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--- courses:ast100:5.4 [2026/03/22 11:28] – [1. Habitable zones] asad
+++ courses:ast100:5.4 [2026/03/22 11:35] (current) – [2. Searching for life] asad
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+Spectroscopy serves as a fundamental tool in astronomy by analyzing the interaction between light and matter to determine the physical properties and chemical composition of celestial objects. As illustrated in the diagram, a hot and dense light source produces a continuous spectrum, which contains a seamless range of all visible wavelengths. When a cloud of gas is energized, it generates an emission spectrum characterized by distinct bright, colored lines at specific wavelengths that correspond to the unique energy transitions of the atoms within that gas. Alternatively, when light from a continuous source passes through a cooler cloud of gas, an absorption spectrum is created, revealing dark lines or gaps where the gas has filtered out specific wavelengths. By analyzing these spectral "fingerprints," astronomers can identify the elemental makeup, temperature, and density of distant stars and nebulae without ever needing to visit them.
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+A transmission spectrum is a specialized astronomical tool used to determine the chemical composition of an exoplanet’s atmosphere by analyzing starlight as it passes through the planet's gaseous envelope during a transit. As illustrated in the diagram, when an exoplanet moves in front of its host star, different gas molecules in its atmosphere absorb specific wavelengths of light, spanning from the visible red end of the spectrum into the mid-infrared range of sixteen microns. These absorption events manifest as distinct dips in the graph, representing the "lesser" amount of light that reaches the observer at those specific frequencies. By identifying these unique spectroscopic fingerprints for molecules such as water vapor, methane, and carbon dioxide, researchers can effectively map the invisible gases surrounding a distant world and determine its atmospheric density.
+Beyond identifying simple atmospheric components, transmission spectroscopy is the primary method for detecting biosignatures, which are chemical indicators of potential biological activity on an exoplanet. The presence of specific gases, such as molecular oxygen and ozone depicted in the figure, is particularly significant because these molecules are often linked to photosynthesis or other life-sustaining processes on Earth. Finding a combination of water, methane, and carbon dioxide in chemical disequilibrium provides compelling evidence for a habitable or even inhabited environment. By analyzing these complex spectral profiles, astronomers can distinguish between barren, geologically dead worlds and planets with active biospheres, marking a transformative step in our quest to find life elsewhere in the galaxy.