Observatoire de Paris
<-   A choice between a star, a brown dwarf or a planet   ->
Between star and planet
Temperature and diameters of several brown dwarves, compared to the Sun and Jupiter.
Copyright : ESA / Medialab. Data from R. Rebolo and Serge Jodra
  • If the mass of the central core is small, , M < 3*10^28 kg i.e.13*M_J (Jupiter's mass), it becomes a gas sphere and forms a giant planet (see what is an exoplanet ?).
  • If 13*M_J < M < 100*M_J (10 percent of the mass of the Sun), a brown dawrf forms, where the nuclear combustion only concerns the fusion of the deuterium (D) into helium (He).
  • If M > 0.1*M_soleil (solar mass), the central temperature becomes greater than 5*10^6K and the fusion of hydrogen (H) into helium (He) starts, lighting up a star.

These three scenarios can occur in the neighbourhood of a forming star. This leads to a system of double stars (which are numerous in the Universe), or to a system associating a star with a gas giant or a star with a brown dwarf.

If the scenario above concerns an isolated gas condensation, an isolated star forms. If the mass is small, an isolated brown dwarf, or even an isolated planet, can form.

Some isolated brown dwarves have been detected. However, it is difficult to detect them since they are faint, and shine for a short time in the infrared.

The formation of isolated planets (called floating planets) is theoretically possible. However, nothing has been detected yet. These objects do not have any energy sources, and do not emit any radiation.

Now, we consider a case where the body formed is a star, and look at what happens in its environment. Because that's where the "real" planets form in a circumstellar disk, as the Earth did.