  The ability to make independent detections of the signatures of exoplanets
with complementary telescopes and instruments brings a new potential for robust
identification of exoplanets and precision characterization. We introduce PEXO,
a package for Precise EXOplanetology to facilitate the efficient modeling of
timing, astrometry, and radial velocity data, which will benefit not only
exoplanet science but also various astrophysical studies in general. PEXO is
general enough to account for binary motion and stellar reflex motions induced
by planetary companions and is precise enough to treat various relativistic
effects both in the solar system and in the target system. We also model the
post-Newtonian barycentric motion for future tests of general relativity in
extrasolar systems. We benchmark PEXO with the pulsar timing package TEMPO2 and
find that PEXO produces numerically similar results with timing precision of
about 1 ns, space-based astrometry to a precision of 1 $\mu$as, and radial
velocity of 1 $\mu$m/s and improves on TEMPO2 for decade-long timing data of
nearby targets, due to its consideration of third-order terms of Roemer delay.
PEXO is able to avoid the bias introduced by decoupling the target system and
the solar system and to account for the atmospheric effects which set a
practical limit for ground-based radial velocities close to 1 cm/s. Considering
the various caveats in barycentric correction and ancillary data required to
realize cm/s modeling, we recommend the preservation of original observational
data. The PEXO modeling package is available at GitHub
(https://github.com/phillippro/pexo).

Earth and Planetary Astrophysics;Instrumentation and Methods for Astrophysics;Solar and Stellar Astrophysics