analysis. For instance, the spectra of the solar system’s giant planets differ 2. THERMAL EVOLUTION strongly from blackbodies. 2.1. Method Planet Nine, if it exists, may have a mass between that of Earth and Uranus. Exoplanets in this mass range are ex- In order to estimate the possible radius and intrinsic lu- tremely common (Borucki et al. 2011). Should Planet Nine minosity of Planet Nine we have run Neptune-like thermal be detected, the characterization of the planet would either evolution models, where we varied several structure param- confirm or refute studies such as the one we present here, and eters around those appropriate for Neptune. These models are somewhat similar to those of Linder & Mordasini (2016), although we make different choices about particular compo- 1 Department of Astronomy & Astrophysics, University of California, Santa Cruz, [email protected] sitions to investigate. Here we assume planet masses be- 2 NASA Ames Research Center, Mountain View, CA, United States tween 5 and 50 M⊕ (MNep = 17.15 M⊕) and two composi- 3 University of Rostock tionally different layers consisting of a H/He envelope atop 4 Bay Area Environmental Research Institute a core that is either ice-rich (2:1 ice:rock) or ice-poor (1:4 5 Dordt College ice:rock). We varied the core mass fraction within 0.1– 2 Fortney et al. 0.3 for the most massive models, representing a sub-Saturn TABLE 1 mass gas giant, 0.30–0.84 for a 15M⊕ series of models, and 0.6–0.9 for the least massive models of 5–8 M⊕, represent- Planet Core H/He Env. I:R Planet Teff T1bar Tint ing a sub-Neptune. For reference, constraints from Neptune Mass (M⊕) Mass Mass Ratio Radius (R⊕) (K) (K) (K) suggest ∼ 0.76–0.90 ice/rock by mass (Helled et al. 2011; 5 3 2 2:1 4.12 36.7 51.6 36.5 Nettelmann et al. 2013), and detailed modelingsuggests a ice- 5 4.5 0.5 2:1 2.94 42.2 54.6 41.7 enriched H/He envelope atop very ice/rock rich deep interior 5 4.5 0.5 1:4 2.71 38.9 48.1 36.8 (Hubbard et al.
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