Last week, a team of astronomers reported the first potential discovery of an exomoon — a satellite orbiting a planet around another star. Part of what is so striking about the report is the scale of this possible planet-moon system. In this case, the “moon” appears to be about the size of Neptune; the planet it orbits is some 10 times the mass of Jupiter, or about 3,000 times the mass of Earth!
The system pushes at the limits of how we normally categorize objects in space and invites questions about where we stand in the scale of things. What is the biggest possible planet? Viewed through the full range of possibilities, is Earth a big planet or a small one?
