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Exoplanet Types, Classification, Taxonomy

The classification of exoplanet types is unfortunately arbitrary and there is no concensus on preceise definitions. In the media and in the scientific literature you now see an evergrowing list of names referring to different types of exoplanets. You have your “hot Jupiters,” “cold Neptunes,” “mini-Neptunes,” “super-Earths,” “ice giants,” “gas giants,” and more. It is already terribly confusing. The problem is that the new discoveries demand an understanding of how planet formation gives rise to these different types of planets in order to come up with a classification scheme that reflects something fundamental about the structure of the planets. That understanding remains elusive so the classification of exoplanets is currently somewhat ad hoc. There are no quantitative definitions.

Here I will give an overview that will enable you to figure what is meant by a particular term for a type of exoplanet, even if it is new. As usual, anything that is hyperlinked will take you a page with more details.

Terrestrial Planet. A terrestrial planet generally refers to one that is rocky, like Earth, Mars, Venus, and Mercury. It is generally thought that the size of a terrestrial planet cannot be much larger than a couple of times the size of the Earth, and certainly not as large or as massive as Uranus, Neptune, Saturn, and Jupiter. This is not a fact. It is speculation based upon theoretical simulations that involve many assumptions and “hand-tweaking” (see core accretion theory page for a discussion of the problems with planet-formation theories.

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Solar System Planets in the Name. Whenever the name of a planet in the solar system appears in the name of a type of exoplanet, as you might expect, the implication is that there is some similar characteristic that is shared by the exoplanet and the planet in the solar system. Usually this refers to a similar composition, but it could refer to mass and/or size. It is very vague and fuzzy. Prefixes like “giant,” “super,” or “mini” could refer to the mass and/or size of an exoplanet compared to a named solar system planet. For a given density there is obviously a correlation between mass and size.

Size Prefixes. The prefix “hot” usually refers to a planet's relative distance from its host star. If a planet has no internal heat source of its own and no atmosphere, then its temperature is affected by its distance from the host star (and some assumptions about how incident radiant energy is reprocessed by the exoplanet). Thus, if two planets are similar, the planet-star distances are used as proxies for the surface temperatures of the planets. We can mathematically balance the heating from the star with the heat loss from the planet to come up with a so-called “effective equilibrium temperature.” The calculation must make use of the host star's properties, as well as some properties of the planet that may have to be estimated (in particular, the reflectivity, or albedo).

Hot and Cold. Although this calculated temperature may have nothing to do with the actual temperature of the planet, it serves as a quantitative “benchmark.” Obviously, the further a planet is from its host star, the lower is the effective temperature. Thus, the prefix “hot” in the name of an exoplanet type refers to the fact that the planet is in some sense closer to the host star than was expected before that type of planet was discovered. For example, a “hot Jupiter” is a planet with a mass (“fuzzily speaking”) of the order of, or larger than, that of Jupiter (Jupiter's mass is about 317.828 times that of Earth), but is closer to its host star than Earth is to our Sun.

Thus we have a hot Jupiters, hot Neptunes, and super-Earths as commonly discussed classes or types of exoplanet.

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