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450 BC: Greeks ponder the existence of other worlds. 1400 - 1600: Nicholas of Cusa (1401-1464) and Giordano Bruno (1548-1600) dispute the Sun's uniqueness and singularity in the universe. Bruno is burned at the stake in 1600 for, among other things, his claim that there are other stars and solar systems besides our own. 1543: Nicolaus Copernicus becomes the first modern astronomer to suggest that the Earth, along with the other planets, orbits the sun, and not the other way around. His ideas, though controversial at the time, are the beginning of a new way of thinking about the design of the universe. 1609: Galileo Galilei introduces the telescope to astronomy. The new instrument helps reveal the universe in greater detail than ever before - Galileo uses it to observe the moons of Jupiter, providing further evidence of worlds besides our own. 1698: Christiaan Huygens (1629-1695) publishes Cosmotheros, a book that imagines what life would be like on other planets. 1750: Thomas Wright becomes the first to postulate that the Milky Way may be a massive disk of stars, and that some of the other objects in the sky may be disks of stars similar to our own. 1781: William Herschel discovers Uranus, the first new planet discovered since ancient times. 1855: Capt. W. S. Jacob of the East India Observatory in Madras, India, finds orbital anomalies in the binary star 70 Ophiuchi that he claims are evidence of an extrasolar planet - the first exoplanet false alarm. The "discovery" begins a 140-year period of other exoplanet discovery false alarms, but no actual planets are discovered. 1920: A. A. Michelson and Francis Pease use a stellar interferometer for the first time to measure the diameter of the star Betelguese at the Mt. Wilson observatory in California. Decades later, an interferometer at the Keck Observatory in Hawaii will play an important role in understanding the environments in which exoplanets form. 1925: Using the Hooker 100" Telescope in California, Edwin Hubble proves that galaxies are groups of stars like the Milky Way. The discovery is groundbreaking and forever alters our view of the universe. 1960: Dr. Frank Drake creates an equation that predicts the number of extraterrestrial civilizations in our galaxy that we might come in contact with - his answer is 2.31. The work is considered a groundbreaking method of pondering the likelihood of intelligent life elsewhere in the universe. 1966: Debut of television series "Star Trek." The show introduces exoplanets and alien life into pop culture, with numerous spin-offs, movies, and merchandise. Show creator Gene Roddenberry will later endorse 40 Eridani A - which may have an Earth-like planet - as the home solar system for Mr. Spock. 1990: The Hubble Space Telescope is launched in April. This influential mission will later aid in the identification and study of exoplanets, by watching for the dimming caused when a planet moves in between us and the planet's star. Hubble will also eventually take one of the first pictures of an exoplanet. 1992: Aleksander Wolszczan and Dale Frail discover two rocky planets orbiting PSR B1257+12, a pulsar in the constellation Virgo. Because they are constantly bombarded by radiation from the dead neutron star that they orbit, these rocky planets cannot support organic life as we know it. 1993: Planet PSR B1620-26 b is found orbiting a binary system composed of a pulsar and a white dwarf. Located 1,170 light-years from Earth, the planet, which is about two and a half times the size of Jupiter, takes over 100 earth years to complete a single orbit. It is also the first planet found in a star cluster, in this case, globular cluster M4. 1995: Michel Mayor and Didier Queloz find the first planet orbiting a main-sequence star, 51 Pegasi, a discovery confirmed a week later. The planet, which is half the size of Jupiter, orbits so close to 51 Peg that it practically grazes the surface, a revelation that baffles astronomers. 1999: Researchers from San Francisco State University and the Harvard-Smithsonian Center for Astrophysics working independently announce the discovery of a second and third planet orbiting the star Upsilon Andromedae in the constellation Andromeda. 1999: First transiting exoplanet observed. Research teams led by David Charbonneau and Greg Henry independently observe a planet passing across the front of the star HD 209458. The transit method of observation will later be used to analyze the atmospheres of exoplanets. 2001: Teams led by David Charbonneau and Timothy Brown use the spectrometer on the Hubble Space Telescope to probe the atmospheric composition of a planet orbiting the star HD 209458. Sodium was detected in the planet's atmosphere. Later observations would reveal a 200,000 km tail of hydrogen, carbon, and oxygen streaming from the planet. 2001: Astronomers from Geneva University discover HD 28185 b, a planet that orbits about the same distance from its star as the Earth does from our own Sun. The planet, which is nearly six times as massive as Jupiter, is the first to be found in the so-called "habitable zone" around a star where liquid water can exist. 2002: Astronomers discover a planet orbiting the giant star Iota Draconis, which is 13 times the diameter of the Sun and 40 times as bright. It is the first time an exoplanet has been found orbiting a giant star. 2002: After observing numerous solar systems with gigantic planets that orbit at extremely close to their stars or elliptical, astronomers Geoffrey Marcy and Paul Butler announce the discovery of a Jupiter-like planet orbiting the star 55 Cancri at a similar distance to Jupiter in our own solar system. 2003: The Spitzer Space Telescope and the Canadian space telescope MOST launch during the summer. Spitzer's infrared capabilities will later contribute to many exoplanet milestones. 2005: Astronomers use Spitzer Space Telescope to capture infrared light emanating from exoplanets HD 209458b and TrES-1. The findings mark the first time direct light from an exoplanet has been observed. 2006: The French CoRoT space telescope, which detects planets as they transit their host stars, is launched in December. 2007: NASA's Spitzer Space Telescope finds water vapor in the spectrum of the planet HD 189733b. It is the first time water has been detected on an exoplanet. 2007: NASA's Spitzer Space Telescope takes the first spectroscopic measurements of an exoplanet's atmosphere. Later, data from Spitzer is used to create the first temperature map of an exoplanet. 2008: On the same day, two separate groups of astronomers release the first ever visible-light exoplanet images, using the Hubble, Keck, and Gemini telescopes. 2008: Astronomers using the Hubble Space Telescope measure the first sign of an organic molecule on an exoplanet. The discovery is made by analyzing the spectrum from HD 189733 b. 2009: NASA's Kepler mission launches in March. Kepler observes thousands of stars at once, looking for transiting Earth-like planets. The results will tell astronomers how plentiful planets like Earth are in the galaxy. THE FUTURE: The hunt for exoplanets continues at a rapid pace, and the discovery of a world much like Earth could be right around the corner. This timeline will continue to update with the latest exoplanet milestones, so check back to see how the story continues...
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This interactive multimedia timeline traces the search for extrasolar planets, from ancient philosophical speculation to modern discoveries.