Exoplanet Water Worlds: What are they and where can we find them?

More than 5000 planets orbiting stars other than the sun have been discovered to date. These extrasolar planets or exoplanets are a diverse population, with a stunning variety of sizes, temperatures, and planetary system architectures. While some groups of exoplanets such as cold Jupiters resemble the planets of the solar system, new archetypes such as super-Earths or mini-Neptunes have also appeared. By studying these new classes of planets we can gain a deeper understanding of how planets form an evolve, what they are made of, and what new discoveries might still await us as planet detection and analysis techniques improve. Water worlds are one such class of exoplanets whose existence has been predicted based on planet formation models but for which there is no unambiguous evidence so far. These exoplanets would resemble the icy moons of the outer solar system in bulk composition, with equal amounts of rock and water ice in their interior. Because they are also much larger, hotter, and closer to the star they would be detectable with current observations. In this talk I will explain how we constrain the nature and compositions of transiting exoplanets from the population statistics as a whole. Depending on how water is distributed within exoplanets -- in the form of liquids, ices, or steam -- between 15% and 50% of the transiting small planets could be water worlds. I will show how these planets have escaped direct detection so far, and why we should be able to identify water worlds in the coming years.