[Paper Review] Optical and Near-infrared View of Planet-forming Disks and Protoplanets

M. Benisty, C. Dominik+7 more|arXiv (Cornell University)|Mar 18, 2022

Molecular Spectroscopy and Structure24 citations

TL;DR

The chapter reviews high-resolution optical and near-IR imaging of planet-forming disks, highlighting substructures, protoplanet detections, and the synergy with ALMA to study disk–planet co-evolution.

ABSTRACT

In this chapter of the Protostars and Planets VII, we review the breakthrough progress that has been made in the field of high-resolution, high-contrast optical and near-infrared imaging of planet-forming disks. These advancements include the direct detection of protoplanets embedded in some disks, and derived limits on planetary masses in others. Morphological substructures, including: rings, spirals, arcs, and shadows, are seen in all imaged infrared-bright disks to date, and are ubiquitous across spectral types. These substructures are believed to be the result of disk evolution processes, and in particular disk-planet interactions. Since small dust grains that scatter light are tightly bound to the disk's gas, these observations closely trace disk structures predicted by hydrodynamical models and serve as observational tests of the predictions of planet formation theories. We argue that the results of current and next-generation high-contrast imaging surveys will, when combined with complementary data from ALMA, lead to a much deeper understanding of the co-evolution of disks and planets, and the mechanisms by which planets form.

Motivation & Objective

• Summarize the breakthrough progress in high-resolution, high-contrast optical and near-IR disk imaging.
• Highlight the ubiquity and morphology of disk substructures (rings, spirals, arcs, shadows) in IR-bright disks.
• Explain how scattered light traces small dust grains and tests disk evolution and planet-formation theories.
• Discuss how ALMA data complement optical/near-IR imaging to constrain disk gas and dust structure.
• Outline future directions for high-contrast imaging surveys and their potential to illuminate disk–planet co-evolution.

Proposed method

• Describe observational advancements enabling diffraction-limited imaging (AO, extreme AO) and high-contrast capabilities.
• Summarize differential imaging techniques (Reference Differential Imaging, Angular Differential Imaging, Polarization Differential Imaging, Spectral Differential Imaging) and their trade-offs.
• Explain the scattering geometry and polarization formalism (Stokes parameters, Q_phi, U_phi) used to interpret disk imagery.
• Discuss the physical interpretation of disk substructures as signatures of disk evolution and planet–disk interactions.
• Integrate multi-wavelength perspectives (optical/near-IR scattering vs ALMA gas/dust tracers) to construct a 3D view of disks.

Experimental results

Research questions

• RQ1 What disk substructures (rings, gaps, spirals, shadows) are observed in high-resolution optical/near-IR images across disk types?
• RQ2 How do disk substructures relate to planet–disk interactions and potential embedded protoplanets?
• RQ3 How does multi-wavelength imaging (optical/near-IR vs ALMA) constrain the vertical and radial distribution of gas and dust?
• RQ4 What are the limitations and biases of current imaging techniques (RDI, ADI, PDI, SDI) in recovering disk signals and detecting protoplanets?
• RQ5 What are the prospects and expected scientific gains from future high-contrast imaging surveys for disk and planet formation?

Key findings

• Protoplanets have been directly detected in some disks using high-contrast imaging.
• Substructures such as rings, spirals, arcs, and shadows are common in infrared-bright disks and reflect disk evolution and planet–disk interactions.
• Small dust grains traced by scattered light couple to the gas and map the disk surface layers, enabling tests of hydrodynamical models and planet-formation theories.
• Combining high-contrast optical/near-IR imaging with ALMA data yields deeper insights into disk–planet co-evolution and formation mechanisms.
• Advances in AO, coronagraphy, and differential imaging (RDI, ADI, PDI, SDI) have matured to allow characterization of inner disk regions and potential protoplanets, with surveys expanding in scope and capability.