THE DEFINITION OF THE TERM “EXOPLANET”
The new definition of the term “exoplanet”, adopted in 2018 by the International Astronomical Union (IAU), presents two novelties compared to the one adopted in 2003: beyond objects orbiting a star or stellar remnant, now objects orbiting brown dwarfs (whose minimum mass is maintained at 13 times the mass of Jupiter) can be considered as exoplanets, and exoplanets must orbit an object that is at least 25 times more massive. Alain Lecavelier des Etangs from the Institut d’astrophysique de Paris (IAP) and Jack Lissauer from the NASA Ames Research Center, describe the history and the rationale of this definition in an article published in New Astronomy Reviews.
Since the mid-1990s, nearly 5000 extrasolar planets, or “exoplanets”, have been discovered orbiting stars other than the Sun. The discovered objects are often very different from the planets of the Solar System, so that the question is raised to know under which conditions one can consider that they are planets. In order to clarify the situation, in 2003 a working group was formed within the International Astronomical Union, the international organization that has authority for the naming of celestial bodies and the definition of astronomical terms, and proposed a definition of the term “exoplanet” which specified that a planet must be in orbit around a star or a stellar remnant of a star and must have a mass less than 13 times the mass of Jupiter. Indeed, beyond this mass the deuterium can fuse in the core of the object and the object is then called a brown dwarf, in the case where there is no hydrogen fusion, or even a star if there is hydrogen fusion, which occurs for a mass greater than about 80 times the mass of Jupiter (itself about 1000 less massive than our star the Sun).
Since this first definition, many new exoplanets with very diverse characteristics have been discovered. In view of these observations, the IAU Commission “Exoplanets and Solar System” examined in 2018 the classification of objects populating extrasolar systems and the conditions that define the category of objects belonging to the class of “exoplanets”. After a debate and a vote within this commission of nearly 400 researchers working on this topic, a definition was adopted. This definition is presented in the article entitled “The IAU working definition of an exoplanet”, written by Alain Lecavelier des Etangs (IAP) and Jack Lissauer (NASA Ames Research Center), and published by the journal New Astronomy Reviews.
With the new definition, the two major changes with respect to the provisional definition of 2003 are the addition among the exoplanets of objects orbiting brown dwarfs, as well as the appearance of a condition on the mass ratio between the orbiting object and the object around which it orbits; below this mass ratio, that depends on the distance to the central object, the orbiting body is not considered any more as an exoplanet. The main point of this new definition is that to be considered as an exoplanet, the object must be in orbit around a central body, which requires the existence of a hierarchical configuration between the central object and the planet in orbit. Thus, when the mass of the orbiting object is not negligible compared to the mass of the central object, it can no longer be considered as an orbiting body, but both objects must be considered as a pair of objects orbiting each other around their common center of gravity, as is observed for many binary stars. In order for the notion of orbit to exist, it is also necessary to have stability in the environment of the orbiting object. To constrain this stability, the criterion retained is that the mass ratio between the planet and the central object allows the existence of the Lagrange points L4 and L5 (see Appendix 2) and corresponds to the fact that the exoplanet has a mass lower than 1/25 of the mass of its central star. In other words, an exoplanet must be in orbit around an object at least 25 times more massive.
The criterion of a maximum mass of 13 times the mass of Jupiter for an exoplanet, as adopted in the 2003 definition, is moreover kept in the new definition. Finally, the new definition adds brown dwarfs among the central objects around which exoplanets can orbit. This change implies that five known objects are now considered as exoplanets.
By introducing a mass ratio defined by the stability of the orbits, the new definition of the term “exoplanet” gives an increased importance to dynamics, and thus to motion. This directly echoes the historical definition of the Greek term planet “πλανήτησ” (wandering star) given during antiquity to objects in motion relative to the fixed stars of the sky. This dynamic criterion appears to be a generalization of the concept of planet from the Copernican revolution describing the planets of the Solar System as objects in orbit around the Sun.
The article written by Alain Lecavelier des Etangs and Jack Lissauer insists on the fact that this is a working definition, in the sense that knowledge about exoplanets is changing rapidly, and new discoveries may result in having to modify this definition. Indeed, the work of classification and definition of terms is a work that transcribes the state of our knowledge at a given moment on the nature of objects, and the relations that link them to each other. For example, the definition of the term “planet” for the Solar System adopted in 2006 by the IAU had excluded Pluto from this classification because it is not massive enough. Today, the new definition adopted by the IAU in 2018 reflects the great wealth of exoplanet discoveries of the last decades and concretizes a new step in the classification of objects constituting planetary systems.
 The International Astronomical Union (IAU) is an international non-governmental organization that brings together nearly 14,000 professional astronomers from around the world (90 countries), and promotes their cooperation. It is the internationally recognized authority for the naming of celestial bodies (stars, planets, asteroids, etc.) and structures on the surface of solar system bodies (craters, plains, volcanoes, etc.).
 A stellar remnant is what remains when the star has exhausted its gas supply and can no longer produce energy through thermonuclear fusion. It is a white dwarf for stars of less than 8 solar masses, a neutron star for more massive stars, even a black hole in the most extreme cases where nothing can prevent the collapse of the star under the effect of its own gravity. At the end of its evolution the Sun will become a white dwarf.
 Brown dwarfs are bodies not massive enough to start the whole chain of thermonuclear fusion reactions that define stars, and which, despite periods of fusion and light emission, fade and contract under the effect of gravitation.
 Pluto’s orbit also contains many asteroids that make up the Kuiper Belt. It is therefore not massive enough to evacuate these objects from its orbit and be considered a planet, which led to it being defined as a dwarf planet.
Adopted definition for the “exoplanet” term
- Objects whose mass is lower than the limit mass for thermonuclear fusion of deuterium (currently estimated at 13 times the mass of Jupiter for objects of the same initial composition as the Sun in elements heavier than helium) and which are in orbit around stars, brown dwarfs or star remnants, whose mass ratio with the central object is lower than the ratio leading to the instability of the L4 and L5 Lagrange points (M/Mcentral <2/(25+√621) ≈ 1/25) are “planets”, regardless of their mode of formation.
- The minimum mass required for an extrasolar object to be considered a planet must be the same as that used in our Solar System, i.e., sufficient mass both for self-gravity to overcome the internal forces of rigid bodies and to clear the vicinity of the object’s orbit of other bodies that might disrupt that orbit.
The L4 and L5 Lagrange points
The L4 and L5 Lagrange points of a planet in orbit around a central object are located on the vertices of equilateral triangles having the planet and the central object as other vertices. These points are thus 60° ahead and behind the planet in its orbit (see figure below).
At the L4 and L5 Lagrange points, low mass objects can have stable orbits if the planet has a mass less than 1/25 of the mass of the central object. Thus, we find asteroids trapped in the Lagrange points L4 and L5 of Jupiter; these objects constitute the family of Trojan asteroids.
Writing and contact
- Alain Lecavelier
Institut d’astrophysique de Paris, CNRS, Sorbonne Université
alain.lecavelier_des_etangs [at] iap [dot] fr
Layout: Jean Mouette