Minor Planet Evidence for Water in the Rocky Debris of a Disrupted Extrasolar MInor Planet
, 218 (2013);342Science
et al.J. Farihi
Evidence for Water in the Rocky Debris of a Disrupted Extrasolar
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and morphometric measurements of the tissues
in the developing mouse gut (Fig. 6C). Using
these measurements as inputs in our model suf-
fices to quantitatively predict the formation of
villi (supplementary materials, Fig. 6D, and movie
S3). Compared with the chick, where the endo-
derm is more than 10 times stiffer than the ad-
jacent mesenchyme, the mouse endoderm is only
about 1.5 times as stiff as the mesenchyme (fig.
S3). Our simulations show that the soft endoderm
in mouse is essential for the initial folding that oc-
curs in endoderm alone and for the direct formation
of an array of previllous bumps, rather than zig-
zags, which are qualitatively similar to sulcus for-
mation on biaxially compressed gel surfaces that
lack a stiff top layer (24). The spacing of bumps
and, consequently, the spacing of villi are compa-
rable to the thickness of the whole endoderm-
mesenchyme composite (Fig. 6C), similar to chick.
The process of villification occurs before the
differentiation of the gut endoderm into various
epithelial cell types (25–27) and well before the
postnatal process of crypt formation. In vitro cul-
ture of intestinal stem cells results in the forma-
tion of intestinal organoids that reproduce crypt
structure (28). These organoids consist of an
inner epithelium with villuslike cell types and
outwardly projecting cryptlike structures. How-
ever, no morphological structures are present in
these in vitro cultures resembling the physical
villi. These results suggest that crypt formation
likely does not require the same muscle-driven
compression that is necessary for villi to form.
Additionally, further study is needed to un-
derstand whether structural differences in the
lumen of different regions of the gut are attrib-
utable to distinctions in the parameters we have
measured. For example, the short, wide villi that
coat large longitudinal folds of the chick colon
may be attributable to the thicker muscle layers
of the colon. Consistent with the muscle playing
such a role, studies have shown that transposi-
tion of a ring containing all radial layers of the
colon into regions of the small intestine preserve
villi morphology (29).
Our previous work provided a mechanical
basis for the diversity of macroscopic looping
patterns of the gut based on geometry, differen-
tial growth, and tissue mechanics (30), and our
present results demonstrate that the same phys-
ical principles drive morphological variation on
the luminal surface of the gut. Further, we see
that relatively minor changes in the geometry,
growth, and physical properties of the develop-
ing tissue in the guts of various species can
substantially alter both the process and the form
of villus patterning. A deep understanding of how
patterns vary requires us to combine our knowl-
edge of biophysical mechanisms with the genetic
control of cell proliferation and growth; indeed
this variation can occur in an organism as a func-
tion of its diet, across species, and over evolu-
tionary time scales via natural selection.
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Acknowledgments: We thank M. Kirschner for providing
Xenopus tadpoles and O. Pourquie for providing snake embryos.
D.L.K. and Tufts University hold a series of patents that cover the
processing of silk into material structures, including those used
in the research reported here. T.T. acknowledges the Academy of
Finland for support. Computations were run at CSC–IT Center
for Science, Finland. C.J.T. acknowledges the support of a grant
from NIH RO1 HD047360. L.M. acknowledges the support of
the MacArthur Foundation.
Materials and Methods
Figs. S1 to S11
Movies S1 to S3
8 April 2013; accepted 13 August 2013
Published online 29 August 2013;
Evidence for Water in the Rocky Debris
of a Disrupted Extrasolar Minor Planet
* B. T. Gänsicke,2
The existence of water in extrasolar planetary systems is of great interest because it constrains the
potential for habitable planets and life. We have identified a circumstellar disk that resulted
from the destruction of a water-rich and rocky extrasolar minor planet. The parent body formed
and evolved around a star somewhat more massive than the Sun, and the debris now closely orbits
the white dwarf remnant of the star. The stellar atmosphere is polluted with metals accreted
from the disk, including oxygen in excess of that expected for oxide minerals, indicating that the
parent body was originally composed of 26% water by mass. This finding demonstrates that
water-bearing planetesimals exist around A- and F-type stars that end their lives as white dwarfs.
