It was earlier this month that a team of astronomers revealed the first discovery of a potential exo moon – nothing more than a satellite gravitating to a planet close to another star. But the most striking of this discovery is the scale of this possible planet-moon system. In this case, the “moon” appears to be the size of Neptune, while the planet it orbits is about 10 times the mass of Jupiter, about 3000 times the mass of Earth.
The system pushes the limits of how we normally classify objects into space and leaves us questioning the scale of things. What will be the largest possible size for a planet to have? Considering the full range of possibilities, is the Earth a large planet or will it be small?
There are different ways of answering this question if we think about the size of a planet in terms of mass, so there is a specific, though quite technical, response. The planets are defined as bodies that do not generate their own energy from nuclear fusion. That is, any planet with more than about 13 times the mass of Jupiter (4000 masses of Earth, roughly) generates enough heat and pressure in its nucleus to trigger limited fusion reactions of deuterium, a heavy isotope of hydrogen. There, the object is considered a brown dwarf instead of a planet.
The boundary of nuclear ignition between the planet and the brown dwarf is based on hidden internal processes, yet these processes are not at all obvious from the outside. The critical mass for fusion also depends on the combination of elements within the object. For a plausible range of compositions, the cut-off point may be 11 to 16 times the mass of Jupiter.
Being outside this gray area that makes it difficult to know if it is a brown planet or dwarf, everything becomes clearer. That is, anything below the lower limit of 11 masses of Jupiter (3500 Earth masses, more or less) is unquestionably planetary.
However, anything above this value is an object in space capable of even creating its own energy, which removes any standard astronomical definition of the planet.