> In that no, these are not the first water worlds we've discovered. And the author knows that since they directly compare to Europa in the subtitle and second

> In that no, these are not the first water worlds we've discovered. And the author knows that since they directly compare to Europa in the subtitle and second paragraph. As explained in subsequent sentences/paragraphs, these are different from Europa in that they very likely have liquid water surfaces rather than being encased in ice. That's what makes them water worlds instead of ice worlds.