  Gaia is undertaking a deep synoptic survey of the Galaxy, but photometry from
individual epochs has, as of yet, only been released for a minimal number of
sources. We show that it is possible to identify variable stars in Gaia Data
Release 2 by selecting stars with unexpectedly large photometric uncertainties
given their brightness and number of observations. By comparing our results to
existing catalogs of variables, we show that information on the amplitude of
variability is also implicitly present in the Gaia photometric uncertainties.
We present a catalog of about 9.3 million candidate variable stars, and discuss
its limitations and prospects for future tests and extensions.
