  Recent discoveries of young exoplanets within their natal disks offer
exciting opportunities to study ongoing planet formation. In particular, a
planet's mass accretion rate can be constrained by observing the
accretion-induced excess emission. So far, planetary accretion is only probed
by the H$\alpha$ line, which is then converted to a total accretion luminosity
using correlations derived for stars. However, the majority of the accretion
luminosity is expected to emerge from hydrogen continuum emission, and is best
measured in the ultraviolet (UV). In this paper, we present HST/WFC3/UVIS F336W
(UV) and F656N (H$\alpha$) high-contrast imaging observations of PDS 70.
Applying a suite of novel observational techniques, we detect the planet PDS 70
b with signal-to-noise ratios of 5.3 and 7.8 in the F336W and F656N bands,
respectively. This is the first time that an exoplanet has been directly imaged
in the UV. Our observed H$\alpha$ flux of PDS 70 b is higher by $3.5\sigma$
than the most recent published result. However, the light curve retrieved from
our observations does not support greater than 30% variability in the planet's
H$\alpha$ emission in six epochs over a five-month timescale. We estimate a
mass accretion rate of $1.4\pm0.2\times10^{-8}M_{\mathrm{Jup}}/\mathrm{yr}$.
H$\alpha$ accounts for 36% of the total accretion luminosity. Such a high
proportion of energy released in line emission suggests efficient production of
H$\alpha$ emission in planetary accretion, and motivates using the H$\alpha$
band for searches of accreting planets. These results demonstrate
HST/WFC3/UVIS's excellent high-contrast imaging performance and highlight its
potential for planet formation studies.
