Those planets that had less water tended to have much less, to the point where deserts dominated the landscape. Also, size plays a role. Larger habitable planets (including Earth) are more likely to be water worlds thanks to deeper oceans and stronger gravity, according to the calculations, while smaller ones are drier.

If reasonably accurate, the data points to Earth hitting a rare sweet spot, possibly due to unusually deep water basins. And that makes sense at first glance. Despite what Earth looks like, water only occupies a tiny amount of volume compared to the rest of the planet. It wouldn't take much more to inundate the land, or much less to make it barren. You can see for yourself in the video below.

There is reason to be skeptical. Astrophysicist Sean Raymond warns Gizmodo that there are still a number of unknowns that may play an important role in water levels, and recent models suggest that water delivery to planets is relatively "reliable" with fewer surges or shortfalls. However, Simpson is quick to add that his theory should be testable soon. Future instruments (likely including the James Webb Space Telescope) will have enough power to measure the atmospheric compositions of alien planets, giving a clue as to how much water there is on the surface. If nothing else, the study is a reminder that we shouldn't assume a planet is human-friendly just because there's plenty of H2O.