Researchers can make those estimates even from 39 light years away by observing planets as they pass in front of their star. They can get a general idea of a planet's size, but also its density, which tells us a lot about what it’s made of. These were about the size of Earth, but much, much lighter. Not so light that their weight could be attributed to some gaseous atmosphere, but not solid as a rock, either. That leaves water, and presents a world of new questions for planetary scientists.