There are billions of objects found in space with astronomers making discoveries of such objects each year. Planets are some of the most common celestial objects and also some of the most studied. Astronomers have come up with several different classifications used to categorize planets. One such method is the classification of planets based on their mass regime. There are eight distinct categories of planets in this classification which are giant planets, mesoplanets, mini-Neptunes, planemos, planetar, super-earths, super-Jupiters, and sub-earths.
8. Giant Planet
A giant planet is a massive planet that is primarily made up of materials which have low-boiling points which are mainly gases or ices. However, there are several giant planets which are composed of solid matter. The four largest planets in our solar system, Neptune, Uranus, Saturn, and Jupiter, are examples of giant planets. There are numerous other giant planets found orbiting other distant stars with one example being Kepler-10c. Giant planets are characterized by having a thick atmosphere and in most cases, the presence of a molten core. However, giant planets with extremely high temperatures may not have a core as the heat can dissolve and disperse the materials in the core. Giant planets are also known as jovian planets (named after Jupiter) or gas giants. However, the “gas giant” definition is a matter of contention as the composition of some giant planets such as Uranus and Neptune (made up of methane and ammonia) is different from that of actual “gas giants” such as Saturn and Jupiter (made up of helium and hydrogen). The term “giant planet” was introduced by James Blish, a science fiction writer in 1952.
Mesoplanets are the planets whose sizes are larger than that of minor planets but are still smaller than major planets. Mesoplanets can also be defined as planets which are larger than Ceres (the largest known minor planet) but smaller than Mercury (the smallest known major planet). Based on this definition, a mesoplanet has an equatorial diameter ranging between 1,000 km and 5,000 km. The term mesoplanet is derived from Greek term “mesos” which translates to “middle.” The term was introduced in the 1980s by essay writer Isaac Asimov in his essay entitled“What’s in a Name?” published in the Los Angeles Times. Asimov coined the term as a classification of Pluto which had been declassified from being a planet. Asimov argued that such planets would need a distinct classification since they were smaller than major planets but larger than minor planets.
Mini-Neptune is the term used to define a planet that has a smaller mass than that of either Uranus or Neptune. The mass of a Mini-Neptune planet is up to 10 times that of Earth (in comparison, Uranus and Neptune have masses which are 14.5 and 17 times that of earth respectively). Mini-planets share several characteristics with Neptune and Uranus as they are covered by a thick atmosphere made up of hydrogen and helium, and have surfaces covered either by ammonia or water either in liquid or ice form. A Mini-Neptune planet has a small core which is made up of compounds of low volatility. There are no Mini-Neptune planets in the Solar System, but there are several exoplanets which fit the description. These planets are usually orbiting far away from their respective parent stars where low temperatures prevent the thick helium and hydrogen-based atmospheres from evaporation.
A planemo is a term given to all celestial bodies which have the characteristics of a planet. These characteristics are that the celestial body should have enough mass to have a spherical shape formed by its gravity but not massive enough to have core fusion seen in stars. A planemo can also be referred to as a planetary body or a planetary-mass object. All planets in the Solar System fit this description and are therefore all planemos. Other celestial objects in our solar system which can be classified as planemos are dwarf planets such as Pluto, large moons such as Europa, and sub-brown dwarfs.
A planetar is a celestial object which has a mass that exceeds that of the largest gas giant planets but are less massive than the lightest stars. Unlike stars, the masses of planetars cannot sustain nuclear fusion in their respective cores. Planetars are also less luminous than stars at visible wavelengths and would appear magenta to the human eye. Another distinct feature in planetars is the presence of Lithium which is absent in stars. Some planetars orbit around big stars while other planetars are massive enough to have planets orbiting around them with examples of such planetars being MOA-2007-BLG-192Lb, Teide 1, and 2M1207b. Planetars are further divided into two main categories which are brown dwarfs and sub-brown dwarfs. Luhman 16 is the planetar closest to our solar system at a distance of about 6.5 light years.
A super-earth is a term given to an extrasolar celestial object which is more massive than the Earth but still considerably less massive than the two ice giants of the solar system, Neptune and Uranus. The term is exclusively used in relation to a planet’s mass and not its habitability, surface temperature, composition, or conditions of its surface. Super-earths share the same characteristics with mini-neptunes but are less massive than mini-Neptunes. The first super-earth to be discovered was Gliese 876d, which orbits around Gliese 876. This super-earth is 7.5 times more massive than Earth and therefore fit the description.
A super-Jupiter is an extrasolar celestial object which is more massive than the largest planet in our solar system, Jupiter. The planets which are classified as super-Jupiters have their respective densities being directly proportional to their respective surface gravities. The largest super-Jupiter has a mass that is 80 times that of Jupiter. There are about 180 known super-Jupiters in space with one being Corot-3b which has a mass 22 times that of Jupiter. Another example of a super-Jupiter is Kappa Andromedae b.
Sub-earths are defined as planets which are substantially less massive than Venus and Earth. There are only two sub-earth planets in our solar system, and these are Mercury and Mars. Planets which fit the description are characterized by lack of a substantial atmosphere due to the presence of a weak magnetic field and a low gravity force on such a planet. Astronomers state that sub-earth planets are the hardest to discover due to their small sizes.