  Abbreviated. By selecting stars with similar ages and masses, the Young Suns
Exoplanet Survey (YSES) aims to detect and characterize planetary-mass
companions to solar-type host stars in the Scorpius-Centaurus association. Our
survey is carried out with VLT/SPHERE with short exposure sequences on the
order of 5 min per star per filter. The subtraction of the stellar point spread
function (PSF) is based on reference star differential imaging (RDI) using the
other targets in the survey in combination with principal component analysis.
We report the discovery of YSES 2b, a planetary-mass companion to the K1 star
YSES 2 (TYC 8984-2245-1). The primary has a Gaia EDR3 distance of 110 pc, and
we derive a revised mass of $1.1\,M_\odot$ and an age of approximately 14 Myr.
We detect the companion in two observing epochs southwest of the star at a
position angle of 205$^\circ$ and with a separation of $\sim1.05''$, which
translates to a minimum physical separation of 115 au at the distance of the
system. We derive a photometric planet mass of
$6.3^{+1.6}_{-0.9}\,M_\mathrm{Jup}$ using AMES-COND and AMES-dusty evolutionary
models; this mass corresponds to a mass ratio of $q=(0.5\pm0.1)$% with the
primary. This is the lowest mass ratio of a direct imaging planet around a
solar-type star to date. We discuss potential formation mechanisms and find
that the current position of the planet is compatible with formation by disk
gravitational instability, but its mass is lower than expected from numerical
simulations. Formation via core accretion must have occurred closer to the
star, yet we do not find evidence that supports the required outward migration,
such as via scattering off another undiscovered companion in the system. YSES
2b is an ideal target for follow-up observations to further the understanding
of the physical and chemical formation mechanisms of wide-orbit Jovian planets.
