Low mass companions to stars: Implications for models of the formation and evolution of binary and planetary systems

Precise radial velocity observations over the past decade have revealed evidence of very low amplitude, periodic spectroscopic variations in some 90 stars. These variations have been interpreted to be due to similar to 100 unseen low mass companions to these stars. The apparent masses of these companions are low enough that they are generally referred to as extrasolar planets. We here review briefly the current model for how planets and planetary systems form, as well as all facets of the data from the radial velocity searches. The radial velocity data provide unambiguous information regarding orbital period and eccentricity, but inferences regarding mass are uncertain for all but two of the companions. We point out that while the apparent masses suggest that the companions are substellar, the orbital data, period and eccentricity, are highly reminiscent of similar parameters for stellar binary systems. There is nothing in the results to date from these observational studies that necessitate a revision in current models of how the solar system and planetary systems in general form. However, the available data do suggest that the bulk of these companions, regardless of their mass, have more in common with stars than planets. Until the true nature of these companions is made clear by future studies we suggest that they be referred to as "U-dwarfs".