Human civilizations dating back thousands of years left behind structures and records documenting their studies of the stars as they sought to chart the seasons, help travelers find their way and interpret the world around them. Stargazers among the ancient Greeks, Maya, Egyptians, Middle Easterners and Asians likely also pondered if there were other planets like ours among those distant points of light — and if so, what might live there. Over the last century, science-fiction storytellers have used books, movies, comics and television to speculate at great length about contact with creatures from other worlds — to our benefit and our detriment. These creatures have been imagined as sometimes benevolent and sometimes bloodthirsty, and they have come in a wide range of shapes and sizes — from inquisitive "little green men" to human-parasitizing, chest-bursting Xenomorphs in the "Alien" movie franchise. Present-day astronomers have likewise been probing this question, using sophisticated equipment to listen farther and peer deeper into the universe than ever before, to find evidence of our cosmic neighbors. From detecting unexplained radio signals to investigating the atmospheres and liquid water on distant worlds — how are scientists searching for signs of extraterrestrial life? For an alien-seeking scientist, "life" means any living form — including microbes, astronomer Mercedes López-Morales, at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, told Live Science. But even the smallest microbe living on a distant exoplanet — a planet orbiting a star other than our sun — could still broadcast a chemical signal that would be visible to sensitive telescopes, in the form of atmospheric gases that probably wouldn't be there in the absence of life, López-Morales explained. "Life affects the atmosphere of a planet," she said. "You have gases that are only there because they are constantly being replenished by something — otherwise, they would react with other gases and disappear. For that gas or that molecule to be in the atmosphere of a planet, it must have some mechanism that is continuously producing it," López-Morales said. One of the atmospheric gases astronomers are searching for in exoplanets is oxygen, which is plentiful in Earth's atmosphere because it is continuously being replaced by plants through photosynthesis. However, the presence of unusual atmospheric gases doesn't necessarily mean that something living is generating them, López-Morales added.
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