CHAPTER 1 UNEXPECTED PLANETS in the 1980s Earth and the other planets orbiting the sun were the only planets known in the universe Since then astronomers have found thousands of planets circling other stars These planets are called extrasolar planets or exoplanets for short Astronomers had discovered more than 3 500 exoplanets by 2018 Some scientists have found signs that there may be even more planets outside our solar system Many of the first exoplanets scientists discovered were huge gas giants 5
However more research is still needed to confirm that these exoplanets exist Astronomers found the first two exoplanets in 1992 These exoplanets look nothing like Earth They orbit a dead star When a star dies its center sometimes collapses into a dense glowing cinder This type of leftover star is very different from the sun Scientists were surprised to find planets orbiting these remnants Three years later scientists found an exoplanet moving around a star similar to the sun But this exoplanet is quite different from Earth The exoplanet travels around its star in only four days That means it orbits very close to its star Because it is so close to its star the exoplanet is much hotter than Earth In addition the exoplanet weighs as much as Jupiter That means it is more than 300 times as massive as Earth
A KELT 9b right the hottest exoplanet discovered so far is even warmer than some stars This type of large hot planet is known as a hot Jupiter During the next decade astronomers found many other large hot exoplanets Some are so close to their stars that their surfaces are hot enough to melt iron
