To date over 700 planets have been discovered outside our own solar system, a number that seems to grow every day. The majority of these planets are Jovian or super-Jovian planets; a category that includes everything bigger than 40% of the mass of Jupiter. The next most common category are super-Earth or Neptune sized planets, with less than 10% of the mass of Jupiter. We have even discovered a small number of Earthlike planets, of similar size to our planet, or a little smaller. Planets of a similar size to Saturn, between 10% and 40% of the size of Jupiter, are relatively unusual.
This is thought to be largely a product of our sampling methods. Very large planets are easy to detect, since their gravity effects their host stars, causing them to wobble as the planets move around them. Planets close to their stars are also fairly easy to observe, since they transit their stars quite often, causing regular dips in the light output of the star that we can detect. Thus the planets we detect the most easily are very large planets close to their stars, planets we suspect are quite unusual, but which we find readily. Conversely we suspect that planets of similar size to Saturn in similar orbits are fairly common, but this does not make them any easier to find. Saturn-sized planets close to their stars do not appear to be common, but this is not unexpected.
On 24 January 2012, a paper appeared on the arXiv online database at Cornell University Library, in which a team lead by Bun'ei Sato of the Department of Earth and Planetary Sciences at the Tokyo Institute of Technology, describing the discovery of a planet orbiting the star GSC 2314-00559. The paper has also been accepted for publication in the Publications of the Astronomical Society of Japan. The discovery was made using the Hungarian-made Automated Telescope Network (HATnet) which uses telescopes at the Fred Whipple Observatory in Arizona and at the Smithsonian Astrophysical Observatory in Hawaii and back up observations made using Subaru Telescope of the National Astronomical Observatory of Japan, also in Hawaii.
The Subaru Telescope.
GSC 2314-00559 is a G-type yellow dwarf star 812 light years from Earth. It has 89% of the mass of our Sun, and 92% of its radius; it is thought to be about 1.7 billion years old. As the 38th star hosting a planet discovered by the HATNet Project it has been renamed HAT-P-38. Orbiting this at a distance of 0.0523 AU (5.23 % of the distance at which the Earth orbits the Sun, or 13.4% of the distance at which Mercury orbits the Sun) is HAT-P-38B, a planet with 26.7% of the mass of Jupiter and 82.5% of its radius. The planet is highly irradiated due to the nearness of the star, with an estimated equitorial temperature of 1082 K, compared to 134 K for Saturn. Despite being so strongly heated, HAT-P-38b has an average density of 0.59 g cmˉ³, compared to 0.687 g cmˉ³ for Saturn (heated to the same temperature, Saturn would expand considerably, lowering its average density), suggesting that the overall makeup of the planet is not that similar to Saturn; Sato et al. suggest a large rocky core may be raising the overall density.
See also A planet disintegrating in the heat of its own sun, Wasp-19b: a Highly Irradiated Hot Jupiter, A new study of the HAT-P-13 planetary system and Exoplanets on Sciency Thoughts YouTube.