# Debris Disks

Mark C. Wyatt

##  1 Introduction

Debris disk is a catch-all term that can be used to refer to any component of
a planetary system which is not an actual planet. In the Solar System this
refers to the asteroids and comets in the Asteroid and Kuiper belts as well as
the dust and gas derived from them, such as the zodiacal cloud. Studying the
structure of extrasolar debris disks provides unique constraints on the
underlying planetary system and on the processes of planet formation and
protoplanetary disk evolution. Debris disks also have important implications
for processes affecting the planets, such as impact events that may strip an
atmosphere or deliver volatiles. Their presence or absence also has
consequences both positive and negative for the detectability of exoplanets in
the system. Here I describe five of the most important ways in which debris
disks will contribute to our understanding of extrasolar planetary systems in
the coming years.

##  2 Placing the Solar system in context

Debris disks were first discovered over 30 years ago from the far-infrared
emission of circumstellar dust heated by the star [7]. This continues to be
the main method of discovery and several hundred such cold debris disks are
known, with temperatures that place the dust at 10s of au from their stars
(i.e., at distances comparable with our own Kuiper belt). Most recently
Spitzer and Herschel surveys have quantified the incidence of detectable cold
emission as $\sim 20$% across the spectral type range A-K [23, 93, 88] and has
shown that such disks decay in brightness as the stars age [82, 99] in a
manner that can be explained by mass loss due to collisional erosion within a
population of planetesimal belts [104, 54, 30]. However, we are not yet able
to detect true analogues to the Solar System’s debris disk [9, 95]. Thus we
cannot answer how typical the Solar System is in terms of its Kuiper belt and
zodiacal dust levels (i.e., where it sits amongst the $\sim 80$% of stars
without detectable debris). This is important to place our best example of a
habitable planetary system into context, especially if debris played a role in
its habitability and in the evolution of life [e.g., 1, 77, see also §4].

While Herschel had the sensitivity to achieve detections at a few times Solar
system levels [23], such photometric surveys are fundamentally limited in that
a Kuiper belt analogue emits less than 1% of the stellar flux at all
wavelengths, whereas neither the stellar flux nor the instrumental calibration
are known that accurately [e.g., 51]. Essentially this has limited our current
knowledge to the brightest 20% of extrasolar Kuiper belts and the brightest
$\sim 1$% of exozodiacal clouds [i.e., analogues to the cometary dust seen in
the inner Solar system; 44]. To detect true Kuiper belt analogues, these would
have to be resolved from the stellar emission, which requires high resolution
imaging capabilities with well characterised instrumental systematics to reach
the high contrast of the disk relative to the star, as well as techniques to
mitigate background confusion at some wavelengths [e.g., in the far-IR where
there is a high density of extragalactic sources, 74].

Recent improvements in nulling mid-IR interferometric techniques have resulted
in detections of exozodiacal emission with LBTI at 30 times Solar system
levels [25]. However, this still means that it is only the brightest $\sim
20$% of exozodi that have been detected, and inferences about lower dust
levels have to be gleaned by extrapolation. Yet, such low levels are important
not only for their astrophysical context (i.e., how the Solar system’s
cometary dust levels compare with other stars), but also because this dust is
a serious hindrance in our ability to detect emission from exo-Earths and its
presence or absence must be factored in to the design of telescopes such as
HabEx, LUVOIR and LIFE [84, 80]. For now such extrapolations suggest that the
median exozodi level could be as low as 3 times Solar system levels [25],
which would be good news for exo-Earth detection. However, this remains to be
confirmed with more sensitive observations and we can only confidently say
this median is $<27$ times Solar System levels.

