Kepler-16: A Transiting Circumbinary Planet
Laurance R. Doyle1, Joshua A. Carter2, Daniel C. Fabrycky3, Robert W. Slawson1, Steve B. Howell4,
Joshua N. Winn5, Jerome A. Orosz6, Andrej Prˇsa7, William F. Welsh6, Samuel N. Quinn8, David
Latham8, Guillermo Torres8, Lars A. Buchhave9, 10, Geoffrey W. Marcy11, Jonathan J. Fortney12, Avi
Shporer13,14, Eric B. Ford15, Jack J. Lissauer4, Darin Ragozzine2, Michael Rucker16, Natalie Batalha16,
Jon M. Jenkins1, William J. Borucki4, David Koch4, Christopher K. Middour17, Jennifer R. Hall17,
Sean McCauliff17, Michael N. Fanelli18, Elisa V. Quintana1, Matthew J. Holman8, Douglas A.
Caldwell1, Martin Still18, Robert P. Stefanik8, Warren R. Brown8, Gilbert A. Esquerdo8, Sumin
Tang8, Gabor Furesz8,19, John C. Geary8, Perry Berlind20, Michael L. Calkins20, Donald R. Short21,
Jason H. Steffen22, Dimitar Sasselov8, Edward W. Dunham23, William D. Cochran24, Alan Boss25,
Michael R. Haas4, Derek Buzasi26, Debra Fischer27
1Carl Sagan Center for the Study of Life in the Universe, SETI Institute, 189 Bernardo Avenue, Mountain View,
CA 94043, USA, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com,
2Hubble Fellow, Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138,
3Hubble Fellow, Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064,
4 NASA Ames Research Center, Moffett Field, CA 94035, USA, firstname.lastname@example.org,
email@example.com, firstname.lastname@example.org, email@example.com
5Massachusetts Institute of Technology, Physics Department and Kavli Institute for Astrophysics and Space
Research, 77 Massachusetts Avenue, Cambridge, MA 02139, USA, firstname.lastname@example.org
6Astronomy Department, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182-1221,
7Villanova University, Dept. of Astronomy and Astrophysics, 800 E Lancaster Ave, Villanova, PA 19085, USA,
8Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA,
email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org,
email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org,
9Niels Bohr Institute, Copenhagen University, DK-2100 Copenhagen, Denmark, email@example.com
10Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, DK-1350
11Astronomy Department, University of California, Berkeley, CA, 94720, USA, firstname.lastname@example.org
12Department of Astronomy and Astrophysics, University of California, Santa Cruz, Santa Cruz, CA 95064,
13Las Cumbres Observatory Global Telescope Network, 6740 Cortona Drive, Suite 102, Santa Barbara, CA 93117, USA,
14Department of Physics, Broida Hall, University of California, Santa Barbara, CA 93106, USA, email@example.com
15211 Bryant Space Science Center, Gainesville, FL 32611-2055, USA, firstname.lastname@example.org
16Physics Department, San Jose State University, San Jose, CA, 95192, USA, email@example.com,
17Orbital Sciences Corporation/NASA Ames Research Center, Moffett Field, CA 94035, USA,
firstname.lastname@example.org, email@example.com, firstname.lastname@example.org
18Bay Area Environmental Research Inst./NASA Ames Research Center, Moffett Field, CA 94035, USA
19Konkoly Observatory, Konkoly ut 15-17, Budapest, H-1121, Hungary
20Fred Lawrence Whipple Observatory, Smithsonian Astrophysical Observatory, Amado, AZ 85645, USA,
21Mathematics Department, San Diego State University, 5500 Campanile Drive, San Diego, CA USA 92182,
22Fermilab Center for Particle Astrophysics, P.O. Box 500, Batavia IL 60510, USA email@example.com
23Lowell Observatory, Flagstaff, AZ, 86001, USA, Dunham@lowell.edu
24McDonald Observatory, University of Texas at Austin, Austin, TX, 78712, USA, firstname.lastname@example.org
25Carnegie Institute of Washington, Washington, DC 20015 USA, email@example.com
26Eureka Scientific, Inc., 2452 Delmer Street Suite 100, Oakland, CA 94602, USA, firstname.lastname@example.org
27Department of Astronomy, Yale University, New Haven, CT 06511 USA, email@example.com
Submitted to Science: July 7, 2011
Final revision submitted: August 16, 2011
We report the detection of a planet whose orbit surrounds a pair of low-mass stars.
