Exoplanets Defined

There is a new official definition of an exoplanet, roughly speaking, a planet in a solar system other than our own.

In antiquity, all of the enduring celestial bodies that were seen to move relative to the background sky of stars were considered planets. During the Copernican revolution, this definition was altered to objects orbiting around the Sun, removing the Sun and Moon but adding the Earth to the list of known planets. The concept of planet is thus not simply a question of nature, origin, composition, mass or size, but historically a concept related to the motion of one body around the other, in a hierarchical configuration. 

After discussion within the IAU Commission F2 "Exoplanets and the Solar System", the criterion of the star-planet mass ratio has been introduced in the definition of the term "exoplanet", thereby requiring the hierarchical structure seen in our Solar System for an object to be referred to as an exoplanet. Additionally, the planetary mass objects orbiting brown dwarfs, provided they follow the mass ratio criterion, are now considered as exoplanets. Therefore, the current working definition of an exoplanet, as amended in August 2018 by IAU Commission F2 "Exoplanets and the Solar System", reads as follows:

- Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars, brown dwarfs or stellar remnants and that have a mass ratio with the central object below the L4/L5 instability (M/Mcentral<2/(25+621‾‾‾‾√)≈1/25) are "planets", no matter how they formed.

- The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System, which is a mass sufficient both for self-gravity to overcome rigid body forces and for clearing the neighborhood around the object's orbit.

Here we discuss the history and the rationale behind this definition.

A. Lecavelier des Etangs, Jack J. Lissauer, "The IAU Working Definition of an Exoplanet" arXiv:2203.09520 (March 17, 2022) (Accepted for publication in New Astronomy Reviews).

The key bit of analysis in the body text is as follows:

2.5. The mass ratio

At the time of the previous amendments to the exoplanet definition in 2003, no planetary mass objects had been found in orbit about brown dwarfs, and such objects were not considered in the 2003 definition. Members of this class have subsequently been found, and they have typically been referred to as exoplanets. Most planetary mass objects orbiting brown dwarfs seem to fall cleanly into one of two groups: (1) very massive objects, with masses of the same order as the object that they are bound to and (2) much lower mass objects. The high-mass group of companions appear akin to stellar binaries, whereas the much less massive bodies appear akin to planets orbiting stars. There is a wide separation in mass ratio between these two groupings Figures 1 and 2, so any definition with a ratio between ∼1/100 and 1/10 would provide similar results in the classification of known objects. It is noteworthy that the limiting mass ratio for stability of the triangular Lagrangian points, M/Mcentral < 2/(25 + √ 621) ≈ 1/25, falls in the middle of this range. Moreover, this ratio is a limit based on dynamical grounds, which distinguish between star-planet couples where the star dominates and the planet can “clear the neighborhood around its orbit” (when the mass ratio is below 1/25), and pairs of objects where the more massive body does not dominate the dynamics to the extent that the less massive body can be to a good approximation considered to be orbiting about an immobile primary. 

Therefore, we proposed using this dynamically-based criterion as the dividing point. The triangular Lagrangian points are potential energy maxima, but in the circular restricted three-body problem the Coriolis force stabilizes them for the secondary to primary mass ratio (m2/m1) below 1/25, which is the case for all known examples in the Solar System that are more massive than the Pluto–Charon system. The precise ratio required for linear stability of the Lagrangian points L4 and L5 is m2/m1 < 2/(25 + √ 621) ∼ 1/25 (see Danby, 1988). If a particle at L4 or L5 is perturbed slightly, it will start to librate about these points (i.e., oscillate back and forth, without circulating past the secondary). From an observational point of view, mass ratios are commonly used in statistical studies of exoplanet discoveries by microlensing (e.g., Suzuki et al., 2016, 2018) or by transit photometry (e.g., Pascucci et al., 2018). It appears that the planet-to-star mass ratio is not only the quantity that is best measured in microlensing light curves analysis, but also it may be a more fundamental quantity in some aspects of planet formation than planet mass (Pascucci et al., 2018). It can be considered as a natural criterion to be used in the definition of the term exoplanet.

2.6. The question of unbound planetary mass objects

Motion relative to the “fixed” stars was the defining aspect of the ancient definition of the term “planet”. Motion about the Sun became a requirement for planethood as a result of the adoption of the heliocentric model of the Solar System. Orbiting a star (or a similar object) is a natural extension of this requirement for exoplanets. We therefore agreed to keep that requirement for an object to be considered as an exoplanet to be “orbiting” around a more massive object. The need to orbit around a star or an analogous massive object to be considered as a planet is effectively an extension of the Copernicus revolution. 

For planetary mass objects that do not orbit around a more massive central object, the term “sub-brown dwarf” has not been adopted in the usage by the community; rather, these objects are often referred to as “free floating planetary mass objects”. These two terms are nowadays considered as synonymous. An alternative to the rather ambiguous term “free floating”, to specifically underline the presence or absence of a central object, would be to use the term “unbound”. 

Note that neither of these terms accounts for the situation when the planetary mass object is orbiting a companion whose mass is below the deuterium-burning limit and/or the minimum mass ratio for stability of the triangular Lagrangian points, but the advantages of brevity in terminology may well dictate that one of these terms is nonetheless optimal.