Astronomers say that each of the 100 billion stars in the Milky Way probably has at least one companion planet. Until the first part of the 20th century, scientists believed our home galaxy was the entire universe and, until 1994,that the planets in our solar system were the only ones we knew. With the aid of the Kepler Spacecraft, launched in 2009, an international team of 42 scientists has been surveying millions of stars in the Milky Way. They've discovered that planets may be as plentiful as grains of sand on a beach, and that many stars likely host planets with mass five times that of earth. Some stars are home to gas giants like Jupiter. And some planets may circle not one but two stars--a phenomenon so unlikely it was previously only considered in science fiction. We may need to keep searching for ET, though; none of the planets detected so far appears suitable for conventional carbon-based life as known on Earth.
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