Atmospheric Circulation, Chemistry, and Infrared Spectra of Titan-like Exoplanets around Different Stellar Types

Abstract

With the discovery of ever smaller and colder exoplanets, terrestrial worlds with hazy atmospheres must be increasingly considered. Our Solar System's Titan is a prototypical hazy planet, whose atmosphere may be representative of a large number of planets in our Galaxy. As a step towards characterizing such worlds, we present simulations of exoplanets that resemble Titan, but orbit three different stellar hosts: G-, K-, and M-dwarf stars. We use general circulation and photochemistry models to explore the circulation and chemistry of these Titan-like planets under varying stellar spectra, in all cases assuming a Titan-like insolation. Due to the strong absorption of visible light by atmospheric haze, the redder radiation accompanying later stellar types produces more isothermal stratospheres, stronger meridional temperature gradients at mbar pressures, and deeper and stronger zonal winds. In all cases, the planets' atmospheres are strongly superrotating, but meridional circulation cells are weaker aloft under redder starlight. The photochemistry of hydrocarbon and nitrile species varies with stellar spectra, with variations in the FUV/NUV flux ratio playing an important role. Our results tentatively suggest that column haze production rates could be similar under all three hosts, implying that planets around many different stars could have similar characteristics to Titan's atmosphere. Lastly, we present theoretical emission spectra. Overall, our study indicates that, despite important and subtle differences, the circulation and chemistry of Titan-like exoplanets are relatively insensitive to differences in host star. These findings may be further probed with future space-based facilities, like WFIRST, LUVOIR, HabEx, and OST.

Aerosols in Exoplanet Atmospheres

“…Several studies have investigated the surface temperature (Gilliam & McKay, 2011) and transmission spectra and geometric albedo (Checlair et al, 2016;Robinson et al, 2014) of Titan-like exoplanets, all of which are strongly dependent on the haze properties. In addition, Lora et al (2018) used a photochemical model to show that haze production on Titan-like exoplanets for host stars of various stellar types may be similar, though the photochemical pathways require further laboratory studies.…”

“…In addition, Lora et al. (2018) used a photochemical model to show that haze production on Titan‐like exoplanets for host stars of various stellar types may be similar, though the photochemical pathways require further laboratory studies.…”

Ground-based HCN submillimetre measurements in Titan's atmosphere: an intercomparison with Herschel observations

“…Fur-thermore, our mean HCN profile is also comparable to the photochemical models of Krasnopolsky (2014) and Vuitton et al (2019) below 230 km. The HCN modelled profiles by Loison et al (2015), Loison (2018), andLora et al (2018) appear to have over-predicted the amounts of HCN in atmospheres below 170 km. However, the earlier ones are consistent in shape.…”

“…These photochemistry models require revision, not only in the calculated absolute amount of HCN, but also in its vertical distribution below 170 km and 250 km, and, excluding Loison et al (2015), above 400 km as well. The model from Lora et al (2018) includes HCN modelled in the atmospheres of planets around G stars, with planetary parameters corresponding to Titan (more details in Section 5).…”

The Fundamental Connections between the Solar System and Exoplanetary Science

“…With its active methane cycle (Dalba et al., 2012; Hörst, 2017; Levi & Cohen, 2019; Turtle et al., 2011), Earth‐like shorelines (Lunine & Lorenz, 2009), diverse geological processes (Jaumann et al., 2009), and the potential for prebiotic chemistry (C. He & Smith, 2014; Neish et al., 2009), Titan serves as an analog for ocean planets that are similar to Earth in nature. Haze in the atmospheres of Titan‐like exoplanets could be detected by next‐generation space telescopes (Checlair et al., 2016; Lora et al., 2018; Robinson, Maltagliati, et al., 2014), thereby revealing the atmospheric compositions of numerous ocean planets.…”