Question:
How are extrasolar planets usually classified in terms of their sizes or composition?
Answer:
by Dane Späth, University of Heidelberg
A general classification of exoplanets is difficult to achieve and is heavily discussed among researchers, because the assignment of a planet into a framework is strongly dependent on the point of view of the respective research field. Still, let’s give it a try to present the most important groups of planets inside and outside our Solar System.
Among the most important characteristics of a planet are its size – which we can classify here by its radius, its mass, and its composition (gaseous or solid). To better categorize extrasolar planets, many scientists and journalists use comparisons to better known planets in the Solar System.
Starting with small radii and masses, we begin with the group of terrestrial planets (also known as rocky planets). These are comparatively small and low-mass planets, about the size of Earth or smaller, that are composed mostly from solid materials like rock or silicates and usually feature a heavy iron core. At the surface, there can be liquid or gaseous ingredients like water or an atmosphere. Within the Solar System, all inner planets – Mercury, Venus, Earth, and Mars – are slotted into the group of terrestrial planets.
Towards higher masses, terrestrial planets are followed by Super-Earths. These are admittedly heavier than Earth, but lighter than, for example, the ice giants Uranus and Neptune (see below). The word “Super-Earth” is here defined somewhat vaguely; according to some definitions, it encompasses both the larger rocky planets (which are generally found to have up to about two times the radius for Earth) and planets that correspond to smaller versions of the Solar System planet Neptune, and have radii of about two to four times that of Earth. However, according to other definitions, this second group is also called Mini-Neptunes. According to this definition, the difference between Super-Earths and Mini-Neptunes lies primarily in their respective compositions. While Super-Earths usually have an Earth-like composition out of rock and metal, Mini-Neptunes usually feature a significantly higher amount of gaseous and fluid materials. This shows that even at similar sizes, the formation history of these planets is probably quite different. Unlike for the terrestrial planets, there are no representatives of Super-Earths/Mini-Neptunes in our Solar System.
The class of Neptune-like planets, sometimes also called ice giants, denotes a group of planets that, like Neptune and Uranus probably do, have a centre of rock and ice, but surrounded by a very deep and dense atmosphere. The lower layers of the atmosphere are primarily composed water, methane and ammonia in a supercritical, fluid state. Only the outer layers consist of – similar to gas giants, see below – hydrogen, helium and methane. The atmosphere is many times larger than the core. Some scientists hypothesize that some Super-Earths may have once been Neptune-like or larger gas giant planets that have lost most of their atmosphere due to their stars irradiation.
Finally, The largest and most massive planets are gas giants, which in the Solar System include Jupiter and Saturn. Similar to the ice giants, these planets feature a small solid core, but that is surrounded by an even larger atmosphere consisting almost exclusively of hydrogen and helium. Outside the Solar System, even more massive planets than Jupiter have been found, but they probably have a very similar composition.
Only if they are about 13 times as massive as Jupiter, the planets begin to have the necessary pressure and temperature in their core to initiate the first preliminary forms of nuclear fusion. From this point forward, we usually call them brown dwarfs.
