---
abstract: 'Debris disks are tenuous, dust-dominated disks commonly observed around stars over a wide range of ages. Those around main sequence stars are analogous to the Solar System’s Kuiper Belt and Zodiacal light. The dust in debris disks is believed to be continuously regenerated, originating primarily with collisions of planetesimals. Observations of debris disks provide insight into the evolution of planetary systems; the composition of dust, comets, and planetesimals outside the Solar System; as well as placing constraints on the orbital architecture and potentially the masses of exoplanets that are not otherwise detectable. This review highlights recent advances in multiwavelength, high-resolution scattered light and thermal imaging that have revealed a complex and intricate diversity of structures in debris disks, and discusses how modeling methods are evolving with the breadth and depth of the available observations. Two rapidly advancing subfields highlighted in this review include observations of atomic and molecular gas around main sequence stars, and variations in emission from debris disks on very short (days to years) timescales, providing evidence of non-steady state collisional evolution particularly in young debris disks.'
author:
- 'A. Meredith Hughes,$^1$ Gaspard Duchêne,$^{2,3}$ Brenda C. Matthews$^{4,5}$'
title: 'Debris Disks: Structure, Composition, and Variability'
---

circumstellar disks, planet formation, extrasolar planetary systems, main sequence stars, planetesimals, circumstellar matter

1.  Debris disks are a common phenomenon around main sequence stars, with current detection rates at $\sim 25$% despite the limited sensitivity of instrumentation (for Kuiper Belt analogues, a few $\times$ Solar System levels for even the nearest stars, and orders of magnitude from Solar System levels for exozodis) indicating that this figure is unquestionably a lower limit. Debris disks are more commonly detected around early-type and younger stars, and they frequently show evidence of two dust belts, like the Solar System.

2.  High-resolution imaging of structures in outer debris disks reveals a rich diversity of structures including narrow and broad rings, gaps, haloes, wings, warps, clumps, arcs, spiral arms, and eccentricity. There is no obvious trend in disk radial extent with stellar spectral type or age. Most of the observed structures can be explained by the presence of planets, but for most structures there is also an alternative proposed theoretical mechanism that does not require the presence of planets.

3.  The combined analysis of a debris disk’s SED with high-resolution (polarized) scattered light images and thermal emission maps is a powerful method to constrain the properties of its dust. Despite some shortcomings in current models, a coherent picture arises in which the grain size distribution is characterized by a power law similar to that predicted from collisional models, albeit with a minimum grain size that is typically a few times larger than the blowout size. Beyond the ubiquitous silicates, constraints on dust composition remain weak. Remarkably, dust in most debris disks appears to be characterized by a nearly universal scattering phase function that also matches that observed for dust populations in the Solar System, possibly because most dust grains share a similar aggregate shape.

4.  Many debris disk systems harbor detectable amounts of atomic and/or molecular gas. Absorption spectroscopy of a few key edge-on systems reveals volatile-rich gas in a wide variety of atomic tracers, and there is evidence of an enhanced C/O ratio in at least some systems. Emission spectroscopy reveals that CO gas is common, especially around young A stars. The quantities of molecular gas found in most systems are likely insufficient to strongly affect the dust dynamics or planet formation potential. While the origin of the gas is still a matter of discussion, there is increasing evidence that the sample is not homogeneous. The molecular gas in some systems is clearly second-generation like the dust, while in other systems the disk is likely to be a “hybrid” with second-generation dust coexisting with at least some primordial gas.

5.  Detection of time variable features in debris disk SEDs, images and light curves on timescales of days to years provide a window into ongoing dynamical processes in the disks and potentially the planetesimal belts as well.

6.  Planet-disk interaction is now directly observable in a handful of systems that contain both a directly imaged planet(ary system) and a spatially resolved debris disk. These rare systems provide valuable opportunities to place dynamical constraints on the masses of planets, and thereby to calibrate models of planetary atmospheres.

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1.  [*M star exploration:*]{} Most studies of debris disk structure and composition have so far focused on F, G, K, and A stars. While the incidence of M star debris disks appears to be low, the known disks include some of the most iconic systems including AUMic, and their gas and dust properties are poorly understood. Given then high frequency of terrestrial planets around M dwarfs, understanding their debris disks is a high priority.

2.  [*Physics of Debris Disk Morphology:*]{} Multiwavelength imaging is beginning to untangle the underlying physical mechanisms sculpting debris disk structure. Dynamically-induced disk structures from planets or stellar flybys should affect even the largest grains, whereas those caused by the ISM or radiation effects tend to produce the largest effect on the smallest grains. As limits on gas emission improve, gas becomes a less plausible mechanism for sculpting narrow and eccentric rings in many systems. Imaging of structures across the electromagnetic spectrum, particularly comparison of ALMA thermal imaging with high-contrast scattered light imaging, will be key for moving from categorization of structure to conceptualization of the underlying physics.

3.  [*The connection between hot, warm and cold dust belts:*]{} While multiple components are often detected in debris disk systems, identifying correlations between them has not yet delivered a fully coherent picture. Understanding the relative importance of in situ dust production and dust migration, as well as the physical mechanisms explaining the latter, remains an open question. Future observations sensitive enough to detect dust at intermediate radii between separate belts will help in clarifying this issue.

4.  [*Variability:*]{} While studies of gas absorption variability have a long and fascinating history – revealing the dynamics and composition of falling evaporating bodies – studies of the variability of dust, both directly in emission and indirectly through stellar variability, are in their infancy. Currently there are a handful of known instances, each of which has multiple proposed explanations ranging from planetary dynamics to collisional avalanches. While large variations in integrated infrared light are rare, they are important for understanding stochastic events. Furthermore, the recent detection of fast-moving scattered light features in the dust around AUMic suggests that at least in some cases systems previously assumed to be static might be variable on more subtle spatial and flux contrast scales. Debris disk variability is an area that is wide open for discovery and modeling, particularly in the approaching epoch of [*JWST*]{}.

5.  [*Gas statistics and chemistry:*]{} As the number of gas detections in debris disks increases, the opportunity for characterization of atomic and molecular abundances of the gas also grows. Clear opportunities include surveys designed to measure incidence of gas emission in a statistically meaningful way, in addition to detection of molecules other than CO and the corresponding opportunity to characterize the composition of exocometary gas. It would also be beneficial to amass rich data sets comparable to that of $\beta$Pic for a sample of several different objects so that modeling of both the atomic and molecular components can begin to explore the diversity of exosolar gas composition and better distinguish between primordial and secondary gas origins.

DISCLOSURE STATEMENT {#disclosure-statement .unnumbered}
====================

The authors are not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review.

ACKNOWLEDGMENTS {#acknowledgments .unnumbered}
===============

The authors wish to thank the following people for providing feedback and commentary on the article: Christine Chen, Kevin Flaherty, Paul Kalas, Grant Kennedy, Sasha Krivov, Luca Matra, Aki Roberge, Kate Su and Ewine van Dishoeck. The authors thank Eugene Chiang and Mark Wyatt for points of clarification. The authors are also grateful to the following people who agreed to share data used in the preparation of the figures in this review: Dan Apai, John Carpenter, Cail Daley, Bill Dent, Carsten Dominik, Jane Greaves, Paul Kalas, Markus Kasper, Mihoko Konishi, Meredith MacGregor, Sebastian Marino, Julien Milli, Johan Olofsson, Glenn Schneider. A.M.H. gratefully acknowledges support from NSF grant AST-1412647, and G.D. from NSF grants AST-1413718 and AST-1616479.