Data from the Kepler spacecraft reveal transits of the planet across both stars, in addition to
the mutual eclipses of the stars, giving precise constraints on the absolute dimensions of all
three bodies. The planet is comparable to Saturn in mass and size, and is on a nearly
circular 229-day orbit around its two parent stars. The eclipsing stars are 20% and 69% as
massive as the sun, and have an eccentric 41-day orbit. The motions of all three bodies are
confined to within 0.5° of a single plane, suggesting that the planet formed within a
A planet with two suns is a familiar concept from science fiction. However, the
evidence for the existence of circumbinary planets—those that orbit around both members of
a stellar binary—has been limited. A few good cases have been made for circumbinary
planets based upon timing of stellar eclipses (see, e.g., refs. 1-3), but in no previous case have
astronomers obtained direct evidence of a circumbinary planet by observing a planetary
transit (a miniature eclipse as the planet passes directly in front of a star). Detection of a
transit greatly enhances confidence in the reality of the planet, and provides unusually precise
knowledge of its mass, radius, and orbital parameters (4).
Here we present the detection of a transiting circumbinary planet around a binary star
system based on photometric data from the NASA Kepler spacecraft. Kepler is a 0.95m
space telescope that monitors the optical brightness of about 155,000 stars within a field
encompassing 105 square degrees in the constellations Cygnus and Lyra (5-8).
Star number 12644769 from the Kepler Input Catalog was identified as an eclipsing
binary with a 41-day period, from the detection of its mutual eclipses (9). Eclipses occur
because the orbital plane of the stars is oriented nearly edge-on as viewed from Earth.
During primary eclipses the larger star, denoted “A”, is partially eclipsed by the smaller star
“B”, and the system flux declines by about 13%. During secondary eclipses B is completely
occulted by A, and the resulting drop in flux is only about 1.6% because B is relatively small
and has a lower surface brightness (Figure 1).
This target drew further attention when three additional drops in brightness were
detected outside of the primary and secondary eclipses, separated by intervals of 230.3 and
221.5 days (10). These tertiary eclipses could not be attributed to the stars alone, and
indicated the presence of a third body. The differing intervals between the tertiary eclipses
are simply explained if the third body is in a circumbinary orbit, because stars A and B would
be in different positions in their mutual orbit each time the third body moved in front of them
(11, 12). In contrast, there would be no ready explanation for the shifting times of the
tertiary eclipses if they were produced by a background star system or some other unrelated
During tertiary eclipses the total light declines by 1.7%. Because this is larger than
the 1.6% decline during secondary eclipses (when star B is completely concealed), the
tertiary eclipses had to be transits of the third body across star A. This interpretation was
supported by the subsequent detection of weaker 0.1% quaternary eclipses, which were
consistent with the passage of the third body across star B. The observed time of this
quaternary eclipse was used to predict two other times of quaternary eclipses that should
have been present in the data, and these two events were subsequently detected (Figure 1).
Because the third body covers only 1.7% of the area of star A, which was determined
to be smaller than the Sun based on its broad band colors (10), the circumbinary body was
suspected to be either a planet, or a third star with grazing eclipses. Decisive evidence that
it is a planet came from investigation of the timing of the stellar eclipses. The primary and
secondary eclipse times were found to depart from strict periodicity by deviations of order
one minute. A third body causes timing variations in two ways. Firstly there is a light travel-
time effect: the third body induces a periodic motion of the center of mass of the stellar
binary, causing periodic variations in the time required for the eclipse signals to reach the
Earth (13, 14). Secondly there is a dynamical effect: the gravitational attraction of each star
to the third body varies with time due to the changing positions of all three bodies, causing
perturbations in the stars' orbital parameters and therefore in the eclipse times (15, 16). Both
effects depend on the mass of the third body. Therefore we could constrain the mass of the
third body by fitting the eclipse data with a numerical model of three-body gravitational
interactions. This model, described below in detail, showed that the third body must be less
massive than Jupiter.
Hence, based on the depth of the tertiary eclipses, and on the magnitude of the eclipse
timing variations, the third body was shown to be a transiting circumbinary planet.
The model was based on the premise that the three bodies move under the influence
of mutual Newtonian gravitational forces. For this purpose we modified the computer code
that was used to model the triple star system KOI-126 (17, SOM). The leading-order
relativistic correction to the force law was included, although it proved to be unimportant.
