When Pluto was downgraded from being called a planet, it made you ponder over the criteria for the classification of a planetary object. In reality, the proper definition of a planet is more important than just the semantics. It can provide astronomers finding new celestial objects with a basis of classification, thus making it easier to identify if the object under investigation is a planet or not. This distinction is important for scientists as it provides indirect ways of inferring properties of those “planets” by using ideal comparisons with closer, and easier to observe, planets.

Kevin Schlaufman, an astrophysicist at Johns Hopkins University, has proposed an upper limit on the mass of space objects that qualify as a “planet”. According to his most recent study, this upper bound of a mass should be between four and ten times the mass of Jupiter. However, this is only an exclusionary criterion, disqualifying anything heavier than that as a planet. Anything lower in weight would need further investigation in order to classify. This finding is significant because it helps to distinguish a very large planet from a brown dwarf – an object whose mass lies between that of a very large planet and a star. These are self-luminescent, and hotter than a planet due to deuterium and lithium fusion, but they are not massive enough to support the nuclear fusion of protium (hydrogen).

“Another criterion for deciding class is the chemical make-up of closest stars”

According to Schlaufman, the current classification is insufficient and leaves ambiguity in results. He proposed another criterion to settle the debate: the chemical make-up of the closest stars. This argument roots from the process of the formation of stars. The giant planets such as Jupiter form from a bottom-up process – a rocky base is built, which is then enveloped by a gaseous atmosphere. Since planets cannot support the fusion process, they depend on their neighbouring sun to provide these rocks to form the heavy elements. But the brown dwarf forms due to gravitational instability which leads to the mass of clouds to collapse over itself. This process is like the formation of a star.

So how do the planetary limits become relevant? Well, Schlaufman analysed 146 planetary systems and inferred a few facts about space objects: those greater than ten times the size of Jupiter are seldom found near these heavy metal enriched stars who provide resources for their development. Thus, any celestial object lying above the upper limit identified should be categorised as a brown dwarf as opposed to a planet. Though this may end up opening a fierce debate – similar to the exclusion of Pluto as a planet from our solar system – but the development of a robust definition eases the identification of new planets and their study. As our picture of the universe gets bigger, a more methodical definition is required to help provide a clearer view.