  Discovering exoplanets orbiting young Suns can provide insight into the
formation and early evolution of our own solar system, but the extreme magnetic
activity of young stars obfuscates exoplanet detection. Here we monitor the
long-term magnetic field and chromospheric activity variability of the young
solar analogue V889 Her, model the activity-induced radial velocity variations
and evaluate the impacts of extreme magnetism on exoplanet detection
thresholds. We map the magnetic field and surface brightness for 14 epochs
between 2004 and 2019. Our results show potential 3-4 yr variations of the
magnetic field which evolves from weak and simple during chromospheric activity
minima to strong and complex during activity maxima but without any polarity
reversals. A persistent, temporally-varying polar spot coexists with weaker,
short-lived lower-latitude spots. Due to their different decay time-scales,
significant differential rotation and the limited temporal coverage of our
legacy data, we were unable to reliably model the activity-induced radial
velocity using Gaussian Process regression. Doppler Imaging can be a useful
method for modelling the magnetic activity jitter of extremely active stars
using data with large phase gaps. Given our data and using Doppler Imaging to
filter activity jitter, we estimate that we could detect Jupiter-mass planets
with orbital periods of $\sim$3 d. A longer baseline of continuous observations
is the best observing strategy for the detection of exoplanets orbiting highly
active stars.
