The boundaries of the planetary domain

• Planets or stars?

The planets can be placed on a mass-radius diagram of all the massive celestial objects.

• The planets are widely dispersed on a diagram such as this, with their position depending on their mass, temperature and chemical composition.

• Stars follow a complicated path during the course of their lifetime. Once it has exhausted all of its fuel, the Sun will swell to become a red giant 100 times its current size, and will disperse some of its atoms into a planetary nebula before collapsing and ending its life as a white dwarf ^? .
• Between the two, the brown dwarfs ^? are bright for a short period and then become as invisible as planets.

The boundaries between these three families depend on the chemical composition of the bodies and the theoretical models. The minimum mass for the fusion of deuterium ranges from 11 to 16 masses of Jupiter. The fusion of hydrogen ranges from 90 masses of Jupiter for a gas with very few metals (atoms other than hydrogen and helium) to 70 masses of Jupiter for a star with the metal levels of the Sun and with the presence of clouds.

What is certain, as shown by the Trappist-1 star with its mass of 0.089M_sol and its 7 planets, is that there is nothing to stop a small star from having a large family of planets.

The above drawing does not show objects smaller than planets. We could say that the smallest possible mass for a planet is the mass below which the body is not spherical, which corresponds to objects of about 200 km. This family of objects includes satellites ^? , comets ^? , planetesimals ^? , discs and rings.

• Free-floating planetary mass objects

The most sensitive instruments such as the JWST re capable of exploring star clusters and of detecting sub-stellar objects, despite their low brightness. By studying their spectra, they can even estimate their masses. We thus know that for 10 stars, between 2 and 5 brown dwarfs ^? are formed with a mass less than 15 M_J (limit of deuterium fusion). It would appear that the method of formation by collapse is no longer possible below 5 M_J.
With a mass of between 5 and 15 times the mass of Jupiter, the free-floating objects in the interstellar medium ^? (ISM) are either substars, formed by the collapse of small clouds of ISM, or planets, formed in a circumstellar disc and then ejected into the ISM. These Free-Floating Planetary Mass Objects could account for 10% of the objects in star clusters and 1 to 5% could be ejected planets.

• Exoplanetary systems

The Solar System is defined by all the objects linked to the Sun by gravitation. By extension, we use the term exoplanetary system to refer to a group of objects, planets, stars, discs, comets ^? , satellites ^? , etc., linked together by gravitation.
Even this apparently simple definition raises questions of identity: these small comets ^? found close to the Sun, such as ’Oumuamua, but whose velocity shows that they are not linked to the Sun by gravitation, are thus visitors from the interstellar medium ^? , or even from another system.
It should also be noted that certain systems may not contain a star and cannot therefore be detected in the visible wavelengths.