Kepler K2 Campaign 9: II. First space-based discovery of an exoplanet
  using microlensing
  We report on the discovery of a bound exoplanetary microlensing event from a
blind search of data gathered from Campaign 9 of the Kepler K2 mission (K2C9).
K2-2016-BLG-0005Lb is a densely sampled, binary caustic-crossing microlensing
event with caustic entry and exit points that are resolved in the K2C9 data,
enabling the lens-source relative proper motion to be measured. We have fitted
a binary microlens model to the K2 dataset, and to simultaneous observations
from the Optical Gravitational Lensing Experiment (OGLE-IV),
Canada-France-Hawaii Telescope (CFHT), Microlensing Observations in
Astrophysics (MOA-2), the Korean Microlensing Telescope Network (KMTNet), and
the United Kingdom InfraRed Telescope (UKIRT). Whilst the ground-based data
only sparsely sample the binary caustic, they provide a clear detection of
parallax that allows us to break completely the microlensing
mass-position-velocity degeneracy and measure the planet's mass directly. We
find a host mass of $0.58\pm0.03 ~{\rm M}_\odot$ and a planetary mass of $1.1
\pm 0.1 ~{\rm M_J}$. The system lies at a distance of $5.2 \pm 0.2~$kpc from
Earth towards the Galactic bulge. The projected physical separation of the
planet from its host is found to be $4.2 \pm 0.3~$au which, for circular
orbits, corresponds to $a = 4.4^{+1.9}_{-0.4}~$au and period $P =
13^{+9}_{-2}~$yr, making K2-2016-BLG-0005Lb a close Jupiter analogue. Though
previous exoplanet microlensing events have included space-based data, this
event is the first bound microlensing exoplanet to be discovered from
space-based data. Even through a space telescope not designed for microlensing
studies, this result highlights the advantages for exoplanet microlensing
discovery that come from continuous, high-cadence temporal sampling that is
possible from space. (Abridged).
