Undergraduate Honors Theses

Thesis Defended

Spring 2019

Document Type

Thesis

Type of Thesis

Departmental Honors

Department

Astronomy

First Advisor

Prof. David Brain

Second Advisor

Dr. Eric Wolf

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

Abstract

In order to better direct future exoplanetary research, we must be able to accurately predict what telescopes like the James Webb Space Telescope (JWST) will be able to detect. To this effect, NASA’s Planetary Spectrum Generator (PSG) can be used to simulate observations, both during exoplanet transits and throughout an exoplanet’s year. Such predictions allow us to make informed decisions about how to spend time on JWST and which techniques will allow for different types of analysis. In order to produce the most realistic results from the Planetary Spectrum Generator (PSG), self-consistent three-dimensional climate models of planets in the TRAPPIST-1 system are first used to simulate atmospheres on these tidally locked exoplanets, and understand how they might be different from the climate of Earth.

The TRAPPIST-1 exoplanets are assumed to be tidally locked with the host star. Recent climate modeling studies indicate that there are three distinct types of tidally locked exoplanet atmospheres; slow rotators, fast rotators, and an intermediate regime. Slow rotators have large substellar clouds that remain constant over time. Fast rotators will have a significant Coriolis force, resulting in much smaller substellar clouds and more intense zonal winds which advect clouds to the eastern side of the substellar point. In all cases, the thermal emission of a planet depends strongly on the spatial distribution of clouds, which would change from the perspective of a distant observer over the exoplanet’s year.

I have constructed a data pipeline that uses climate models as inputs, then uses the PSG to produce spectra. This pipeline can use a number of climate models on a number of different planets to create transit spectra or thermal phase curves.

When studying terrestrial exoplanets, the most exiting cases are potentially habitable planets. The TRAPPIST-1 system is one of the most likely systems to host a habitable exoplanet. TRAPPIST-1 e is the most likely habitable planet in the system. The most effective methods for JWST are the transit method and thermal phase curves. The transit method is the most widely used method for detecting exoplanets, and JWST will have high resolution spectrographs that will be able to detect many atmospheric species including CO2 and H2O. Additionally, accurate spectra may enable us to infer the surface temperature of a planet. Transit spectra can tell us a lot about the terminator profile of an atmosphere, but it cannot tell us about other parts of the planet’s surface.

Transit spectra can also be prone to extinction by clouds or aerosols. Thermal phase curves will observe how the thermal emission of an exoplanet will change over the course of its year, and may be able to detect features like the nature of the substellar cloud, if a planet has entered a runaway greenhouse effect, or a snowball state. Thermal phase curves will look dramatically different for fast rotators like TRAPPIST-1 e compared to slow rotators, and these results can only be simulated with global circulation models used in conjunction with accurate radiative transfer models. Thermal phase curves are also very sensitive to the planet’s temperature, and can therefore help determine if a planet is habitable. Future predictions for JWST should incorporate the results from global circulation models, and those models should be accurate to the exoplanets in question. Future observations should also consider thermal phase curves because they will be able to detect a range of different features than transit spectra and will enable us to more effectively understand potentially habitable terrestrial exoplanets.

Available for download on Wednesday, April 08, 2020

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