Gaia Data Release 2. Summary of the contents and survey properties
  We present the second Gaia data release, Gaia DR2, consisting of astrometry,
photometry, radial velocities, and information on astrophysical parameters and
variability, for sources brighter than magnitude 21. In addition epoch
astrometry and photometry are provided for a modest sample of minor planets in
the solar system.
  A summary of the contents of Gaia DR2 is presented, accompanied by a
discussion on the differences with respect to Gaia DR1 and an overview of the
main limitations which are still present in the survey. Recommendations are
made on the responsible use of Gaia DR2 results.
  Gaia DR2 contains celestial positions and the apparent brightness in G for
approximately 1.7 billion sources. For 1.3 billion of those sources, parallaxes
and proper motions are in addition available. The sample of sources for which
variability information is provided is expanded to 0.5 million stars. This data
release contains four new elements: broad-band colour information in the form
of the apparent brightness in the $G_\mathrm{BP}$ (330--680 nm) and
$G_\mathrm{RP}$ (630--1050 nm) bands is available for 1.4 billion sources;
median radial velocities for some 7 million sources are presented; for between
77 and 161 million sources estimates are provided of the stellar effective
temperature, extinction, reddening, and radius and luminosity; and for a
pre-selected list of 14000 minor planets in the solar system epoch astrometry
and photometry are presented. Finally, Gaia DR2 also represents a new
materialisation of the celestial reference frame in the optical, the Gaia-CRF2,
which is the first optical reference frame based solely on extragalactic
sources. There are notable changes in the photometric system and the catalogue
source list with respect to Gaia DR1, and we stress the need to consider the
two data releases as independent.
