HOW TO DISTINGUISH BETWEEN CLOUDY MINI-NEPTUNES AND WATER/VOLATILE-DOMINATED SUPER-EARTHS

By Björn Benneke, Sara Seager. 2013

165 citations recorded by the source corpus.

One of the most profound questions about the newly discovered class of low-density super-Earths is whether these exoplanets are predominately H2-dominated mini-Neptunes or volatile-rich worlds with gas envelopes dominated by H2O, CO2, CO, CH4, or N2. Transit observations of the super-Earth GJ 1214b rule out cloud-free H2-dominated scenarios, but are not able to determine whether the lack of deep spectral features is due to high-altitude clouds or the presence of a high mean molecular mass atmosphere. Here, we demonstrate that one can unambiguously distinguish between cloudy mini-Neptunes and volatile-dominated worlds based on wing steepness and relative depths of absorption features in moderate-resolution near-infrared transmission spectra (R ∼ 100). In a numerical retrieval study, we show for GJ 1214b that an unambiguous distinction between a cloudy H2-dominated atmosphere and cloud-free H2O atmosphere will be possible if the uncertainties in the spectral transit depth measurements can be reduced by a factor of ∼3 compared to the published Hubble Space Telescope Wide-Field Camera 3 and Very Large Telescope transit observations by Berta et al. and Bean et al. We argue that the required precision for the distinction may be achievable with currently available instrumentation by stacking 10–15 repeated transit observations. We provide a scaling law that scales our quantitative results to other transiting super-Earths and Neptunes such as HD 97658b, 55 Cnc e, GJ 3470b and GJ 436b. The analysis in this work is performed using an improved version of our Bayesian atmospheric retrieval framework. The new framework not only constrains the gas composition and cloud/haze parameters, but also determines our confidence in having detected molecules and cloud/haze species through Bayesian model comparison. Using the Bayesian tool, we demonstrate quantitatively that the subtle transit depth variation in the Berta et al. data is not sufficient to claim the detection of water absorption.

Subfields: Astronomy and Astrophysics, Instrumentation

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In Astronomy and Astrophysics

1. N. Madhusudhan
2. Jon Fortney
3. M. Line
4. A. Showman
5. Laura Kreidberg
6. V. Parmentier
7. J. D'esert
8. H. Knutson

In Instrumentation

1. N. Madhusudhan
2. Jon Fortney
3. M. Line
4. A. Showman
5. Laura Kreidberg
6. J. D'esert
7. V. Parmentier
8. H. Knutson

Citations in the corpus

• HUBBLE SPACE TELESCOPE NEAR-IR TRANSMISSION SPECTROSCOPY OF THE SUPER-EARTH HD 97658B (2014, methodology): “…al. 2010), the most recent near-IR observations obtained by Kreidberg et al. (2014) using the Wide Field Camera 3 (WFC3) instrument on the Hubble Space Telescope (HST) are precise enough to rule out cloud-free models over a broad range of atmospheric metallicities (also see Benneke & Seager 2013).”
• THE INFLUENCE OF NONUNIFORM CLOUD COVER ON TRANSIT TRANSMISSION SPECTRA (2015, background): “The degeneracy between uniform clouds and high mean molecular weight has already been investigated in a variety of planets (Benneke & Seager 2013; Knutson et al. 2014; Fraine et al. 2014).”
• CHARACTERIZING TRANSITING EXOPLANET ATMOSPHERES WITH JWST (2015, background): “…of molecules such CO, CO2, and CH4 in addition to H2O in ∼ 10 exoplanet atmospheres (e.g., Madhusudhan & Seager 2009; Madhusudhan et al. 2011; Lee et al. 2012; Line et al. 2012; Barstow et al. 2013a,b; Benneke & Seager 2012, 2013; Line et al. 2013a, 2014a; Benneke 2015; Waldmann et al. 2015).”
• A DETECTION OF WATER IN THE TRANSMISSION SPECTRUM OF THE HOT JUPITER WASP-12b AND IMPLICATIONS FOR ITS ATMOSPHERIC COMPOSITION (2015, background): “See also Benneke & Seager (2013) for an application of nested Bayesian model comparisons as applied to super-Earth atmospheres.”
• HD 209458b in new light: evidence of nitrogen chemistry, patchy clouds and sub-solar water (2017, methodology): “Values of at least Bi j = 3, 12, 150 are often interpreted as ‘weak’, ‘moderate’ and ‘strong’ detections in favour of Mi over Mj (Trotta 2008; Benneke & Seager 2013).”
• SPITZER OBSERVATIONS CONFIRM AND RESCUE THE HABITABLE-ZONE SUPER-EARTH K2-18b FOR FUTURE CHARACTERIZATION (2016, background): “Eventually the detectability of the K2-18b’s atmosphere will depend on the mean molecular mass of the atmosphere and presence of high-altitude clouds (Miller-Ricci et al. 2009; Benneke & Seager 2013).”
• NO THERMAL INVERSION AND A SOLAR WATER ABUNDANCE FOR THE HOT JUPITER HD 209458B FROM HST/WFC3 SPECTROSCOPY (2016, methodology): “…straddles the line between the classic “fully data driven” (Madhusudhan & Seager 2009; Lee et al. 2012; Line et al. 2012; Barstow et al. 2012; Benneke & Seager 2013; Line et al. 2014) and “a priori imposition of physical processes” (Knutson et al. 2014, Fraine et al. 2014, Kreidberg et al.…”