he enormous recent progress in the dis-
covery of exoplanetary systems provides a
growing understanding of their frequency
and nature, but our knowledge is still limited in
many respects. There is now observational evi-
dence of rocky exoplanets (1, 2), and the mass
and radius (and hence density) of these planets
can be calculated from transit depth and radial
velocity amplitude; however, estimates of their
bulk composition remain degenerate and model-
dependent. Transit spectroscopy offers some in-
formation on giant exoplanet atmospheres (3), and
planetesimal debris disks often reveal the signa-
ture of emitting dust and gas species (4), yet both
techniques only scratch the surface of planets, as-
teroids, and comets. Interestingly, white dwarfs—
the Earth-sized embers of stars like the Sun—offer
a unique window onto terrestrial exoplanetary sys-
tems: These stellar remnants can distill entire
Institute of Astronomy, University of Cambridge, Cambridge
CB3 0HA, UK. 2
Department of Physics, University of Warwick,
Coventry CV5 7AL, UK. 3
Institut für Theoretische Physik und
Astrophysik, University of Kiel, 24098 Kiel, Germany.
*Corresponding author. E-mail: firstname.lastname@example.org
11 OCTOBER 2013 VOL 342 SCIENCE www.sciencemag.org218
planetesimals into their constituent elements,
thus providing the bulk chemical composition for
the building blocks of solid exoplanets.
Owing to high surface gravities, any atmo-
spheric heavy elements sink rapidly as white
dwarfs cool below 25,000 K (5), leaving be-
hind only hydrogen and helium in their outer-
most layers—a prediction that is corroborated
by observation (6). Those white dwarfs with rocky
planetary system remnants can become con-
taminated by the accretion of small, but spec-
troscopically detectable, amounts of metals (7).
Heavy element absorption lines in cool white
dwarfs are a telltale of external pollution, often
implying either ongoing mass accretion rates
(8) or large asteroid-sized masses
of metals within the convection zone of the
In recent years, metal-rich dust (10, 11) and gas
(12) disks, likely produced by the tidal disruption
of a large asteroid (13), have been observed to
be closely orbiting 30 cool white dwarfs [e.g.,
(14–19)] and provide a ready explanation for
the metal absorption features seen in their atmo-
spheres (20). The circumstellar material being
gradually accreted by the white dwarf can be
directly observed in the stellar photosphere to
reveal its elemental abundances (21). These plan-
etary system remnants offer empirical insight
into the assembly and chemistry of terrestrial exo-
planets that is unavailable for any exoplanet or-
biting a main-sequence star.
Until now, no white dwarf has shown re-
liable evidence for the accretion of water-rich,
rocky planetary material. Unambiguous signa-
tures of icy asteroids at white dwarfs should
include (i) atmospheric metal pollution rich in
refractory elements; (ii) trace oxygen in excess
of that expected for metal oxides; (iii) circum-
stellar debris from which these elements are ac-
creted; and, where applicable, (iv) trace hydrogen
(in a helium-dominated atmosphere) sufficient
to account for the excess oxygen as H2O. The
presence of a circumstellar disk signals that ac-
cretion is ongoing, identifies the source material,
and enables a confident quantitative assessment
of the accreted elemental abundances, which in
turn allows a calculation of the water fraction of
the disrupted parent body.
The metal-enriched white dwarfs GD 362 and
GD 16 both have circumstellar disks and relatively
large trace hydrogen abundances in helium-
dominated atmospheres (22), but as yet no as-
sessment of photospheric oxygen is available
(21, 23). These two stars have effective temper-
atures below 12,000 K, and their trace hydrogen
could potentially be the result of helium dredge-
up in a previously hydrogen-rich atmosphere (24).
The warmer, metal-lined white dwarfs GD 61
and GD 378 have photospheric oxygen (25), but
the accretion history of GD 378 is unconstrained
(i.e., it does not have a detectable disk), and
without this information, the atmospheric oxygen
could be consistent with that contained in dry min-
erals common in the inner solar system (26). In
the case of GD 61, elemental abundance uncer-
tainties have previously prevented a formally sig-
nificant detection of oxygen excess (27).
