sentences:  Astrometric measurements allows researchers to check whether objects that appear to be high mass planets are more likely to be brown dwarfs.A further disadvantage is that the gas envelope around certain types of stars can expand and contract, and some stars are variable.This method is unsuitable for finding planets around these types of stars, as changes in the stellar emission spectrum caused by the intrinsic variability of the star can swamp the small effect caused by a planet.