Extrasolar planet. Artwork of a gas giant planet (lower right) in orbit around a star. A moon of the planet is seen at upper left. Several stars are thought to have planets around them, which have been detected by observing the effect the planet's gravity has upon the star's path. Due to the huge masses necessary to make such effects observable, however, all such predicted planets have been gas giants, planets similar to Jupiter and Saturn. These have no solid surface, instead they are composed of gases which become denser and denser the deeper into the planet they get.