We used the Cosmic Origins Spectrograph
(COS) onboard the Hubble Space Telescope to
obtain ultraviolet spectroscopy of the white dwarf
GD 61, and, together with supporting ground-
based observations, we derived detections or lim-
its for all the major rock-forming elements (O,
Mg, Al, Si, Ca, Fe). These data permit a con-
fident evaluation of the total oxygen fraction
present in common silicates within the parent
body of the infalling material, and we identified
excess oxygen attributable to H2O as follows.
(i) The observed carbon deficiency indicates that
this element has no impact on the total oxygen
budget, even if every atom is delivered as CO2.
(ii) The elements Mg, Al, Si, and Ca are as-
sumed to be carried as MgO, Al2O3, SiO2, and
CaO at the measured or upper-limit abundance.
(iii) The remaining oxygen exceeds that which
can be bound in FeO, and the debris is interpreted
to be water-rich. By this reasoning, we found oxy-
gen in excess of that expected for anhydrous min-
erals in the material at an H2O mass fraction of
0.26 (Table 1 and Fig. 1).
Because we have assumed the maximum al-
lowed FeO, and because some fraction of metal-
lic iron is possible, the inferred water fraction of
the debris is actually bound between 0.26 and
0.28. Although this makes little difference in the
case of GD 61, where the parent body material
appears distinctly mantle-like (27), there are at least
two cases where metallic iron is a major (and
even dominant) mass carrier within the parent
bodies of circumstellar debris observed at white
dwarfs (28). Overall, these data strongly suggest
that the material observed in and around polluted
white dwarfs had an origin in relatively massive
and differentiated planetary bodies.
We have assumed a steady state between ac-
cretion and diffusion in GD 61. However, a typ-
ical metal sinking time scale for this star is 105
years, and thus the infalling disk material could
potentially be in an early phase of accretion where
material accumulates in the outer layers, prior
to appreciable sinking (27). In this early-phase
scenario, the oxygen excess and water fraction
would increase relative to those derived from
the steady-state assumption, and hence we confi-
dently conclude that the debris around GD 61
originated in a water-rich parent body. Although
the lifetimes of disks at white dwarfs are not
robustly constrained, the best estimates imply
Table 1. Oxide and water mass fractions in
the planetary debris at GD 61. We adopt the
steady-state values, which assume accretion-diffusion
Oxygen carrier Steady state Early phase
CO2 <0.002 <0.002
MgO 0.17 0.18
Al2O3 <0.02 <0.02
SiO2 0.32 0.27
CaO 0.02 0.01
FeO* 0.05 0.02
Excess 0.42 0.50
H2O in debris 0.26 0.33
*All iron is assumed to be contained in FeO; some metallic Fe
will modestly increase the excess oxygen.
Fig. 1. Oxygen budget in GD 61 and terrestrial bodies. The first two columns are the early phase
(EP) and steady-state (SS) fractions of oxygen carried by all the major rock-forming elements in GD 61,
assuming that all iron is carried as FeO. Additional columns show the oxide compositions of the bulk
silicate (crust plus mantle) Earth, Moon, Mars, and Vesta (35). Their totals do not reach 1.0 because trace
oxides have been omitted. The overall chemistry of GD 61 is consistent with a body composed almost
entirely of silicates, and thus appears relatively mantle-like but with substantial water. In contrast, Earth is
relatively water-poor and contains approximately 0.023% H2O (1.4 × 1024
www.sciencemag.org SCIENCE VOL 342 11 OCTOBER 2013 219
that the chance of catching GD 61 in an early
phase is less than 1% (17, 29–31).
The helium-rich nature of GD 61 permits an
assessment of its trace hydrogen content and
total asteroid mass for a single parent body. The
total metal mass within the stellar convection
zone is 1.3 × 1021
g, roughly equivalent to that
of an asteroid 90 km in diameter. However, be-
cause metals continuously sink, it is expected
that the destroyed parent body was substantially
more massive, unless the star is being observed
shortly after the disruption event. In contrast, hy-
drogen floats and accumulates, and thus places
an upper limit on the total mass of accreted water-
rich debris. If all the trace hydrogen were deliv-
ered as H2O from a single planetesimal, the total
accreted water mass would be 5.2 × 1022
g, and a
26% H2O mass fraction would imply a parent
body mass of 2 × 1023
g, which is similar to that
of the main-belt asteroid 4 Vesta (32).
