About 35% of all known exoplanets which are bigger than Earth should be water-rich. These water worlds likely formed in similar ways to the giant planet cores Jupiter, Saturn, Uranus, Neptune in our own solar system. The newly-launched TESS mission will find many more of them, with the help of ground-based spectroscopic follow-up. The next generation space telescope, the James Webb Space Telescope, will hopefully characterize the atmosphere of some of them.
Many of the known exoplanets may contain as much as 50% water. This is much more than the Earth’s 0.02% (by weight) water content.
Their surface temperature is expected to be in the 200 to 500 degree Celsius range. Their surface may be shrouded in a water-vapor-dominated atmosphere, with a liquid water layer underneath. Moving deeper, one would expect to find this water transforms into high-pressure ices before we reaching the solid rocky core.
The radii of over 4000 exoplanet candidates have been precisely measured by the NASA Kepler Mission, along with their orbital periods and other parameters. The exoplanets sizes have a bi-modal distribution, with the main and
secondary peaks likely corresponding to Earth-like rocky planets (1.5 times the radius of the Earth) and larger intermediate-sized planets (2.5 times the radius of the earth). The masses of planets can be determined by groundbased spectroscopic observations, but only for planets orbiting the brightest stars. These observations, allow calculations of average densities and, thus, constraining their bulk compositions and internal structures. An important question about the compositions of the planets ranging from 2 to 4 Earth radii (RÅ) still remain. They may either have a rocky core enveloped in a massive H2-He gas (gas dwarfs) or contain a significant amount of multi-component, H2O-dominated ices/fluids (water worlds). The growth model tracks how mass and radius change when a planet population grow from rocky core and subsequently accrete either O-H-C-N-ices or H2-He gas. Because their composition cannot be uniquely constrained, we use growth model and Monte Carlo simulation for these planets to argue that many intermediate-sized planets are “water worlds”.
Aliens will not be coming for Earth’s water
Solar system is full of water moons
The five icy moons of Jupiter and Saturn show strong evidence of oceans beneath their surfaces: Ganymede, Europa and Callisto at Jupiter, and Enceladus and Titan at Saturn. The Asteroid Ceres has a water ocean under ice.