In-Between Planets

Over the past five years, the Kepler space telescope has discovered thousands of possible planets in other star systems. Some are small, like Earth. Others are big and fluffy, like Jupiter, the giant of our solar system. For veteran planet hunter Geoff Marcy, though, the big surprise is what’s in between.
MARCY: Perhaps the most remarkable discovery by Kepler are these planets between the size of Earth and four times the size of Earth. This is a size range that dominates the planet inventory from Kepler.... These are planets we never expected based on our own solar system. [:19]
Kepler discovered the planets as they passed in front of their parent stars. These “transits” revealed how big the planets are. Astronomers then used ground-based telescopes to measure the gravitational pull of some of these planets, which revealed how massive they are. The combination revealed how dense the planets are, which provided a rough idea of what they’re made of.
MARCY: Those smaller than twice the si