These data imply that water in planetesi-
mals can survive post–main sequence evolution.
One possibility is that solid or liquid water is
retained beneath the surface of a sufficiently large
(diameter >100 km) parent body (26), and is
thus protected from heating and vaporization
by the outermost layers. Upon shattering during
a close approach with a white dwarf, any ex-
posed water ice (and volatiles) should rapidly
sublimate but will eventually fall onto the star;
the feeble luminosities of white dwarfs are in-
capable of removing even light gases by radia-
tion pressure (31). Another possibility is that a
substantial mass of water is contained in hydrated
minerals (e.g., phyllosilicates), as observed in main-
belt asteroids via spectroscopy and inferred from
the analysis of meteorites (33). In this case, the
H2O equivalent is not removed until much higher
temperatures are attained, and such water-bearing
asteroids may remain essentially unaffected by
the giant phases of the host star.
The white dwarf GD 61 contains the unmis-
takable signature of a rocky minor planet anal-
ogous to the asteroid 1 Ceres in water content
(34) and probably analogous to Vesta in mass.
The absence of detectable carbon indicates that
the parent body of the circumstellar debris was
not an icy planetesimal analogous to comets, but
was instead similar in overall composition to
asteroids in the outer main belt. This exoplan-
etary system originated around an early A-type
star that formed large planetesimals similar to
those in the inner solar system that were the
building blocks for Earth and other terrestrial
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Acknowledgments: This work is based on observations
made with the Hubble Space Telescope, which is operated
by the Association of Universities for Research in Astronomy
under NASA contract NAS 5-26555. These observations are
associated with program programs 12169 and 12474. Some
of the data presented herein were obtained at the W. M. Keck
Observatory, which is operated as a scientific partnership
among the California Institute of Technology, the University
of California, and NASA. The Observatory was made possible
by the generous financial support of the W. M. Keck
Foundation. J.F. acknowledges support from the UK Science
and Technology Facilities Council in the form of an Ernest
Rutherford Fellowship (ST/J003344/1). The research leading
to these results has received funding from the European Research
Council under the European Union’s Seventh Framework
Programme (FP/2007-2013)/ERC Grant Agreement no. 267697
(WDTracer). B.T.G. was supported in part by the UK Science and
Technology Facilities Council (ST/I001719/1). Keck telescope time
for program 2011B-0554 was granted by NOAO through the
Telescope System Instrumentation Program, funded by NSF.
Materials and Methods
Tables S1 and S2
References (36, 37)
22 April 2013; accepted 15 August 2013
of Lattice Dynamics in
* S. Boutet,1
G. J. Williams,1
A. E. Gleason,3
M. M. Seibert,1,4
D. C. Swift,5
W. E. White,1
J. S. Wark2
The ultrafast evolution of microstructure is key to understanding high-pressure and strain-rate
phenomena. However, the visualization of lattice dynamics at scales commensurate with those
of atomistic simulations has been challenging. Here, we report femtosecond x-ray diffraction
measurements unveiling the response of copper to laser shock-compression at peak normal elastic
stresses of ~73 gigapascals (GPa) and strain rates of 109
per second. We capture the evolution
of the lattice from a one-dimensional (1D) elastic to a 3D plastically relaxed state within a few tens
of picoseconds, after reaching shear stresses of 18 GPa. Our in situ high-precision measurement of
material strength at spatial (<1 micrometer) and temporal (<50 picoseconds) scales provides
a direct comparison with multimillion-atom molecular dynamics simulations.
