Anyone growing up in the 1990s or earlier would recollect that our solar system had nine planets, but did you ever wonder if planets exist outside the solar system? Planets found outside of our solar system are called extrasolar planets or exoplanets. Approximately 5,600 exoplanet candidates have been discovered since 1993, and nearly 2,000 exoplanets have been confirmed since 1995. Now we know that billions of stars in our galaxy have one or more planets orbiting them. Since the universe has billions of galaxies and trillions of stars, there are at least trillions of planets in our universe.
Astronomers have long wondered how these planets form. According to the “solar nebula” theory, the gravitational collapse of a cloud of gas leads to the formation of a star and planets orbiting it. When such a gas-cloud collapses under its own gravity, most of its mass accretes into the central region of the cloud. Then nuclear fusion begins producing energy in this very high-density and high-pressure central region, and thus, a star is born.