Fortunately there is a range of instrumentation coming on-line that will help
progress towards the detection of true analogues to the Solar system’s debris
disk, and so our understanding of the Solar system’s place within the wider
population. The LBTI techniques and instrumentation can be improved, and JWST
provides high resolution and sensitivity mid-IR imaging that is imminent.
Shortly thereafter, even higher resolution capabilities will become available
with ELT and coronagraphic imaging with WFIRST. Further in the future far-IR
observatories such as Spica and the Origins Space Telescope, if approved,
would have the capability of detecting true Kuiper belt analogues towards the
nearest stars [85]. In the meantime, studies of the brightest 20% of debris
disks also provide a perspective on how the Solar system may have looked early
in its history, before the system-wide instability that depleted the Kuiper
belt [9, see also §3], or shortly after giant impact events like that which
formed the Moon [39, see also §4].

##  3 Constraints on planetary system architecture and evolutionary history

The proximity of nearby stars (10s of pc) and the large size of most debris
disks (typical radii in the range 30-150 au) means that, for the brightest
disks their emission can be spatially resolved providing a map of the dust
distribution in the system. Indeed the first debris disk was imaged around the
star $\beta$ Pictoris soon after its discovery [90], though it was not until
14 years later that the next five debris disk images were taken [36, 32, 41].
By now, well over 100 debris disks have been spatially resolved, at a range of
wavelengths from the optical, at which starlight scattered by the dust is
seen, and where the stellar emission has to be suppressed by coronagraphic
techniques, to the sub-mm, at which thermal emission from mm- to cm-sized dust
is seen that is thought to trace the parent planetesimals [for a review, see
38].

In general the structures observed in debris disks are well described as
planetesimal belts. However, these belts range in width from narrow rings
($\Delta r/r\approx 0.1$) to broad disks ($\Delta r/r>1$). Some of those which
are broad have radial sub-structure such as gaps which may be carved by
planets, [e.g., 59, 62, 63], reminiscent of similar structures seen in
protoplanetary disks [3], albeit over a narrower range of radii. The inner
edges of many disks are found to be sharp, which has been inferred to be
evidence for sculpting by a planet at the inner edge [15]. However, there are
other possible interpretations, such as the radial profile being set by where
planetesimals were able to form within protoplanetary disks, such as at
snowlines [e.g., 86]. Indeed there is emerging evidence that the radii of
debris belts may correlate with snowline locations [66].

The vertical structure of disks that are not face-on can also now be resolved,
with scale heights of order a few % being inferred and linked to stirring
processes by bodies embedded within or near to the disks [17, 67]. Non-
axisymmetric structures in the disks also provide evidence for planet-sized
bodies, the most unambiguous of which is the warp in the $\beta$ Pictoris
disk, since that may be linked to the known planet in the system [50, 5].
Other asymmetric structures, such as eccentric disks, clumps and spirals [98,
43, 21], have also been interpreted as evidence for planets. However, more
examples of systems where known planets can be linked to observed disk
structures would boost confidence in what can be inferred from disk structures
about underlying planetary systems. Nevertheless, we can be confident that
even small planets (e.g., as small as Neptune or even Earth) would leave an
imprint in the structure of a debris disk [e.g., 101, 79, 87], moreover one
that would be readily detectable with current imaging capabilities. Thus high
resolution observations of disk structure have the potential to pinpoint the
location of planets that would otherwise be extremely hard to detect using any
other techniques (e.g., Neptune analogues), and also can inform about their
evolutionary history [e.g., providing evidence of past migration; 97].

We are truly in an era of high resolution disk imaging, with ALMA continuing
to image new disks that can be followed up at even higher resolution. JWST
looks set to provide further high resolution images both of exo-Kuiper belts
and of any (mid-planetary system) dust structures that may lie inside these.
Longer term, coronagraphic imaging with WFIRST and ELT will provide further
advances. These disk images provide our best window into outer planetary
systems and the mechanisms involved in their formation and evolution.
Theoretical modelling techniques will also continue to be developed to find
ways to extract more concrete evidence for the presence of planets from the
images, e.g., using kinematic structures in the gas [21, 92], and this field
will further benefit when direct imaging capabilities allow the detection of
planets toward more than a few % of stars.