he distinct properties of materials at high-
pressure and/or strain-rate conditions lead
to a broad range of phenomena in fields
such as high-energy-density physics (1), Earth
and planetary sciences (2, 3), aerospace engi-
neering (4), and materials science (5, 6). For the
latter, a predictive understanding and control
of mechanical properties, enabled by the di-
rect comparison of experiments with large-scale
atomistic simulations, is the ultimate goal. Where-
as the bulk material behavior can be inferred
by macroscopic measurements (7, 8), key infor-
mation on the mechanical properties requires
knowledge of the physics embedded at the
lattice level. Such knowledge has traditionally
been obtained via nanosecond-resolution x-ray
11 OCTOBER 2013 VOL 342 SCIENCE www.sciencemag.org220
Supplementary Materials for
Evidence for Water in the Rocky Debris of a Disrupted Extrasolar Minor
J. Farihi,* B. T. Gänsicke, D. Koester
*Corresponding author. E-mail: email@example.com
Published 11 October 2013, Science 342, 218 (2013)
This PDF file includes:
Materials and Methods
Tables S1 and S2
Supporting Online Material for
Evidence for Water in the Rocky Debris
of Disrupted Extrasolar Minor Planets
, B. T. G¨ansicke2
, D. Koester3
Institute of Astronomy, University of Cambridge, Cambridge CB3 0HA, UK
Department of Physics, University of Warwick, Coventry CV5 7AL, UK
Institut f¨ur Theoretische Physik und Astrophysik, University of Kiel, 24098 Kiel, Germany
STFC Ernest Rutherford Fellow
To whom correspondence should be addressed; E-mail: firstname.lastname@example.org
We describe here in detail the observations and analyses supporting the main paper, speciﬁcally
the spectroscopy of the metal-enriched white dwarf atmosphere and the analytical link to the
elemental abundances of the infalling planetary debris.
1 Summary of the Observations and Datasets
GD 61 exhibits infrared excess consistent with circumstellar dust orbiting within its Roche limit
(26), and bears the unambiguous signature of debris accretion via its metal-polluted atmosphere.
The white dwarf was observed with the Cosmic Origins Spectrograph (COS) during Hubble
Space Telescope Cycle 19 on 2012 January 28. The ultraviolet spectra were obtained with a
total exposure time of 1600 s (split between two FP-POS positions) using the G130M grating
and a central wavelength setting at 1291 ˚A, covering 1130−1435 ˚A at R ≈ 18 000. The COS
data were processed and calibrated with CALCOS 2.15.6, and are shown in Figure S1. Optical
spectroscopy of GD61 was obtained on 2011 October 24 with the Keck II Telescope and the
Echelle Spectrograph and Imager (36, ESI) in echelle mode, effectively covering 3900−9200 Å
at R 13 000. The spectra were obtained in a series of 16 exposures of 900 s each, for a total
exposure time of 4 hr, and reduced using standard tasks in IRAF1
2 Derivation of Photospheric and Debris Abundances
Elemental abundances for GD61 were derived from the COS and ESI data by fitting white dwarf
atmospheric models (37) to the observed spectra. For these calculations, Teff = 17 280 K and
log g = 8.20 are adopted, based on a published analysis of low-resolution optical spectra (24).
The resulting photospheric abundances and upper limits are listed in Table S1 together with
previous measurements from the Far Ultraviolet Spectroscopic Explorer (24, FUSE) and Keck I
HIRES (26). Notably, all heavy element abundances agree well, despite being derived using
separate instruments and with multiple absorption lines across distinct wavelength regimes.
The transformation between the heavy element abundances in the white dwarf
atmospheres and those within the infalling planetary debris are calculated assuming a steady
state balance between accretion and diffusion. An early (or build-up) phase of accretion is
theoretically possible in GD61, but this is unlikely (see main paper). Importantly, in this case an
early phase would imply a larger oxygen excess and H2O fraction, and therefore the more
conservative, and most probable, assumption is made.
