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Recent discoveries of young exoplanets within their natal disks offerexciting opportunities to study ongoing planet formation. In particular, aplanet's mass accretion rate can be constrained by observing theaccretion-induced excess emission. So far, planetary accretion is only probedby the H$\alpha$ line, which is then converted to a total accretion luminosityusing correlations derived for stars. However, the majority of the accretionluminosity is expected to emerge from hydrogen continuum emission, and is bestmeasured 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 70b 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 imagedin 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 fromour observations does not support greater than 30% variability in the planet'sH$\alpha$ emission in six epochs over a five-month timescale. We estimate amass 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 highproportion of energy released in line emission suggests efficient production ofH$\alpha$ emission in planetary accretion, and motivates using the H$\alpha$band for searches of accreting planets. These results demonstrateHST/WFC3/UVIS's excellent high-contrast imaging performance and highlight itspotential for planet formation studies.

Hubble Space Telescope UV and H$\alpha$ Measurements of the Accretion Excess Emission from the Young Giant Planet PDS 70 b