THE ATMOSPHERIC SIGNATURES OF SUPER-EARTHS: HOW TO DISTINGUISH BETWEEN HYDROGEN-RICH AND HYDROGEN-POOR ATMOSPHERES

By E. Miller-Ricci, Sara Seager, D. Sasselov. 2008

188 citations recorded by the source corpus.

Extrasolar super-Earths (1–10 M⊕) are likely to exist with a wide range of atmospheres. Some super-Earths may be able to retain massive hydrogen-rich atmospheres. Others might never accumulate hydrogen or experience significant escape of lightweight elements, resulting in atmospheres more like those of the terrestrial planets in our solar system. We examine how an observer could differentiate between hydrogen-rich and hydrogen-poor atmospheres by modeling super-Earth emission and transmission spectra, and we find that discrimination is possible by observing the transmission spectrum alone. An Earth-like atmosphere, composed of mostly heavy elements and molecules, will have a very weak transmission signal due to its small atmospheric scale height (since the scale height is inversely proportional to molecular weight). However, a large hydrogen-rich atmosphere reveals a relatively large transmission signal. The super-Earth emission spectrum can additionally constrain the atmospheric composition and temperature structure. Super-Earths with massive hydrogen atmospheres will reveal strong spectral features due to water, whereas those that have lost most of their hydrogen (and have no liquid ocean) will be marked by CO2 features and a lack of H2O. We apply our study specifically to the low-mass planet orbiting an M star, Gl 581c (Msin i = 5 M⊕), although our conclusions are relevant for super-Earths in general. The ability to distinguish hydrogen-rich atmospheres might be essential for interpreting mass and radius observations of planets in the transition between rocky super-Earths and Neptune-like planets.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/5611b7d7aec49611e8d3b17e602e4ecf24c55010

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In Astronomy and Astrophysics

1. Jon Fortney
2. N. Madhusudhan
3. Sara Seager
4. A. Showman
5. M. Line
6. D. Charbonneau
7. D. Deming
8. H. Knutson

Citations in the corpus

• HOW TO DISTINGUISH BETWEEN CLOUDY MINI-NEPTUNES AND WATER/VOLATILE-DOMINATED SUPER-EARTHS (2013, background): “Miller-Ricci et al. (2009) showed that cloud-free, hydrogen-dominated atmospheres would display absorption features in the transmission spectrum that are several times larger than those expected for atmospheres bbenneke@mit.edu dominated by water vapor, CO2, CO, CH4, or N2 due to the lower mean…”
• A FRAMEWORK FOR CHARACTERIZING THE ATMOSPHERES OF LOW-MASS LOW-DENSITY TRANSITING PLANETS (2013, background): “As was suggested by Miller-Ricci et al. (2009) and Miller-Ricci & Fortney (2010), probing the outer atmosphere of the low-mass low-density (LMLD) planets can in principle constrain the composition of the gaseous envelope.”
• The Transit Light Source Effect: False Spectral Features and Incorrect Densities for M-dwarf Transiting Planets (2017, background): “The scale of the signal depends inversely on the square of the stellar radius (Miller-Ricci et al. 2009), prompting a focus on studying exoplanets around M dwarf stars.”
• ATMOSPHERIC RETRIEVAL FOR SUPER-EARTHS: UNIQUELY CONSTRAINING THE ATMOSPHERIC COMPOSITION WITH TRANSMISSION SPECTROSCOPY (2012, methodology): “…SOLUTION TO INFER THE MEAN MOLECULAR MASS For thin or cloudy atmospheres the change in the transit depth across the spectrum, ∆D, as proposed by Miller-Ricci et al. (2009), cannot be used to uniquely constrain the mean molecular mass because clouds, hazes, and a surface also affect the…”
• The minimum-mass extrasolar nebula: in situ formation of close-in super-Earths (2012, background): “Differences in the mean molecular weight of an atmosphere lead to differences in atmospheric scale height, and these can be distinguished by measuring transit depths as a function of wavelength (Miller-Ricci et al. 2009).”
• The Detectability and Characterization of the TRAPPIST-1 Exoplanet Atmospheres with JWST (2019, background): “…features, but will be challenging for transmission spectroscopy if molecular absorption is suppressed by a high mean molecular weight atmosphere (Miller-Ricci et al. 2009), the presence of clouds and hazes (Berta et al. 2012; Ehrenreich et al. 2014; Knutson et al. 2014; Kreidberg et al. 2014;…”
• Exoplanet Atmospheres: Physical Processes (2010, background): “In other words, using transmission spectroscopy during transit, atmospheres with large scale heights are easier to detect than are atmospheres with small scale heights (Miller-Ricci et al. 2008).”
• Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life (2017, background): “ntial component of H 2 or He, they will be significantly more detectable in transit spectroscopy because they will increase the scale height and therefore the transmission depths of spectral features (Miller-Ricci et al., 2009; Seager et al., 2013b; Ramirez and Kaltenegger, 2017). Manyreducing,H-bear”
• Equilibrium chemistry down to 100 K (2017, background): “…constants based on partition functions (Allard & Hauschildt 1995; Gustafsson et al. 2008; Barklem & Collet 2016; Van Eck et al. 2017), and (iii) minimisation of the system Gibbs free energy (Lodders 2003; Ackerman & Marley 2001; Miller-Ricci et al. 2009; Mbarek & Kempton 2016; Blecic et al. 2016).”
• C/O RATIO AS A DIMENSION FOR CHARACTERIZING EXOPLANETARY ATMOSPHERES (2012, methodology): “For this purpose, we use the equilibrium chemistry code originally developed by Seager et al. (2000), and subsequently used in several recent works (Seager et al. 2005; Miller-Ricci et al. 2009; Madhusudhan & Seager 2011).”
• THREE POSSIBLE ORIGINS FOR THE GAS LAYER ON GJ 1214b (2009, background): “Discriminating between a hydrogen-rich envelope and a water vapor atmosphere on the basis of the atmospheric scale height (Miller-Ricci et al. 2009) might be more promising than discriminating on the basis of the presence or absence of spectral lines.”
• CHEMISTRY OF SILICATE ATMOSPHERES OF EVAPORATING SUPER-EARTHS (2009, background): “So far, atmospheric models of super-Earths have focused on traditional compositions containing H, C, and N (e.g., Miller-Ricci et al. 2009, Elkins-Tanton & Seager, 2008, Ehrenreich et al. 2006).”
• HOW THERMAL EVOLUTION AND MASS-LOSS SCULPT POPULATIONS OF SUPER-EARTHS AND SUB-NEPTUNES: APPLICATION TO THE KEPLER-11 SYSTEM AND BEYOND (2012, background): “Since hydrogen-rich atmospheres have much larger scale heights at a given temperature, near-infrared water and methane absorption features will be much more prominent for planets with hydrogen/helium envelopes (Miller-Ricci et al. 2009; Miller-Ricci & Fortney 2010).”
• Characterization of the K2-18 multi-planetary system with HARPS: A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit (2017, background): “…of ∆F/F ∼ 10Hrp/R2s ∼ 230 ppm where H = kBTeq/µg is the atmospheric scale height, Teq is the planet’s equilibrium temperature set by the stellar insolation, µ is the mean molecular weight of the atmosphere, and g is the surface gravity (Miller-Ricci et al. 2009; Kaltenegger & Traub 2009).”