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15: Life in the Universe

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    As we have learned more about the universe, we have naturally wondered whether there might be other forms of life out there. The ancient question, “Are we alone in the universe?” connects us to generations of humans before us. While in the past, this question was in the realm of philosophy or science fiction, today we have the means to seek an answer through scientific inquiry. In this chapter, we will consider how life began on Earth, whether the same processes could have led to life on other worlds, and how we might seek evidence of life elsewhere. This is the science of astrobiology.

    The search for life on other planets is not the same as the search for intelligent life, which (if it exists) is surely much rarer. Learning more about the origin, evolution, and properties of life on Earth aids us in searching for evidence of all kinds of life beyond that on our planet.

    • 15.1: The Cosmic Context for Life
      Life on Earth is based on the presence of a key unit known as an organic molecule, a molecule that contains carbon, especially complex hydrocarbons. Our solar system formed about 5 billion years ago from a cloud of gas and dust enriched by several generations of heavier element production in stars. Life is made up of chemical combinations of these elements made by stars. The Copernican principle suggests that there is nothing special about our place in the universe.
    • 15.2: Astrobiology
      The study of life in the universe, including its origin, is called astrobiology. Life appears to have spread around our planet within 400 million years after the end of heavy bombardment, if not sooner. The actual origin of life—the processes leading from chemistry to biology—is not completely understood. Once life took hold, it evolved to use many energy sources, including first a range of different chemistries and later light, and diversified across a range of environmental conditions.
    • 15.3: Searching for Life beyond Earth
      The search for life beyond Earth offers several intriguing targets. Mars appears to have been more similar to Earth during its early history than it is now, with evidence for liquid water on its ancient surface and perhaps even now below ground. This offers the exciting potential to directly examine ancient and modern samples for evidence of life. Recent research on exoplanets leads us to believe that there may be billions of habitable planets in the Milky Way Galaxy.
    • 15.4: The Search for Extraterrestrial Intelligence
      Some astronomers are searching for extraterrestrial intelligent life (SETI). Because other planetary systems are so far away, traveling to the stars is either very slow or extremely expensive (in terms of energy required). Despite many UFO reports and tremendous media publicity, there is no evidence that any of these are related to extraterrestrial visits. If we do find a signal someday, deciding whether to answer and what to answer may be two of the greatest challenges humanity will face.
    • 15.5: Life in the Universe (Exercises)

    Thumbnail: In this fanciful montage produced by a NASA artist, we see one roadmap for discovering life in the universe. Learning more about the origin, evolution, and properties of life on Earth aids us in searching for evidence of life beyond our planet. Our neighbor world, Mars, had warmer, wetter conditions billions of years ago that might have helped life there begin. Farther out, Jupiter’s moon Europa represents the icy moons of the outer solar system. Beneath their shells of solid ice may lie vast oceans of liquid water that could support biology. Beyond our solar system are stars that host their own planets, some of which might be similar to Earth in the ability to support liquid water—and a thriving biosphere—at the planet’s surface. Research is pushing actively in all these directions with the goal of proving a scientific answer to the question, “Are we alone?” (credit: modification of work by NASA).

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