Waterworlds: Structures / Properties and Discoveries of Ocean Planets Andria C. Schwortz Department of Physics and Astronomy, University of Wyoming, 1000 E. University Ave., Laramie, WY, 82071 aschwort@uwyo.edu ABSTRACT In recent years a number of surveys, including the MEarth project (located in Ari- zona and run out of Cambridge, MA), and the COROT and Kepler space missions, have been dedicated to the discovery of extrasolar planets through transits. The properties (including structure and composition) and formation histories of many of the newly discovered exoplanets remains an open question. Driven by the search for an Earth-like planet, we have discovered a class of super-Earths or sub-Neptunes which may not fall into either of the traditional categories of terrestrial or gas giant planets, but instead form a new category of ocean planets. In this review paper, I outline the history of this field by looking at major theoretical breakthroughs and outlining recent discoveries of candidate ocean planets. 1. Definition When the general public thinks of a water world or ocean planet, they typically think of the film Waterworld starring Kevin Costner (Reynolds 1995). The film’s eponymous Waterworld planet is revealed in the opening sequence to be the planet Earth at some indeterminate time in the future when the polar ice caps have completely melted and the entire surface of the planet is covered with water. Approximately 2/3 of the surface of the Earth is currently covered with liquid water, with the fraction increasing if ice caps and glaciers are also included; (ignoring issues of conservation of mass) the fictional Waterworld is essentially 100% covered by water. However, either Earth or Waterworld are still fundamentally terrestrial planets: most of their mass is comprised of either metals (referred to as the core 1 henceforth) or of rocky material (mantle 1 ), with essentially 0% of their mass in either liquid water or ice form. 2 1 In this paper, the terms “core” and “mantle” are used in the same way as for terrestrial planets, i.e., based upon their composition, with “core” meaning the metal-dominated region and “mantle” meaning the rocky region. Some other papers in the field of planetary science use the terms in the context of gas giants, where “core” refers to both metals and rocky materials, and “mantle” refers to the icy layer, i.e., based upon their role within the structure of the planet. 2 The vast majority of water ice on Earth is in the phase known as ice I h , also called ice-I or hexagonal ice. Some atmospheric ice takes the form of ice Ic or cubic crystalline ice, and further phases of ice exist through ice XV. While