##  4 Planetary bombardment

As noted in §2, there remains uncertainty about the level of cometary activity
in the inner few au of nearby stars, as traced by their level of exozodiacal
dust. However, it is already clear that for 20% of stars, the level of dust in
their inner regions is orders of magnitude higher than in the Solar system.
These exozodi detections correlate with the presence of an outer belt [71,
25], but it is debated whether this dust is dragged inwards from the outer
belt [e.g., 83] or if the material arrives in the inner regions on larger
planetesimals through scattering amongst a planetary system before being
released by sublimation or disintegration [e.g., 22, 58]. It will soon be
possible to test such models by using JWST (and later WFIRST and ELT) to
search for dust at intermediate $\sim 10$ au separations. However, it is
already evident that dust levels in some systems are too high to be explained
by drag processes and so planetesimal delivery is required [20, 83].

If exozodis are replenished by planetesimals this opens up the possibility
that some of these planetesimals may collide with planets in the system, an
interaction with implications for conditions on the planets’ surfaces. For
example, planetesimal bombardment can both deplete a pre-existing atmosphere
and deliver complex organic molecules and water, as well as volatiles that can
be released to supply a secondary atmosphere [69, 16]. The level of
planetesimal bombardment in a system depends on a number of factors, most of
which are completely unknown, such as the architecture of the outer planetary
system. While this means there is potential for constraining that planetary
system from observations of exozodiacal dust and its relation to any outer
belt [i.e., providing complementary information to that discussed in §3, e.g.,
27, 100, 64], there are a number of theoretical hurdles. Notably, there is
still not a complete model that links the level of zodiacal dust to its origin
in comets [73]. Clearly there is scope for improved models that include all of
the processes at play in debris disks, including dynamical interactions,
collisions, radiation forces, gas release, gas drag, etc. [e.g., 46, 78].

While the exozodiacal dust observations show that it is plausible that
planetesimal bombardment is ongoing in a significant fraction of systems, it
should be acknowledged that bombardment does not need to be ongoing to have
(or have had) a significant effect on a planet. High levels of warm dust are
more common around stars in the first few 100 Myr of their lives [e.g., 89,
44], which could be indicative of an early bombardment phase as planetesimal
belts are being cleared. Indeed, the Solar system’s terrestrial planets are
thought to have experienced high bombardment levels early in their history
[72]. In general what matters for a planet is its integrated bombardment, the
timing being less critical, and so it will be important to understand what
these young bright warm disks tell us about typical bombardment histories.

This evidence for bombardment is opening up new lines of research, since it is
now possible to consider for example how this would affect a planet’s
propensity for habitability. It has been shown that there is a shoreline in
planet mass versus semi-major axis space that divides planets that would be
expected to grow secondary atmospheres under bombardment, and those that would
be expected to have their atmospheres eroded [102]. This process would be
competing with the effects of evaporation due to irradiation by the star, but
also with secondary outgassing as well as geological processes. Such
interactions have been considered for Solar system bodies where the
bombardment history is better constrained, but it is becoming relevant to
consider such processes in an exoplanetary context, given the growing body of
constraints on exoplanetary atmospheres and geology.

Whereas systems with bright hot dust were discussed above in the context of
external bombardment onto the planets, it has also been interpreted as
evidence for ongoing planet formation processes [105]. Specifically, this
could be the dust created in the final giant impact stage of terrestrial
planet growth. There is evidence for this in one system from the silica
composition of its dust inferred from the infrared spectrum [52]. The rapid
variability in some systems [70] is also explained in this context [40, 91].
If so, observations of these extreme debris disks tell us something about the
timing and duration of the giant impact phase, and potentially about the
frequency of terrestrial planet formation [e.g., 39]. For example, for Sun-
like stars this dust is seen within a few 0.1 au of the star, which could be
because this is where planets tend to form for such stars. This is perhaps
unsurprising given the prevalence of close-in super-Earth systems seen in
transit surveys, but this could mean that Solar system-like configurations are
rare. The coming years will see more examples of such extreme disks, as well
as studies of their variability. We can also expect advances in the models for
their interpretation which need to include a wider range of physical effects
(e.g., due to the optical depth of the dust). There are also puzzles that for
now defy easy explanation, such as the rapid dispersal of one disk [68], and
the high fraction of stars with significant quantities of $\sim 1000$ K dust
[e.g., 24]. Solutions to these puzzles may come in the form of theoretical
insights [e.g., 81, 78] or from further observational characterisation of the
phenomena.

