EXOPLANET ALBEDO SPECTRA AND COLORS AS A FUNCTION OF PLANET PHASE, SEPARATION, AND METALLICITY

By Kerri L. Cahoy, M. Marley, Jon Fortney. 2010

136 citations recorded by the source corpus.

First generation space-based optical coronagraphic telescopes will obtain images of cool gas- and ice-giant exoplanets around nearby stars. Exoplanets lying at planet–star separations larger than about 1 AU—where an exoplanet can be resolved from its parent star—have spectra that are dominated by reflected light to beyond 1 μm and punctuated by molecular absorption features. Here, we consider how exoplanet albedo spectra and colors vary as a function of planet–star separation, metallicity, mass, and observed phase for Jupiter and Neptune analogs from 0.35 to 1 μm. We model Jupiter analogs with 1× and 3× the solar abundance of heavy elements, and Neptune analogs with 10× and 30× the solar abundance of heavy elements. Our model planets orbit a solar analog parent star at separations of 0.8 AU, 2 AU, 5 AU, and 10 AU. We use a radiative–convective model to compute temperature–pressure profiles. The giant exoplanets are found to be cloud-free at 0.8 AU, possess H2O clouds at 2 AU, and have both NH3 and H2O clouds at 5 AU and 10 AU. For each model planet we compute moderate resolution (R = λ/Δλ ∼ 800) albedo spectra as a function of phase. We also consider low-resolution spectra and colors that are more consistent with the capabilities of early direct imaging capabilities. As expected, the presence and vertical structure of clouds strongly influence the albedo spectra since cloud particles not only affect optical depth but also have highly directional scattering properties. Observations at different phases also probe different volumes of atmosphere as the source–observer geometry changes. Because the images of the planets themselves will be unresolved, their phase will not necessarily be immediately obvious, and multiple observations will be needed to discriminate between the effects of planet–star separation, metallicity, and phase on the observed albedo spectra. We consider the range of these combined effects on spectra and colors. For example, we find that the spectral influence of clouds depends more on planet–star separation and hence atmospheric temperature than metallicity, and it is easier to discriminate between cloudy 1× and 3× Jupiters than between 10× and 30× Neptunes. In addition to alkalis and methane, our Jupiter models show H2O absorption features near 0.94 μm. While solar system giant planets are well separated by their broadband colors, we find that arbitrary giant exoplanets can have a large range of possible colors and that color alone cannot be relied upon to characterize planet types. We also predict that giant exoplanets receiving greater insolation than Jupiter will exhibit higher equator-to-pole temperature gradients than are found on Jupiter and thus may exhibit differing atmospheric dynamics. These results are useful for future interpretation of direct imaging exoplanet observations as well as for deriving requirements and designing filters for optical direct imaging instrumentation.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/a710e6fecaf1e1519a48ce8787e2e16fc2b9afd2

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In Astronomy and Astrophysics

1. Jon Fortney
2. N. Madhusudhan
3. C. Mordasini
4. Th. Henning
5. V. Parmentier
6. A. Showman
7. M. Marley
8. Y. Alibert

In Instrumentation

1. Jon Fortney
2. N. Madhusudhan
3. C. Mordasini
4. A. Showman
5. V. Parmentier
6. Th. Henning
7. M. Marley
8. C. Morley

Citations in the corpus

• Atmospheric characterization of Proxima b by coupling the Sphere high-contrast imager to the Espresso spectrograph (2016, background): “This value is justified as follows: planets that can be angularly resolved are mostly temperate to cool and are expected to have rather high albedos owing to the presence of reflective clouds (e.g. Cahoy et al. 2010).”
• A FRAMEWORK FOR CHARACTERIZING THE ATMOSPHERES OF LOW-MASS LOW-DENSITY TRANSITING PLANETS (2013, background): “T) profiles for the atmosphere of Titan (McKay et al. 1989), Uranus (Marley & McKay 1999), brown dwarfs (Marley et al. 1996; Burrows et al. 1997; Saumon & Marley 2008), and irradiated exoplanets (Fortney et al. 2005, 2008; Cahoy et al. 2010; Morley et al. 2013).”
• TRANSITIONS IN THE CLOUD COMPOSITION OF HOT JUPITERS (2016, background): “…model solves the radiative transfer equations for the hemispherically averaged radiation field and is unable to provide information regarding the direction of the radiation escaping a given atmospheric column, contrary to more sophis- ticated models (e.g. Cahoy et al. 2010; Webber et al. 2015).”
• Ice structures, patterns, and processes: A view across the icefields (2012, background): “The existence of ice on the newly detected exoplanets is already being considered; observations are being carried out to investigate this possibility, for instance through surface albedo and color (Cahoy et al., 2010).”
• Characterization of exoplanets from their formation - I. Models of combined planet formation and evolution (2012, methodology): “In future work, we will use an albedo that depends on planet parameters (Cahoy et al. 2010).”