The Kepler Mission was designed to answer one question: how many planets like Earth are out there? Are worlds like our own a common outcome of planet formation, or are they rare?
While the Kepler Mission was designed to answer this question about Earth-sized planets in Earth-like orbits, it was also able to answer this question for planets bigger than Earth and for planets closer to their stars than Earth. We can ask more broadly, what other kinds of planets are out there? What types of planets are common, and what types of planets are rare?
Kepler answered these questions with the power of statistics. By staring at 150,000 stars for four years and searching for transiting exoplanets, Kepler allowed scientists to survey what kinds of planets do and do not exist.
The statistical analysis is easy at first. We begin with counting how many planets of a certain size Kepler found. How many Jupiter-sized planets? How many Neptune-sized planets? How many Earth-sized planets? What about planets of in-between sizes? How many of each of these planets do we find at a variety of different orbits? We can cross-cut and tally up the Kepler planet harvest to get a first glimpse of which planets are common and which are rare.
But there is a twist: some planets are easier for Kepler to find than others. Planets closer to their stars are more likely to transit than planets farther away, so our simple tally has under-counted the number of far-out planets. Also, planets that make deep transits are easier to find than planets that make shallow transits, and so our tally has under-counted the number of small planets. With some robust geometry, algebra, and signal processing, my colleagues Petigura, Howard, and Marcy (2013) corrected for these counting missteps, creating a clearer picture of which planets are common and which are rare.
One important caveat to this survey is that planet size does not necessarily correspond to planet composition. Just because a planet is Earth-sized and in the habitable zone does not mean that it is a rocky world, let alone a world with continents, oceans, and life. To learn more about these attributes, we will need to design and build new telescopes.