##  5 Planetesimal composition

A fundamental property of the circumstellar material is its composition. This
could provide information on where and when the planetesimals replenishing
debris disk dust formed, as well as how those bodies may have been
subsequently altered by physical and geological processes.

The spectrum of the dust emission can in some cases be used to infer what it
is made of. Spectral features in the mid-IR associated with silicates are
often used in this way. However, most debris disks are too cold to emit in the
mid-IR. Such spectral features are also sensitive to the size of the dust,
being present only if the dust is smaller than the mid-IR wavelengths that
characterise the feature. Nevertheless, this has been used to show that the
bright warm dust in a few systems is silicate-rich and bears a similarity to
dust compositions found in the Solar system [53], while in one case the
abundance of silica implied formation in a recent giant impact [52]. Herschel
opened up a new possibility for the colder disks which was to identify
forsterite features at 69 $\mu$m, allowing the fraction of Fe and Mg to be
determined [18]. However, this was only achieved for 2 disks. Future far-IR
observatories like Spica could make such measurements routine. Scattered light
observations also have the potential to probe the dust composition, e.g.,
through its colour or phase function [e.g., 19], although such inferences are
again prone to sensitivity to the size of the dust and are limited by how well
we can model dust optical properties. Nevertheless, those optical properties
can be characterised using techniques like the discrete dipole approximation
[e.g., 6], and empirical comparisons with Solar system objects are also
possible [34]. The current set of observations with instruments like GPI and
SPHERE have lead to a recent increase in the number of scattered light
detections [75, 26], and these will gain momentum with JWST.

The volatile content of the planetesimals is harder to probe through spectral
features, though there are mid- and far-IR features due to ice that could be
detected by JWST and Spica [45]. However, ALMA is providing a growing body of
observations of gas in debris disks which is thought to have a secondary
origin, i.e., to have been released from volatile-rich planetesimals, rather
than being a remnant of the protoplanetary disk [21, 61, 60]. Most commonly
detected is CO, but there were a few detections of atomic carbon and oxygen
with Herschel [e.g., 12], and neutral carbon detections with ALMA are becoming
more prevalent [35, 13, 48, 14]. The paradigm within which these observations
are interpreted is that of release of volatiles during the collisional cascade
process, with those molecules photodissociating relatively quickly to produce
an atomic gas disk that spreads viscously [47, 49]. The relative abundances of
the observed gas and dust constrain their abundances on planetesimals which
looks to be similar to Solar system comets [61, 65]. This paradigm is not
without its challenges, for example there remains debate about whether the gas
is produced stochastically rather than in steady state [13, 14]. We can expect
significant observational progress on this in the coming years, first with
ALMA and SOFIA, but in the future with Spica. These observational advances
will have to go hand-in-hand with improvements in the models to accurately
couple the hydrodynamical evolution of the gas with its thermodynamics and
chemistry.

Perhaps the most direct measurement of planetesimal composition comes from
observations of white dwarf atmospheres [e.g., 28]. These are frequently seen
to be polluted with metals which have short sinking times, and so require
constant replenishment which is inferred to be from planetesimals that are
tidally disrupted before forming a disk that accretes onto the star [42, 37].
Multiple species have been observed in the atmospheres of many stars allowing
the bulk composition of the pollutants to be measured, which can then be
interpreted in terms of the temperature at which the planetesimal formed and
the differentiation processes that it underwent since formation [8]. While
such inferences are possible, there are still many unknowns; e.g., there is no
direct evidence for the location of the parent planetesimal belt [29], the
mechanism which caused the planetesimal to arrive at the star is not known, as
are the details of the tidal disruption and disk accretion process [94]. With
large numbers of white dwarfs being discovered in the era of large all-sky
surveys like Gaia, SDSS, ZTF and LSST [31], and follow-up spectroscopy being
performed to characterise any pollution, we can anticipate new observational
results in the coming years, as well as significant theoretical progress on
the various steps needed for the interpretation.

