Temperature has the largest bearing on a planet’s environment. The prerequisite of life as we know it requires water to exist in liquid form \citep{forget_probability_2013}. Therefore, the habitability of a planet is largely determined by whether the temperature of the planet is suitable to develop and sustain Earth-like life. The stability of surface temperatures is dependent on atmospheric circulation. The atmosphere is responsible for the circulation and redistribution of heat. The atmosphere provides a buffer between the stellar radiation and the surface. The planet’s albedo and atmospheric composition factor into the amount of protection for the surface. This is due to the mixture of greenhouse gases that control the distribution of heat that can contribute to extreme heating or cooling of the planet as well as providing effective methods of reducing the amount of harmful or damaging stellar rays.
To search for life on extrasolar planets, few detection methods are feasible. The most practical is to detect biosignatures via remote sensing. Life can alter a planet’s atmosphere, surface, and appearance over time on a global scale. These changes in environmental processes can be detected in the spectrum of the planet. It becomes critical to learn to differentiate between planets with and without life based entirely on the interpretation of remote sensing observations.
The amount of light being absorbed by a particular material can vary widely just as the surface and atmospheric composition of a planet can be very different. Light is also reflected and scattered differently depending on the material \citep{sterzik_biosignatures_2012}. Different compounds and materials lend themselves to being conducive environments for life and thus the change in albedo as a function of wavelength can be used to determine the habitability of exoplanet.
In reality, the albedo we measure is related to the average albedo of a planet. We breakdown the different components into the albedo contributions from surface and atmospheric sources. Surface reflection contributions can be broken into many components depending on the variety of planetary surfaces \citep{Coakley_2003}. For example, oceans, lakes and forests on Earth reflect small fractions of the incident sunlight resulting in a low albedo. On the other hand, snow, sea ice, and deserts reflect relatively large fractions of the incident sunlight and produce large albedos. However, this surface albedo can be influenced by other factors. One of the factors is topographic: areas of steep slope produce lower albedo than flat areas. For land and forests composed of deciduous trees and shrubbery, the albedo can fluctuate depending on the season. The fraction of the area covered by snow and sea ice and the thickness of snow and ice layers alter the albedo as well. Also, old snow reflects less light than fresh snow. However, the contribution of surface albedo to the planetary albedo is relatively low compared to atmosphere albedo due to the attenuation of light as it passes through the atmosphere \citep{aaron_donohoe_atmospheric_2011}.
The reflectance spectra of a planet is strongly influenced by its atmosphere \citep{marley_reflected_1999}. The albedo contribution from clouds is largely affected by the cloud condensation nuclei meaning the size, density, and source of the nuclei. The reflectance of light varies by cloud type, which depend on liquid water content and cloud location. For example, a stratocumulus cloud is low level thick cloud, located at 0-2 km above the ground. These clouds produce high albedos. Thin clouds, such as cirrus clouds, located at high levels in the atmosphere reflect less light and thus produce low albedo. \citet{heng_understanding_2013} presented a few scenarios of clouds in irradiated exoplanets and possible interpretations based on the optical phase curve of the planet. Exoplanets that produce high albedo with sinusoidal optical phase curves contain large cloud particles or dust grains (\(\sim\)10µm) in the atmosphere. Exoplanets that produce low albedo with flat optical phase curves, have atmospheres that contain small cloud particles (\(<<\)1µm). The majority of our planets exhibit a low albedo and flat optical phase curve resulting in the latter scenario.
Reflectivity is also a wavelength dependent phenomenon. For example, peaks can be seen in the infrared region of reflectance spectra which can be associated with byproducts of life such as water, carbon dioxide, ozone, methane, ammonia and nitrous oxide molecules \citep{des_marais_remote_2002}. In addition, it is also possible to detect biosignatures through the analysis of transmission spectra. For example, the so called “red edge” spectroscopic feature on Earth due to the chlorophyll production in vegetation.