<P> The first confirmed discovery of an extrasolar planet orbiting an ordinary main - sequence star occurred on 6 October 1995, when Michel Mayor and Didier Queloz of the University of Geneva announced the detection of an exoplanet around 51 Pegasi . From then until the Kepler mission most known extrasolar planets were gas giants comparable in mass to Jupiter or larger as they were more easily detected . The catalog of Kepler candidate planets consists mostly of planets the size of Neptune and smaller, down to smaller than Mercury . </P> <P> There are types of planets that do not exist in the Solar System: super-Earths and mini-Neptunes, which could be rocky like Earth or a mixture of volatiles and gas like Neptune--a radius of 1.75 times that of Earth is a possible dividing line between the two types of planet . There are hot Jupiters that orbit very close to their star and may evaporate to become chthonian planets, which are the leftover cores . Another possible type of planet is carbon planets, which form in systems with a higher proportion of carbon than in the Solar System . </P> <P> A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way . </P> <P> On December 20, 2011, the Kepler Space Telescope team reported the discovery of the first Earth - size exoplanets, Kepler - 20e and Kepler - 20f, orbiting a Sun - like star, Kepler - 20 . </P>

Which of the following is a characteristic of the early solar system