Figure 1 shows a schematic of the Planetary Haze Research (PHAZER) experimental setup at Johns Hopkins University (He et al. 2017; Hörst et al. 2018). The initial gas mixtures for our experiments are calculated from the chemical-equilibrium models of Moses et al. (2013), who examined the possible thermochemistry and photochemistry in the atmospheres of Neptune-sized and sub-Neptune-sized exoplanets. Most of the known super-Earth and mini-Neptune size planets known to date were discovered by the Kepler mission (Borucki et al. 2011; Fressin et al. 2013). However, follow-up of Kepler targets to determine planetary mass and atmospheric characterization has been limited due to the brightness of their host stars. The TESS mission (Ricker et al. 2014) will target stars that are 10–100 times brighter than those targeted with the Kepler mission and yield a large sample of super-Earth and mini-Neptune class planets with equilibrium temperatures (Teq) 1000 K (Sullivan et al. 2015). This planetary size (Rp  4 REarth) and temperature (Teq  1000 K) phase space has been recently explored by exoplanet atmospheric characterization efforts for a handful of planets such as GJ 1214b, HD 97658b, GJ 436b, and GJ 3470b, which all indicate that aerosols are shaping the planetary spectrum (e.g., Knutson et al. 2014a, 2014b; Kreidberg et al. 2014; Dragomir et al. 2015). Theoretical studies predict a diverse range of possibilities for the composition of atmospheric envelopes of mini-Neptunes and super-Earths (e.g., Fortney et al. 2013; Howe & Burrows 2015; Wolfgang & Lopez 2015). Broadly speaking, we expect that our hydrogen-rich gas mixtures are more relevant to larger mini-Neptune class planets and carbon dioxide-rich gas mixtures are more relevant to smaller super-Earth class planets, with water-rich gas mixtures being relevant to both classes of planets. However, further observational, theoretical, and laboratory work will be required to confirm or refute the potential diversity in atmospheric compositions for these classes of planets that do not exist in our solar system. The equilibrium calculations relevant to this investigation were performed at conditions of 300, 400, and 600 K at 1 mbar for 100×, 1000×, and 10,000× solar metallicity. Smaller planets are less efficient at accreting the H- and He-rich gas from the protoplanetary disk, and the lighter elements have a greater chance of escaping from smaller planets, which enhances the overall metallicity of planets that are Neptune-sized and smaller. As an example, the O/H metallicity on Uranus and Neptune is estimated to be 160 and 540 times solar, respectively (Cavalié et al. 2017). Various evolutionary processes could drive the relative abundances of the heavy elements away from solar, but these high-metallicity chemical-equilibrium models provide a reasonable starting point for our study. As listed in Table 1, we included gases with a calculated abundance of 1% or higher to maintain a manageable level of experimental complexity; this resulted in no nitrogen-bearing species in two cases and the exclusion of sulfur-bearing species. Sulfur-bearing species, like H2S, could be important for haze formation (Gao et al. 2017) and will be an avenue of future work. The pressure, temperature, and gas compositions used in the experiments are self-consistent based on the model calculations.
Citation: Dragomir et al. 2015
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