The bodies’ positions were calculated with a Bulirsch-Stoer algorithm and corrected for the
finite propagation speed of light across the system before comparing to the data. The loss of
light due to eclipses was calculated by assuming the disks of stars A and B to be circular,
with a quadratic law describing the decline in intensity toward the limb (18). We also
allowed for an additional time-independent source of light to account for any possible
background stars within the Kepler photometric aperture. In practice this parameter was
found to be consistent with zero, and bounded to be less than 1.3% of the total light of the
We fitted all of the photometric data within 6 hours of any eclipse or transit. Before
fitting, a linear trend was removed from each segment, to correct for the slow starspot-
induced variations evident in Figure 1. A successful model had to be compatible with the
timings, durations, and depths of the primary and secondary stellar eclipses, as well as the
transits of the planet across both stars. The model also had to account for the slight
departures from strict periodicity of the stellar eclipses. Furthermore, to pin down the stellar
masses and provide an absolute distance scale, we undertook spectroscopic observations to
track the radial velocity variations of star A (Figure 2, top panel).
The model parameters were adjusted to fit the photometric and radial-velocity data
(Table 1). Figures 1 and 2 show the very good match that was achieved between the model
and the data. Uncertainties in the parameters were determined with a Differential Evolution
Markov Chain Monte Carlo simulation (21, SOM).
Due to the presence of uniquely three-body effects (namely, the shifts in eclipse times
and transit durations), the masses, radii, and orbital distances of this system are well
determined in absolute units, and not just in relative units. The eclipse timing variations are
dominated by the effects of dynamical perturbations, with light-time variations contributing
only at the level of one second. The third body’s dimensions are well within the planetary
regime, with a mass of 0.333 ± 0.016 and a radius of 0.7538 ± 0.0025 those of Jupiter.
Following the convention of Ref. 22, we can denote the third body Kepler-16 (AB)-b, or
simply “b” when there is no ambiguity.
Considering its bulk properties, the planet is reminiscent of Saturn but with a higher
mean density (0.964 g cm–3, compared to the Saturnian density of 0.687 g cm–3). This
suggests a greater degree of enrichment by heavy elements. With a mass and radius one can
begin to model a planet’s interior structure, which will depend on age because planets cool
and contract with time. Usually the stellar age is used as a proxy for the planetary age, but in
this case the stellar age is not unambiguous. The primary star is a slow rotator (with a period
of about 35.1 days, judging from the out-of-eclipse variations), usually indicative of old age.
In contrast, the level of starspot activity and chromospheric emission (Mt. Wilson S value =
1.10) are indicative of youth. The spectroscopic determination of star A’s heavy-element
fraction ([m/H] = –0.3 ± 0.2) is also relatively uncertain, making it more difficult to estimate
the age with theoretical evolutionary models. Nevertheless, for any age greater than 0.5 Gyr,
the planet’s interior would include 40-60 Earth masses of heavy elements according to
standard planetary models (23). This would imply a composition of approximately half gas
(hydrogen and helium) and half heavy elements (presumably ice and rock). Saturn, by
contrast, is at least two-thirds gas by mass (24).
To investigate the long-term (secular) changes in the orbital parameters, and check on
the system’s stability, we integrated the best-fitting model forward in time by two million
years. Within the context of our gravitational three-body model, secular variations occur on
a timescale of about 40 years, without any significant excursions in orbital distance that
would have led to instability. The planet’s orbital eccentricity reaches a maximum of about
0.09. Likewise, the planet’s line-of-sight orbital inclination changes by 0.2°, which is large
enough that transits are only visible from Earth about 40% of the time (averaged over
centuries). In particular, the planetary transits across star A should cease in early 2018, and
return some time around 2042. The planetary transits across star B are already grazing, and
are predicted to disappear for 35 years beginning in May 2014.
The planet experiences swings in insolation due to the motion of the stars on short
timescales, and due to secular changes in the planet’s orbit on long timescales. These
variations are likely to affect the temperature and structure of the planet’s atmosphere. The
planet’s current equilibrium temperature, averaged over several orbits, is between 170 and
200 K, assuming isotropic re-radiation of the stellar flux and a Bond albedo between 0.2-0.5
(in the neighborhood of Saturn’s value of 0.34). Orbital motion of the stars and the planet
are expected to produce seasonal temperature variations of around 30 K.
The planetary orbit is aligned with the stellar orbit to within 0.4°. This extreme
coplanarity suggests that the planet was formed along with the stars, within a circumbinary
protoplanetary disk, as opposed to being captured from another system. Planetesimal
formation around an eccentric binary is a theoretical challenge, because of the large collision
velocities of particles that are stirred by the stellar binary (25), although the detection of
debris disks around close binaries has been interpreted as dust produced by colliding
planetesimals (26). Subsequent stages of planet formation around binaries has been studied
theoretically, both for terrestrial planets (27) and gas giants (28), but these and other
theoretical studies (29) have lacked a well-specified circumbinary planetary system that
could allow such a refinement of models.
