Definition of an exoplanet
One simple definition is: an exoplanet is a planet situated outside the Solar System, in other words which does not orbit around our Sun.
But is everything that orbits a star a planet? And, are there planets which do not orbit a star?
The answers are not simple, not only because there are two approaches to defining planets, but the boundaries between these planetary domains are blurred.
1. First approach, internal structure and radiation
An exoplanet is first of all defined by the fact that it emits no visible light (other than the light it receives from the star and then reflects), whereas a star shines because of the thermonuclear fusion of hydrogen taking place within it. The emissions from the planet itself, which is far colder than the star, are limited to the infrared.
When a cloud in the interstellar medium ? collapses in on itself and becomes sufficiently massive (at least 70 or 90 masses of Jupiter depending on the case), the combustion of hydrogen begins, the cloud becomes a star which will emit visible light for millions or even billions of years. If the cloud is less massive, but more massive than 13 masses of Jupiter, deuterium fusion will take place in it. These objects will be bright (visible light) for a few hundred million years before going out, cooling and becoming invisible. These objects are brown dwarfs ? . If the cloud is even less massive, less than 13 masses of Jupiter, it becomes a planetary object whose intrinsic radiation is limited to the infrared.
Observations do not always give a mass value. In particular, the radial velocities method only gives a lower limit value for the mass. Without other observations, it is impossible to know whether the object is a planet or a brown dwarf ? .
2. Formation scenario
Another approach to defining exoplanets is based on their situation and how they are formed. The scénario de formation des planètes est connu dans ses grandes lignes: they were formed in the disc of gas and dust which surrounded the Sun after its formation and were created by an accumulation of particles of rock and ice (solid planets). For some of them, gas was added to this solid core, to form gas planets.
Observations show that, in many star environments, the exoplanets orbit in a plane perpendicular to the rotation axis of the star. Moreover, the young circumstellar discs are agitated by the presence of observed or suspected planets. These observations show that the above scenario, imagined for the Solar System, is common throughout the history of the stars. But we also know that this history can continue with migrations which eject the exoplanets far from the disc in which they were born, out into the interstellar medium ? , or down into the star.
Several of these ejected exoplanets have been discovered and there is an ongoing debate as to whether to exclude these free-floating objects from the family of planets...
For their part, brown dwarfs ? and stars form from the collapse of gas clouds in the interstellar medium ? , either on their own or in multiple systems. This mechanism can form planets with the mass of Jupiter, or possibly even smaller?
According to this definition, the planets are celestial objects formed in a circumstellar disc. The proof of this method of formation is the presence of a solid core in a planet, whereas stars and brown dwarfs ? have an internal structure with no solid core.
To use this definition, we therefore have to know the density and composition of the objects observed, but these parameters are very hard to obtain.
3. Two definitions
These two definitions do not have the same boundaries, and the populations intermingle: objects formed in a disc can trigger the fusion of deuterium. Objects formed far from any star can have a mass of less than $12 M_{\text{Jup}}$. Objects can be formed in a disc and then ejected and become free-floating planets: should they be called planets?
4. IAU definition
here is as yet no official definition of an exoplanet. An IAU working version is:
Objects which
- have a mass below the mass limit for thermonuclear fusion of deuterium (currently evaluated at 13 times the mass of Jupiter for objects with the same initial composition as the Sun of elements heavier than helium),
- and which are in orbit around stars, brown dwarfs ? or star residues,
- whose mass ratio with the central object is below the ratio leading to the instability of Lagrange points L4 and L5 ($\frac{M}{M_{\text{central}}} < \frac{2}{25 + \sqrt{621}} \simeq \frac{1}{25}$).
are “planets” regardless of their method 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, that is, have sufficient mass for self-gravitation to overcome the internal forces of rigid bodies and to clear out other bodies from the neighbourhood of the object’s orbit to prevent them disturbing this orbit.
5. The Encyclopaedia of Exoplanetary Systems
Faced with the difficulty of defining sufficiently legitimate boundaries between these astrophysical objects, the catalogue of the Encyclopaedia of Exoplanetary Systems lists objects with a mass of up to $60 M_{\text{Jup}}$, the limit beyond which the internal structure of the objects changes, as shown by the following mass-density graph.
6. The classification of exoplanets
We can classify exoplanets according to their properties
and obtain this table