How common are Earths? How common are Jupiters?
  Among the billions of planetary systems that fill the Universe, we would like to know how ours fits in. Exoplanet data can already be used to address the question: How common are Jupiters? Here we discuss a simple analysis of recent exoplanet data indicating that Jupiter is a typical massive planet rather than an outlier. A more difficult question to address is: How common are Earths? However, much indirect evidence suggests that wet rocky planets are common. 
Charles H. Lineweaver, Daniel Grether and Marton Hidas (School of   Physics, University of New South Wales and the Australia...