Alien planet 51 Pegasi b and its sun. Artwork of 51 Pegasi b crossing its parent star 51 Pegasi, as seen from the surface of a moon. 51 Pegasi b is a gas giant that was discovered in 1995. This discovery was the first to find an extrasolar (outside the Solar System) Sun-like star with a planet orbiting it. One orbit of 51 Pegasi takes the planet just 4 days, as it is only 7 million kilometres from the star, equivalent to 1/6th of Mercury's orbit around the Sun. Although 51 Pegasi is a Sun-like star, it is almost twice as old as the Sun, being about 7.5 billion years old. It lies some 50 light years from Earth, in the constellation Pegasus.