Finally, the stars themselves are worthy of attention, independently of the planet. It is
rare to measure the masses and radii of such small stars with such high precision, using
geometrical and dynamical methods independent of stellar evolutionary models. In
particular, Star B, with only 20% the mass of the Sun, is the smallest main-sequence star for
which such precise mass and radius data are available (30). The mass ratio of 0.29 is also
among the smallest known for binaries involving fully convective stars at the low-mass end
of the main sequence (29). With well-characterized low-mass stars, in addition to a transiting
circumbinary planet, this makes Kepler-16 a treasure for both exoplanetary and stellar
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42. NASA’s Science Mission Directorate provided funding for the Kepler Discovery mission. LRD
acknowledges the NASA Kepler Participating Scientist Program (grant number NNX08AR15G) and
helpful discussions with the Kepler Science Team. JAC and DCF acknowledge support for this work
was provided by NASA through Hubble Fellowship grants HF-51267.01-A and HF-51272.01-A
awarded by the Space Telescope Science Institute, which is operated by the Association of
Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. JNW is
grateful for support from the NASA Origins program (NNX09AB33G). GF acknowledges the
support of the Hungarian OTKA grant MB08C 81013. The Kepler data used in this analysis can be
downloaded from http://archive.stsci.edu/prepds/kepler_hlsp.
Figure 1. Photometry of Kepler-16.
Top.—Photometric time series from the Kepler spacecraft of star system Kepler-16
(KIC 12644769, KOI-1611, 2MASS 19161817+5145267, Kepler magnitude = 11.762). Each
data point is the relative brightness at a given time (in barycentric Julian days, BJD). The 1%
variations on ~10-day timescales are likely due to starspots carried around by stellar rotation
(a periodogram gives a rotation period of about 35 days). The sharp dips are eclipses,
appearing as vertical lines in this 600-day plot. They are identified as primary (B-eclipses-A;
blue), secondary (A-occults-B; brown), tertiary (b-transits-A; green) and quaternary (b-
transits-B; red). Because of interruptions in Kepler observing, data are missing from one
primary eclipse at BJD 2,455,089, and one secondary eclipse at BJD 2,455,232. Note in
particular the shifting order of the tertiary (green) and quaternary (red) eclipses: the first and
third pairs begin with the tertiary eclipse, while the second pair leads with the quaternary
eclipse. This is because the stars’ orbital motion places them in different positions at each
inferior conjunction of the planet. The stars silhouette the planet as they move behind it.
Bottom.—Close-ups (narrower scales in time and relative flux) of representative examples of
each type of eclipse, along with the best-fitting model (gray), with parameters from Table 1.
Figure 2. Radial-velocity variations, and perturbations of eclipse times.
Top.—Observed radial-velocity variations of star A as a function of orbital phase, based on
observations with the TRES spectrograph and the Tillinghast 1.5m telescope at the Fred
Lawrence Whipple Observatory on Mt. Hopkins, Arizona (SOM). Solid dots are the data,
and the smooth curve is the best-fitting model. Although only the light from star A could be
detected in the spectra, the model for star B’s motion is also shown. Residuals from the best
model fits are given just below the radial velocity curve.
Middle and Bottom panels.—Deviations of the stellar eclipse times from strict periodicity, as
observed (colored dots) and modeled (open diamonds). As noted previously, one primary
eclipse and one secondary eclipse were missed. The deviations are on the order of one
minute for both primary and secondary stellar eclipses. In the model, the effects of
dynamical perturbations are dominant, with light-time variations contributing only at the
level of one second. If the third body were more massive than a planet (> 13 Jovian masses),
the timing variations would have exceeded 30 minutes. This would have been off the scale
of the diagram shown here, and in contradiction with the observations.
Figure 3: Scale diagram of the Kepler-16 system. The current orbits of the Kepler-16
system are shown as gray curves. The sizes of the bodies (including the Sun, Jupiter and
Saturn) are in the correct proportions to one another, but they are on a scale 20 times larger
than the orbital distance scale. We note that the binary and circumbinary planet orbital planes
lie within 0.4° degree of each other (Table 1) so the orbits are essentially flat, as drawn. The
planet’s orbital eccentricity is nearly zero, while the orbital eccentricity of the binary star
system is presently about 0.16. A “+” symbol marks the center of mass of all three bodies.
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