Classification of Irradiated Exoplanets using Radiative-Convective and Chemical Equilibrium

Dubey, Dwaipayan (2022) Classification of Irradiated Exoplanets using Radiative-Convective and Chemical Equilibrium. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS dissertation of Dwaipayan Dubey (17MS021)) 17MS021_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (2MB)

Abstract

Context: For more than 4000 exoplanets discovered till today, the classification of them has always been a challenge to the astronomy community. The ultimate goal is to investigate and propose an equivalent Hertzsprung – Russell diagram for planets so that planets’ properties in terms of bulk composition can easily be understood. Aim: Average density of exoplanet was the only constraint that we had for the bulk composition of planets. The change of different parameters can lead to the formation of planets with same average densities but with different signatures. Hence, the aim is to constrain planetary Carbon-to-Oxygen ratio as a reliable parameter for the classification of irradiated gaseous exoplanets with distinct spectral features and atmospheric properties. Methods: The average photospheric abundances of Water and Methane have been used for the estimation of transition Carbon-to-Oxygen ratio ((C/O)tr) for exoplanets. I have explored a 3D parameter space (C/O ratio, planetary effective temperature, and metallicity) for the classification of planets, having a temperature span of 400 K - 2600 K. The quantification of transition C/O ratios has been done using cloud free radiative-convective and chemical equilibrium principle with a 1D self-consistent forward modeling code. A radiative-transfer model has also been coupled for the simulation of high-resolution (R = 1000) transmission and emission spectra for all planetary classes. The converged atmospheric structure of planet and star-planet parameters have been used for the generation of the same. In the last part, I have studied the noise budget simulation of planet K2-18 b with JexoSim. It predicts the exact number of transits required for JWST to investigate the molecular signatures present inside the planet. Results: A 4D classification scheme of irradiated exoplanets has been proposed based on the trend of transition C/O ratio with planetary effective temperatures. The classification is also being supported with the leading atmospheric chemistry inside the planet. Contribution function for transmission and emission spectra conclude the planetary photosphere to be the most active part in the atmosphere where every observation should focus for analysis. Simulation with JexoSim predicts almost 100 transits for JWST for the molecular detection inside planet K2 - 18 b. Conclusion: (C/O)tr = 1 was earlier thought to be a global separator between Water and Methane dominated planetary spectra. This study shows that the (C/O)tr is not universal. Rather, it has dependencies on several parameters. Using JexoSim, theoretical predictions on transit observations can be made for detecting molecular signatures inside a specific planet which helps for the further betterment of real life observations.

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