A FRAMEWORK FOR CHARACTERIZING THE ATMOSPHERES OF LOW-MASS LOW-DENSITY TRANSITING PLANETS

By Jon Fortney, C. Mordasini, N. Nettelmann, Eliza Miller-Ricci Kempton, T. Greene, K. Zahnle. 2013

154 citations recorded by the source corpus.

We perform modeling investigations to aid in understanding the atmospheres and composition of small planets of ∼2–4 Earth radii, which are now known to be common in our Galaxy. GJ 1214b is a well-studied example whose atmospheric transmission spectrum has been observed by many investigators. Here we take a step back from GJ 1214b to investigate the role that planetary mass, composition, and temperature play in impacting the transmission spectra of these low-mass low-density (LMLD) planets. Under the assumption that these planets accrete modest hydrogen-dominated atmospheres and planetesimals, we use population synthesis models to show that predicted metal enrichments of the H/He envelope are high, with metal mass fraction Zenv values commonly 0.6–0.9, or ∼100–400+ times solar. The high mean molecular weight of such atmospheres (μ ≈ 5–12) would naturally help to flatten the transmission spectrum of most LMLD planets. The high metal abundance would also provide significant condensible material for cloud formation. It is known that the H/He abundance in Uranus and Neptune decreases with depth, and we show that atmospheric evaporation of LMLD planets could expose atmospheric layers with gradually higher Zenv. However, values of Zenv close to solar composition can also arise, so diversity should be expected. Photochemically produced hazes, potentially due to methane photolysis, are another possibility for obscuring transmission spectra. Such hazes may not form above Teq of ∼800–1100 K, which is testable if such warm, otherwise low mean molecular weight atmospheres are stable against atmospheric evaporation. We find that available transmission data are consistent with relatively high mean molecular weight atmospheres for GJ 1214b and “warm Neptune” GJ 436b. We examine future prospects for characterizing GJ 1214b with Hubble and the James Webb Space Telescope.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/e9b05b7c597827acf7ed74e3a315123ce2eba9d7

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In Astronomy and Astrophysics

1. N. Madhusudhan
2. Jon Fortney
3. A. Showman
4. M. Marley
5. M. Line
6. V. Parmentier
7. D. Deming
8. H. Knutson

In Instrumentation

1. N. Madhusudhan
2. Jon Fortney
3. A. Showman
4. M. Line
5. M. Marley
6. V. Parmentier
7. H. Knutson
8. Sara Seager

Citations in the corpus

• HOW TO DISTINGUISH BETWEEN CLOUDY MINI-NEPTUNES AND WATER/VOLATILE-DOMINATED SUPER-EARTHS (2013, background): “Theoretical studies of the atmosphere loss were conducted attempting to understand the stability of different atmospheres on highly irradiated super-Earths (e.g., Heng & Kopparla 2012; Fortney et al. 2013; Kurokawa & Kaltenegger 2013).”
• THE IMPRINT OF EXOPLANET FORMATION HISTORY ON OBSERVABLE PRESENT-DAY SPECTRA OF HOT JUPITERS (2016, background): “For 100 km sized planetesimals as assumed in this work, we found in population syntheses (see Fortney et al. 2013) that giant planets contain solid cores with masses between 6 and 12 M⊕ at the end of the formation phase.”
• The Complete Transmission Spectrum of WASP-39b with a Precise Water Constraint (2017, background): “Gravitational instability theory suggests that planets will have the same atmospheric metallicity as the central star, while with core accretion theory, lower mass planets will have higher atmospheric metallicity (e.g. Mordasini et al. 2012; Fortney et al. 2013).”
• HUBBLE SPACE TELESCOPE NEAR-IR TRANSMISSION SPECTROSCOPY OF THE SUPER-EARTH HD 97658B (2014, background): “Measurements of the transmission spectra of superEarths allow us to directly estimate the mean molecular weight of the planet’s atmosphere (e.g., Miller-Ricci et al. 2008; Fortney et al. 2013), which in turn provides improved constraints on its interior composition.”
• Transmission spectral properties of clouds for hot Jupiter exoplanets (2014, methodology): “It is also possible to use the MIRI low resolution spectrometer (LRS) which covers 5 – 14 µm with a R∼100 (Fortney et al. 2013).”
• HST PanCET Program: A Cloudy Atmosphere for the Promising JWST Target WASP-101b (2017, background): “This in turn can give insights into planetary formation (e.g., Fortney et al. 2013; Kreidberg et al. 2014; Benneke 2015).”
• A PRECISE WATER ABUNDANCE MEASUREMENT FOR THE HOT JUPITER WASP-43b (2014, background): “Planet population synthesis models aim to match atmospheric metallicity to planet mass (Fortney et al. 2013; Marboeuf et al. 2014), but there are limited data available for planets outside the solar system.”
• UNDERSTANDING THE MASS–RADIUS RELATION FOR SUB-NEPTUNES: RADIUS AS A PROXY FOR COMPOSITION (2013, background): “The atmospheres of Neptune and subNeptune sized planets might be significantly enhanced in metals (Fortney et al. 2013) or host extended clouds that greatly enhance atmospheric opacity (Morley et al. 2013).”
• A Population Study of Gaseous Exoplanets (2017, methodology): “Population synthesis studies based on formation scenarios or statistics from the Kepler Space mission suggest a great diversity in the exoplanet population (e.g. Fortney et al. 2013; Lopez & Fortney 2014; Rogers 2015; Parmentier et al. 2016).”
• Global Climate and Atmospheric Composition of the Ultra-hot Jupiter WASP-103b from HST and Spitzer Phase Curve Observations (2018, background): “Planets near WASP-103b’s mass (1.5MJup) are expected to have metallicities ranging from roughly 1 − 10× solar (Fortney et al. 2013; Mordasini et al. 2016).”
• CHARACTERIZING TRANSITING EXOPLANET ATMOSPHERES WITH JWST (2015, methodology): “The 1000× solar compositions correspond to mean molecular weight µ=15 for the warm Neptune and µ = 16.8 for the warm sub-Neptune atmospheres (see Fortney et al. (2013)).”
• THE MASS–METALLICITY RELATION FOR GIANT PLANETS (2015, background): “…either in bulk composition (Guillot et al. 2006; Burrows et al. 2007; Miller & Fortney 2011; Mordasini et al. 2014), or atmospheric composition (Madhusudhan et al. 2011; Fortney et al. 2013; Konopacky et al. 2013; Kreidberg et al. 2014; Barman et al. 2015) as shown in a schematic way in Figure 1.”
• GASEOUS MEAN OPACITIES FOR GIANT PLANET AND ULTRACOOL DWARF ATMOSPHERES OVER A RANGE OF METALLICITIES AND TEMPERATURES (2014, background): “…enhancements, and subsequent opacity enhancements, that could change as a function of time and location within the accreting H/He envelope (Podolak et al. 1988; Mordasini et al. 2006; Fortney et al. 2013), and could plausibly lead to new, and previously unexplored behavior in formation models.”