We will need a simple way to communicate results as part of the Habitable Exoplanets Catalog. Our Habitable Exoplanet Classification focus only on habitable planets and a broader classification will be appropriate for comparisons, including the Solar System. The only formal classification of planets is the approved on 2006 by the International Astronomical Union where Pluto was demoted. This classification was specifically designed for Solar System objects and does not applies to exoplanets. Many other informal classifications have been proposed based on mass, size, composition, temperature, distance from the Sun, or history.
The most famous classification of planets is the Star Trek's Planetary Classification. Unfortunately, it is quite complex, lacks scientific rigor, and therefore only used in sci-fi contexts. Other obscure examples within the web include the Meghar Scale - Planetary Mass Classification, the exuberant Planetary Classification Scheme by The Science and Exploration Society, the Planetary Mass Classification by Christopher Dybala, and even a metric classification. There is a zoo out there of such schemes.
More formal and scientific classifications suggested the use of mass as the basis for a simpler classiffication (Stern and Levison, 2002), or the use of the abundance of elements more important for life (Lineweaver and Robles, 2006). In any case, any classification should be physically based, determinable on easily observed characteristics, quantitative, uniquely, robust to new discoveries, and be based of the fewest possible criteria (Stern and Levison, 2002).
We decided to use mass as the basis for our classification because it is the most measured planetary parameter for confirmed exoplanets (Figure 1). Missions like Kepler are only able (with a few exceptions) to measure radius, but mass-radius relationships could be used to temporally classify those too until mass measurements are available (Figure 2). We also tried to preserve the current common usage by the exoplanet community but to also extend the scale to include Solar System objects. The name Earth was no used to avoid a more direct comparison with Earth-like planets. The latin term "terra" was preferred because it is of common knowledge as a less-direct terrestrial reference. Table 2 summarizes the classification.
Figure 1. Proposed classification of planets based on mass. There are seven types (red labels) which can be use for both Solar System and Exoplanets. Only the last four, Terran, Superterran, Neptunian, and Jovian are applicable to exoplanets because less massive objects cannot be detected. The current common classification of exoplanets are as Earths (or terrestrial), Super-Earths, Neptunes, and Jupiters (blue labels). These names work fine as plurals but become proper names as singulars. The proposed names have a latin root and work both as singular or plural (i.e. terran or terrans). They correspond to the larger or common object in the Solar System within the mass range (exception: subterran was preferred to martian). The current classification for Solar System objects is shown for reference (green labels) but it mix composition and location. Most of the mass boundaries were selected for physical reasons but they are not hard-limits and could be refined.
Figure 2. Mass-Radius relationship for 46 Solar System objects. Rocky bodies line in red. The icy bodies blue line corresponds to those outside the snow line (~2.7 AU). These curves can be used to predict mass or radius based on the other for extrasolar systems but there are large differences based on composition and distance to the star (i.e. hot jupiters are inflated). Check the scientific literature for more specific relations (i.e. Sotin et al., 2007).
Table 1. Description of the proposed classification of planets based on mass. Radius were given as approximations as they are highly dependent in the composition of the planetary body. The Solar System mass-radius relations from Figure 2 were used to estimate the minimum radius, as rocky composition, and maximum radius, as icy composition, causing overlapping.
* The cold zone is the zone farther from the star after the habitable zone (which includes the snow line), the hot zone is the zone between the star and the habitable zone.
Some additional interesting notes: