n the James Webb Space Telescope and Extremely Large Telescopes era we expect to characterize a number of potentially habitable Earth-like exoplanets. However, the characterization of these worlds... Show moren the James Webb Space Telescope and Extremely Large Telescopes era we expect to characterize a number of potentially habitable Earth-like exoplanets. However, the characterization of these worlds depends crucially on the accuracy of theoretical models. Validating these models against observations of planets with known properties will be key for the future characterization of terrestrial exoplanets. Due to its sensitivity to the micro- and macro-physical properties of an atmosphere, spectropolarimetry will be an important tool that in tandem with traditional flux-only observations will enhance the capabilities of characterizing Earth-like planets. In this presentation we benchmark two separate polarization-enabled radiative transfer codes against each other and against unique linear spectropolarimetric observations of the Earthshine (i.e., sunlight scattered by the dayside of the Earth and reflected back to the planet by the nightside of the Moon) that cover wavelengths from ~ 0.4 μm to ~ 2.3 μm. We find that the results from the two codes agree with each other but both underestimate the level of polarization of the Earthshine. We discuss how we plan to update the two codes to better fit the observations. We also report an interesting discrepancy between our models and the observed 1.27 μm O2 feature in the Earthshine, together with an analysis of potential methods for matching this feature and a discussion on the implications this has for future observations of habitable exoplanets. Show less
Traditional methods of exoplanet characterization that only make use of emitted or reflected flux lack the ability to fully distinguish between different physical features of the target, such as... Show moreTraditional methods of exoplanet characterization that only make use of emitted or reflected flux lack the ability to fully distinguish between different physical features of the target, such as cloud layers, hazes, or surface features. Polarimetry, however, is a powerful, more sensitive technique that has this ability, as it measures light as a vector (by the orientation of the electric field) rather than a scalar intensity. It is therefore extremely sensitive to the composition and structure of the planetary atmosphere and surface, being affected by properties such as the mixing ratios of atmospheric absorbing gases, cloud optical thickness, cloud top pressure, cloud particle size, and surface albedo. Various groups have theoretically studied the optical linear polarimetric signals of Earth-like exoplanets as functions of both orbital phase and wavelength. With this project we assess the accuracy of these theoretical models against observations of the Earthshine, the only known observations of an Earth-like planet thus far. Using data of the atmosphere and surface taken by the MODIS instrument aboard the Terra and Aqua satellites, as well as surface reflectance spectra from the JPL EcoStress Spectral Library, we created a gridded model of the Earth. Then, using this model data as input for three separate radiative transfer algorithms, we generate the flux and linear polarization spectra for the model exoplanet-Earth across the optical to near-infrared wavelengths. We compare the results from all three codes to each other and to the observational linear spectropolarimetric data of the Earthshine obtained by a member of our group. We identify similarities and potential pitfalls between the codes, and make necessary adjustments to them, in an effort to improve our future characterizations of terrestrial exoplanets. Show less
Polarimetry is widely becoming recognized as a powerful technique for enhancing the contrast between a star and an exoplanet, and thus improving upon the direct detection of exoplanets. The real... Show morePolarimetry is widely becoming recognized as a powerful technique for enhancing the contrast between a star and an exoplanet, and thus improving upon the direct detection of exoplanets. The real power of polarimetry, however, is in its ability to characterize the physical properties of these worlds. This is because the state of the polarization of the light from the planet is very sensitive to the composition and structure of the planetary atmosphere and surface, being affected by properties such as the mixing ratios of atmospheric absorbing gases, cloud optical thickness, cloud top pressure, cloud particle size, and surface albedo. Various groups have theoretically studied the optical linear polarimetric signals of Earth-like exoplanets as functions of both orbital phase and wavelength. This project aims to validate the accuracy of these theoretical models against the only known observations of an Earth-like planet thus far: Earthshine. Using atmospheric and surface data taken by the MODIS instrument aboard the Terra and Aqua satellites, as well as surface albedo spectra from the EcoStress Spectral Library, we created a detailed model of the Earth. Then, using this model data as input for three separate radiative transfer algorithms, we generate the flux and linear polarization spectra for the model exoplanet-Earth from the optical to near-infrared wavelengths. We compare the results from all three codes to each other and to observational linear spectropolarimetric data of the Earthshine obtained by a member of our group. We identify similarities and potential pitfalls between these codes in an effort to improve our future characterizations of Earth-like exoplanets. Show less
It is less than 20 years since astronomers discovered the first exoplanet orbiting a Sun-like star. In this short period more than 770 confirmed exoplanets have been detected. With so many... Show moreIt is less than 20 years since astronomers discovered the first exoplanet orbiting a Sun-like star. In this short period more than 770 confirmed exoplanets have been detected. With so many exoplanets the next step is their characterization. What is their atmosphere made of? Does it contain water clouds? Is there water on the planetary surface? Could there be life on these planets? To answer all these questions good and reliable models are necessary for interpreting the signal we observe from the detected exoplanets. In this thesis, Karalidi works with a numerical code to model the flux and polarization properties of starlight reflected by exoplanets with various forms of inhomogeneities. She shows that the rainbow, created by water clouds in the planetary atmosphere, is a powerful ally in our search for water clouds on other planets. In the upcoming years the detection of giant planets will be easier than the detection of terrestrial planets. For this reason Karalidi also describes the influence that various formations, such as zones, spots and polar hazes, have on the signal of Jupiter-like exoplanets. Show less
