The first confirmed discovery of an exoplanet was made in 1995 with the disvcovery of a giant gasplanet 51 Pegasi b. Since then 500 other giant gasplanets were  discovered, but only a few smaller Earth-sized planets. The Kepler spacecraft launched in 2009 has now given astronomers their first broad overview of the structure and diversity of planetary systems in the Milky Way.

Astronomers announced recently that, beyond our solar system, there are hundreds of possible planets in a small region of the Milky Way Galaxy. These potential planets range from gaseous planets much larger than Jupiter to suspected rocky planets a few times more massive than Earth.  As of September 13, researchers had confirmed 20 of these 1.235 candidates are actual planets.

The most dramatic result from Kepler so far comes in two forms: one is the discovery that there are more small planets than large planets in our Milky Way Galaxy. There are almost certainly way more of the smaller planets, almost down to the size of the Earth, than planets the size of Neptune, Saturn or Jupiter. That’s an extraordinary result. It doesn’t speak to diversity, but it certainly carries the profound implication that our Milky Way galaxy has more of the smaller planets that are mini-Neptunes and nearly Earth-size.

The other remarkable discovery is that approximately 120 Kepler stars have been found with two or more planets that transit the star, which indicates that in many cases planetary systems have planets that reside in the same flat plane. This flat structure is exactly what we see in our own solar system. That’s a sort of anti-diversity, if you will, in the sense that flattened planetary systems were expected both from our Solar system and from the flat protoplanetary disks within which planets form. We have learned that planets must have all formed in some kind of flattened disc of gas and dust around young stars.

Sara Seager says: "The diversity of exoplanets has really forced us to reconsider what the habitable zone really is. For example, some of these super-Earths are massive enough that they could retain a different atmosphere than we have on Earth. These super-Earths may hold on to the light gases, hydrogen or hydrogen and helium. In this case, if they have a massive atmosphere they could have a massive greenhouse effect. This could actually increase the range of the habitable zone in a planetary system"

Source: Kavli Istitute