What is an exoplanet? An exoplanet (short for extrasolar planet) is any planet which orbits a star other than our sun. To date, scientists have discovered about 2,000 exoplanets within our Galaxy. Scientists now believe that every star in our Galaxy has a planet or planets. The first planet detected outside of the solar system was in 1992, with the discovery of several terrestrial planets orbiting a pulsar. The first detection of a planet around a main sequence star was in 1995, when scientists discovered a massive gas giant orbiting the star 51 Pegasi. The planet was named 51 Pegasi b. Scientists were stunned by the discovery of this planet, not only because it was the first exoplanet detected, but because it was a new kind of planet that scientists thought couldn't exist. It's known as a hot Jupiter. These planets are massive gas giants like Jupiter that orbit very close to their parent star. Some orbit even closer to their stars than Mercury does to our sun. Scientists originally believed that gas giants could only exist in the outer regions of solar systems, being that the temperatures in the inner solar system are much too hot to allow the formation of gas giants. Scientists proposed the theory or planetary migration, which suggests that planets can migrate inward toward their parent star. 51 Pegasi b and many other exoplanets, were discovered using the radio velocity method. Today, nearly every exoplanet found is found using the transit method. Other methods, such as direct imaging, have been used to find only a handful of exoplanets.
The first confirmed exoplanet to orbit within the habitable zone of its parent star was Kepler 22b, discovered in 2011. Since then, scientists have discovered quite a few Earth-like planets around other stars. Many belong to a new class of planet, known as a super-Earth. Based on new estimates, scientists have estimated that about 1 in 5 stars has an Earth-like planet orbiting within the habitable zone. Scientists calculate that about 11 billion of these orbit sun-like stars. This number, however, is raised to 40 billion once red dwarfs are included. The numbers seem to agree that we may not be alone in this universe.