Exoplanets, also known as extrasolar planets, are planets that exist outside our solar system, orbiting a star other than the Sun. These planets can vary in size, composition, and distance from their host star.

The discovery of exoplanets has revolutionized our understanding of the universe and the prevalence of planetary systems. Before the 1990s, exoplanets were purely theoretical, but with advancements in technology and observational techniques, astronomers have been able to detect and study these distant worlds.

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There are several methods used to detect exoplanets:

1. Radial Velocity Method: This method detects exoplanets by measuring the slight wobble of a star caused by the gravitational tug of an orbiting planet. As the planet orbits its star, it causes the star to move back and forth, which can be observed through changes in the star's spectral lines.

2. Transit Method: The transit method involves detecting the periodic dimming of a star's light as an exoplanet passes in front of it. By measuring the decrease in brightness and the regularity of the dimming, astronomers can determine the presence and characteristics of the exoplanet.

3. Direct Imaging: Direct imaging involves taking pictures of exoplanets. This method is challenging because planets are much fainter than their host stars and are often overwhelmed by the star's brightness. However, advancements in adaptive optics and specialized instruments have made it possible to directly image larger exoplanets.

4. Gravitational Microlensing: Gravitational microlensing occurs when the gravitational field of a star bends and magnifies the light from a more distant star. If a planet orbits the lensing star, it can cause additional magnification, revealing its presence. This method is sensitive to detecting distant and cold exoplanets.

5. Astrometry: Astrometry involves measuring the precise position and motion of a star. The gravitational pull of an orbiting planet causes the star to move slightly, and by tracking these movements, astronomers can infer the presence of an exoplanet.

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Since the discovery of the first exoplanet in 1992, thousands of exoplanets have been detected, and the variety of known exoplanets continues to expand. They can range from rocky, terrestrial planets to gas giants similar to Jupiter. Some exoplanets even reside in the habitable zone of their star, where conditions might be suitable for the presence of liquid water and potentially life as we know it.

The study of exoplanets has provided valuable insights into planetary formation, the diversity of planetary systems, and the potential for habitability beyond our solar system. Ongoing missions and future space telescopes, such as the James Webb Space Telescope, will further enhance our understanding of exoplanets and the search for signs of life elsewhere in the universe.