substellar

Substellar is a term used in astronomy to describe objects whose mass is too low to sustain long-term hydrogen fusion in their cores. The category includes brown dwarfs and planetary-mass bodies. The conventional boundary lies roughly between 13 and 80 Jupiter masses (about 0.012 to 0.08 solar masses). Objects above about 80 Jupiter masses reach sustained hydrogen fusion and are classified as stars; those below the fusion threshold do not, placing them in the substellar domain. Within this range, deuterium burning can occur in some objects above about 13 Jupiter masses but is not a long-term energy source.

Brown dwarfs form primarily by gravitational collapse of gas clouds, like stars, but never accumulate enough mass to sustain hydrogen fusion. Planetary-mass bodies under this threshold may form in circumstellar disks around stars or exist as free-floating objects released from forming systems. The distinction between brown dwarfs and planets is often framed in terms of formation history, though mass plays a major role; free-floating planets are substellar but not bound to a star.

Substellar objects cool and fade over time, emitting most strongly in the infrared. They are spectrally classified into types such as L, T, and Y dwarfs, reflecting decreasing temperatures and characteristic molecular features. Observationally, substellar objects are detected via infrared surveys, direct imaging, microlensing, and companion studies. The study of substellar objects informs models of atmospheric physics at low temperatures and provides insight into the formation and evolution of stars and planetary systems.