Exoplanets that exhibit ongoing accretion activities are excellent targets to probe the ground truth of planet formation. The accretion-induced excess emission enables a constraint on the mass accretion rate, a critical characteristic that regulates planet formation and determines the planet's ultimate mass. Based on theoretical studies, planetary accretion creates a hot shock front that produces strong hydrogen line and ultraviolet (UV) continuum emissions. Therefore, an accurate accretion rate estimate requires measuring both emission components from the planet. Recently, we conducted a Hubble Space Telescope campaign to observe the PDS 70 planetary system and detected the planet PDS 70 b in both the Hα and U bands. This is the first-ever direct-imaging detection of an exoplanet in UV, which enables the first direct measurement of the planet's Balmer continuum emission.
In this talk, I will first introduce the observation and image processing techniques that facilitated these detections. I will then explain how we derive the accretion rate for PDS 70 b and discuss the planet's accretion mechanisms and formation process. Finally, I will illustrate the exciting prospects of studying ongoing planet formation with space-based direct imaging observations.