There is an interesting new paper out about the seven near-Earth-sized exoplanets orbiting the star TRAPPIST-1. According to the study, four of the planets may be true water worlds, although just what form those may take isn’t clear. All seven planets are close in size to the Earth, with some of them in the star’s habitable zone, where temperatures could allow liquid water on rocky surfaces.
From the paper:
“The newly detected TRAPPIST-1 system, with seven low-mass, roughly Earth-sized planets transiting a nearby ultra-cool dwarf, is one of the most important exoplanet discoveries to date… Figure 4 indicates that – to within the errors of our determinations – the four most distant planets are consistent with pure water compositions, and in any event, are substantially less dense either Mars or Venus… The planets orbiting TRAPPIST-1 arguably constitute the most important exoplanetary system yet discovered. The planets’ large observed transit depths, coupled with the occurrence of extensive transit timing variations, present an extraordinary opportunity to discern the masses, the densities, the compositions, and the dynamical architecture of low-mass worlds. As more data are collected, substantial insights will be gained by an evolving comparison of these newly detected planets to the familiar terrestrial worlds of our own Solar System.