For white dwarfs with significant convection zones like GD61, the atmospheric mass
fraction Xz of heavy element z is related to its accretion rate zM via
z cvz zz tX MM (S1)
IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the
Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science
where tz is the sinking timescale for the element and Mcvz is the mass of the stellar convection
zone. The mass fraction is determined from the model atmosphere ﬁts and the sinking timescale
is known from white dwarf diffusion calculations (5). In essence, Equation S1 states that the
accretion rate of element z equals its rate of depletion as it settles below the mixing layer. The
ratio of two heavy elements within the debris (and hence parent body) is either the ratio of their
respective accretion rates in the steady state, or the ratio of their atmospheric mass fractions in
the early phase, and related by
Table S2 lists the relevant quantities of GD 61 for the key elements that determine the total
oxygen budget of the debris. The steady state metal abundances relative to oxygen are taken
from the fourth column. The sinking timescales for GD 61 have been updated following a
correction in the theoretical calculations2
, and they are somewhat different than those presented
in a previous analysis (26). Notably, this correction has strengthened the case for an oxygen
excess in GD 61.
3 Evaluation of Oxygen Excess and Uncertainties
The method for calculating the overall oxygen budget is as follows. We begin with the columns
in Table S2, and in particular the identify the total oxygen budget with: 1) its mass accretion
rate for the steady state or 2) its mass within the stellar convection zone for the early phase.
We calculate the fraction of oxygen that can be absorbed as CO2 based on the upper limit for
carbon, and subtract this from the total available. Next, we perform a similar calculation for the
mass of oxygen in MgO, Al2O3, Si2, CaO, and FeO based on their detections or upper limits,
again subtracting these from the budget. After accounting for all the major oxygen carriers, any
remaining mass is considered excess.
The collective data for GD 61 is robust and comprehensive, comprising four instruments
with each probing distinct wavelength regions and containing multiple transitions for each ele-
ment from the far-ultraviolet to the red optical region. The uncertainties in the metal abundances
of this white dwarf are given as 3σ adopted values in the last column of Table S1. Using a brute
force approach, all 128 possible combinations of abundance values are calculated for C, O, Mg,
Al, Si, Ca, Fe where the abundance values N(X)/N(He) take on each of the values x ± δx.
Evaluating all possible permutations, the dispersion in the resulting oxygen excesses values
(0.068) results in a 6.1σ conﬁdence for the case of steady state accretion.
References and Notes
1. N. M. Batalha, et al., Astrophys. J. 729, 27 (2011)
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Figure S1. The normalized COS spectra of GD 61 (grey), together with the best ﬁtting model
spectra (red). Interstellar absorption features are indicated by vertical grey dashed lines, and
are blueshifted with respect to the photospheric features by 40 km s−1
. Geocoronal airglow of
O I at 1302.2, 1304.9, and 1306.0 ˚A can contaminate COS spectra to some degree, and typical
airglow line proﬁles are shown in the middle panel scaled to an arbitrary ﬂux.
Table S1. Elemental Abundances N(X)/N(He) in GD 61
Element COS FUSE ESI HIRES Adopted
H −3.70 (0.10) −4.00 (0.10) −3.98 (0.10) −3.89 (0.15)
O −6.00 (0.15) −5.80 (0.20) −5.75 (0.20) −5.95 (0.13)
Mg −6.50 (0.30) −6.74 (0.10) −6.65 (0.18) −6.69 (0.14)
Si −6.82 (0.12) −6.70 (0.20) −6.85 (0.10) −6.85 (0.09) −6.82 (0.11)
S −8.00 (0.20) −8.00 (0.20)
Ca −7.77 (0.06) −7.90 (0.19) −7.90 (0.19)
Fe −7.60 (0.30) −7.60 (0.20) −7.60 (0.20)
C −9.10 −8.80
Al −7.80 −7.20
Table S2. Atmospheric and Debris Properties for Key Trace Elements in GD 61
Early Phase Steady State
Element tdiﬀ XzMcvz
H ∞ 5.755
C 1.730 < 0.001 < 0.001
O 1.706 0.802 1.489
Mg 1.808 0.222 0.389
Al 1.735 < 0.019 < 0.035
Si 1.438 0.190 0.419
S 0.952 0.014 0.048
Ca 0.782 0.023 0.091
Fe 0.855 0.063 0.232
Total Z 1.332 2.704
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regimes are derived directly from the values in the third and fourth columns respectively.
The third column is the mass of each element residing in the convection zone of GD 61, and
their total (excluding hydrogen) represents a minimum mass for the parent body due to the
continual sinking of metals.
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