  The detection of emission lines associated with accretion processes is a
direct method for studying how and where gas giant planets form, how young
planets interact with their natal protoplanetary disk and how volatile delivery
to their atmosphere takes place. H$\alpha$ ($\lambda=0.656\,\mu$m) is expected
to be the strongest accretion line observable from the ground with adaptive
optics systems, and is therefore the target of specific high-contrast imaging
campaigns. We present MagAO-X and HST data obtained to search for H$\alpha$
emission from the previously detected protoplanet candidate orbiting AS209,
identified through ALMA observations. No signal was detected at the location of
the candidate, and we provide limits on its accretion. Our data would have
detected an H$\alpha$ emission with $F_\mathrm{H\alpha}>2.5\pm0.3
\times10^{-16}$ erg s$^{-1}$ cm$^{-2}$, a factor 6.5 lower than the HST flux
measured for PDS70b (Zhou et al., 2021). The flux limit indicates that if the
protoplanet is currently accreting it is likely that local extinction from
circumstellar and circumplanetary material strongly attenuates its emission at
optical wavelengths. In addition, the data reveal the first image of the jet
north of the star as expected from previous detections of forbidden lines.
Finally, this work demonstrates that current ground-based observations with
extreme adaptive optics systems can be more sensitive than space-based
observations, paving the way to the hunt for small planets in reflected light
with extremely large telescopes.
