VLT/RISTRETTO Resolves Planetary H-alpha Accretion Lines at 37 mas Separations

The arXiv study models RISTRETTO performance using BT-Settl spectra, HARPS PDS 70 data, and accretion-shock line profiles fed through PyEchelle. Synthetic observations demonstrate that the seven-spaxel layout plus extreme AO separates planetary H-alpha spatially and spectrally from the host star at 37-600 mas, enabling direct constraints on pre-shock density and velocity. This directly visualizes ongoing gas accretion rather than relying on indirect disk gaps.

Existing facilities like HARPS and MUSE lack the combination of resolution, coronagraphic suppression, and IFU sampling needed for these separations. RISTRETTO therefore bridges current reflected-light efforts and future ELT instruments, offering a path to measure accretion rates and flow geometry on individual protoplanets such as PDS 70 b/c and WISPIT 2 b.

The design choices also highlight a key trade-off: the narrow wavelength focus around H-alpha sacrifices broad atmospheric characterization for unmatched kinematic precision on accretion flows. This positions the instrument as a targeted pathfinder rather than a general-purpose exoplanet spectrograph.

Commissioning data will test whether real on-sky performance meets the modeled limits once residual speckle and telluric contamination are fully characterized.