##  6 Protoplanetary disk dispersal

The recent far-IR surveys of Spitzer and Herschel have reinforced the paradigm
in which stars are born with a planetesimal belt that subsequently erodes
through collisional processing that passes mass from large planetesimals down
to dust that is eventually expelled by radiation forces [104, 63]. This model
fits the statistics for the fraction of stars seen to have detectable emission
as a function of age in different wavebands. However, as already noted this
necessarily cannot explain the 80% of stars without detectable disks. Further
analysis of the current detections is also leading to new insights, such as a
correlation of planetesimal belt radius with stellar mass [66]. The model also
avoids the question of how and when the planetesimal belts formed, rather
presupposing that these are present, with a collisional cascade already set
up, as soon as the protoplanetary disk disperses. However, with the discovery
of gas in debris disks, and the knowledge that dust in protoplanetary disks is
not primordial but subject to a continual cycle of growth and destruction, it
is becoming clear that the distinction between the two types of disk is not
well defined [103]. This is particularly noticeable when considering the
youngest systems, $<20$ Myr or so, which means that observations of such young
stars can inform on the processes of protoplanetary disk dispersal and of the
birth of a debris disk.

Studies of young stars generally involve first identifying associations of
stars that appear co-eval and with similar distances and space motions.
However, the relevant kinematic information has until now not been available
for the nearest star forming regions which lie at $\sim 150$ pc. While it has
nevertheless been relatively unambiguous to identify protoplanetary disks in
this way (i.e., the class II stars), this is harder for the young stars
without such an obviously bright disk (i.e., the class III stars), with
background stars often mistakenly falling in this category [57]. Gaia is
changing that, and the coming years will see improved membership of the
different nearby associations from which to study their disk populations [10,
56]. At the same time, ALMA observations of such regions have until now been
relatively shallow, i.e., only sensitive to protoplanetary disks [e.g., 4,
96]. However, deeper ALMA observations can probe for dust emission at levels
of the brightest debris disks in the nearest star forming regions, and can
also search for remnant primordial gas that may have yet to disperse after the
dust has been depleted [e.g., 76]. Thus it is timely now to perform a census
of the youngest debris disks as well as to search for protoplanetary disks in
the final stages of dissipation [e.g., 55]. Far-IR surveys are still more
sensitive than those in the sub-mm, and for those we will have to wait for
Spica which would be able to set limits on the presence of dust that are
comparable with those for nearby stars (except where imaging has been able to
probe fainter dust, see §2).

Significant advances have also been made in our understanding of
protoplanetary disks, both in their observational characterisation and in
theoretical advances in modelling the processes within them [for a recent
review, see 2]. These will continue along with the advances in debris disks
described in this chapter, and together it should be possible to infer the
sequence of events that lead to the depletion of a protoplanetary disk.
Observations of the gas will be particularly telling, since this component
dominates the mass of a protoplanetary disk, and its dynamical influence is
responsible for much of the dust morphology in such disks. Observations of the
dispersal of that component in a disk wind would constrain that process [33],
as well as its impact on dust and planetesimals in the disk [11, 76], and
planet formation more generally.

##  7 Opportunities and challenges

Debris disks represent an opportunity, by providing unique information about
exoplanetary systems. Observationally one challenge is to detect debris disks
as faint as those in the Solar System, particularly given how relatively
bright the host star is. Theoretically the challenge is to extract reliable
information in the face of so many unknowns about the architecture, history
and detailed physics of the underlying planetary system.

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