Puny Star or Pudgy Planet?

Abstract

ASTROPHYSICS The distinction between stars and planets has been blurred by observations of low-mass objects in a variety of astrophysical environments. These objects do not have enough mass to sustain hydrogen burning and cannot be characterized by standard evolutionary diagrams, although they may be stars that never reached thermal equilibrium and could be burning lithium or deuterium at lower temperatures. Some of the very low mass objects probably cannot even burn deuterium and may be planets. However, some of these do not orbit a star, so if they are planets, they must have formed differently from a normal planet or have been ejected from their planetary system. Without substellar evolutionary diagrams or a mechanistic definition of a planet, the observed low-mass objects are difficult to classify. Chabrier et al . have brought some order to this problem by developing evolutionary diagrams for low-mass objects that burn deuterium. They have combined an interior model of deuterium burning in the core and convection of the deuterium to the surface with atmospheric models of deuterated water in order to determine the mass, temperature, luminosity, and spectral characteristics of deuterium-burning stars as a function of age. These models will help to distinguish puny stars from relatively massive planets. — LR Astrophys. J. 542 , L119 (2000).