Is it a star? Is it a planet?
Young substellar objects, like 'Teide 1' and the objects in Sigma Orionis, are much hotter than old ones. They are still forming. Once their formation process is over they will probably be the same size as Jupiter. There are not yet any direct measurements of the size of these objects, so astronomers use models to derive them.
[In this diagram MS is the mass of the Sun, MJ is the mass of Jupiter]
The data used to prepare this diagram was provided by Rafael Rebolo, Instituto de Astrofísica de Canarias (IAC), Spain. Rebolo's group discovered Teide 1, the first brown dwarf confirmed by spectroscopic measurements, in 1995.