arXiv:0908.0241v3 [astro-ph.EP] 5 Aug 2009
Astronomy & Astrophysics manuscript no. 11933
August 5, 2009
c ? ESO 2009
Transiting exoplanets from the CoRoT space mission⋆
VIII. CoRoT-7b: the first Super-Earth with measured radius
A. L´ eger1, D. Rouan2, J. Schneider3, P. Barge4, M. Fridlund11, B. Samuel1, M. Ollivier1, E. Guenther5, M. Deleuil4,
H.J. Deeg6, M. Auvergne2, R. Alonso4, S. Aigrain8, A. Alapini8, J.M. Almenara6, A. Baglin2, M. Barbieri4, H. Bruntt2,
P. Bord´ e1, F. Bouchy7, J. Cabrera9,3, C. Catala2, L. Carone18, S. Carpano11, Sz. Csizmadia9, R. Dvorak10, A. Erikson9,
S. Ferraz-Mello23, B. Foing11, F. Fressin13, D. Gandolfi5, M. Gillon12, Ph. Gondoin11, O. Grasset19, T. Guillot13, A.
Hatzes5, G. H´ ebrard20, L. Jorda4, H. Lammer14, A. Llebaria4, B. Loeillet1,4, M. Mayor, M.12, T. Mazeh17, C. Moutou4,
M. P¨ atzold18, F. Pont8, D. Queloz12, H. Rauer9,22, S. Renner9,24, R. Samadi2, A. Shporer17, Ch. Sotin19, B. Tingley6, G.
Wuchterl5, Adda M.2, Agogu P.16, Appourchaux T.1, Ballans H.,1, Baron P.2, Beaufort T.11, Bellenger R.2, Berlin R.25,
Bernardi P.2, Blouin D.4, Baudin F.1, Bodin P.16, Boisnard L.16, Boit L.4, Bonneau F.16, Borzeix S.2, Briet R.16, Buey
J.-T.2, Butler B.11, Cailleau D.2, Cautain R.4, Chabaud P.-Y.4, Chaintreuil S.2, Chiavassa F.16, Costes V.16, Cuna Parrho
V.2, De Oliveira Fialho F.2, Decaudin M.1, Defise J.-M.15, Djalal S.16, Epstein G.2, Exil G.-E.2, Faur C.16, Fenouillet
T.4, Gaboriaud A.16, Gallic A.2, Gamet P.16, Gavalda P.16, Grolleau E.2, Gruneisen R.2, Gueguen L.2, Guis V.4,
Guivarc’h V.2, Guterman P.4, Hallouard D.16, Hasiba J.14, Heuripeau F.2, Huntzinger G.2, Hustaix H.16, Imad C.2,
Imbert C.16, Johlander B.11, Jouret M.16, Journoud P.2, Karioty F.2, Kerjean L.16, Lafaille V.16, Lafond L.16, Lam-Trong
T.16, Landiech P.16, Lapeyrere V.2, Larqu´ e T.2, Larqu T.16, Laudet P.16, Lautier N.2, Lecann H.4, Lefevre L.2, Leruyet
B.2, Levacher P.4, Magnan A.4, Mazy E.15, Mertens F.2, Mesnager J-M16, Meunier J.-C.4, Michel J.-P.2, Monjoin W.2,
Naudet D.2, Nguyen-Kim K.1, Orcesi J-L.1, Ottacher H.14, Perez R.16, Peter G.25, Plasson P.2, Plesseria J.-Y.15, Pontet
B.16, Pradines A.16, Quentin C.4, Reynaud J.-L.4, Rolland G.16, Rollenhagen F.25, Romagnan R.2, Russ N.25, Schmidt
R.2, Schwartz N.2, Sebbag I.16, Sedes G.2, Smit H.11, Steller M.B.14, Sunter W.11, Surace C.4, Tello M.16, Tiph` ene D.2,
Toulouse P.16, Ulmer B.21, Vandermarcq O.16, Vergnault E.16, Vuillemin A.4, and Zanatta P.2
(Affiliations can be found after the references)
Received February 23, 2009; accepted xxxxx
Aims. We report the discovery of very shallow (∆F/F ≈ 3.410−4), periodic dips in the light curve of an active V = 11.7 G9V star observed by the
CoRoT satellite, which we interpret as caused by a transiting companion. We describe the 3-colour CoRoT data and complementary ground-based
observations that support the planetary nature of the companion.
Methods. We used CoRoT colours information, good angular resolution ground-based photometric observations in- and out- of transit, adaptive
opticsimaging,near-infraredspectroscopy, andpreliminaryresultsfromradial velocitymeasurements, totestthedilutedeclipsingbinaryscenarios.
The parameters of the host star were derived from optical spectra, which were then combined with the CoRoT light curve to derive parameters of
Results. We examined all conceivable cases of false positives carefully, and all the tests support the planetary hypothesis. Blends with separation
> 0.40′′or triple systems are almost excluded with a 8 10−4risk left. We conclude that, inasmuch we have been exhaustive, we have discovered a
planetary companion, named CoRoT-7b , for which we derive a period of 0.853 59 ± 3 10−5day and a radius of Rp= 1.68 ± 0.09 REarth. Analysis
of preliminary radial velocity data yields an upper limit of 21 MEarth for the companion mass, supporting the finding.
Conclusions. CoRoT-7b is very likely the first Super-Earth with a measured radius. This object illustrates what will probably become a common
situation with missions such as Kepler, namely the need to establish the planetary origin of transits in the absence of a firm radial velocity detection
and mass measurement. The composition of CoRoT-7b remains loosely constrained without a precise mass. A very high surface temperature on its
irradiated face, ≈ 1800 – 2600 K at the substellar point, and a very low one, ≈ 50 K, on its dark face assuming no atmosphere, have been derived.
Key words. planetary systems – techniques: photometry – techniques: adaptive optics – techniques:spectroscopy – stars: fundamental parameters
⋆The CoRoT space mission, launched on 27 December 2006,
has been developed and is operated by CNES, with the contribu-
tion of Austria, Belgium, Brazil, ESA, Germany, and Spain. First
CoRoT data are available to the public from the CoRoT archive:
http://idoc-corot.ias.u-psud.fr. The complementary observations were
obtained with MegaPrime/MegaCam, a joint project of CFHT and
CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) which
is operated by NRC in Canada, INSU-CNRS in France, and the
University of Hawaii; ESO Telescopes at the La Silla and Paranal
Observatories under programme ID 081.C-0413(C), DDT 282.C-5015;
the IAC80 telescope operated by the Instituto de Astrof´ ısica de Tenerife
at the Observatorio del Teide; the Isaac Newton Telescope (INT), op-
erated on the island of La Palma by the Isaac Newton group in the
Spanish Observatorio del Roque de Los Muchachos of the Instituto
de Astrofisica de Canarias; and at the Anglo-Australian Telescope
that have been funded by the Optical Infrared Coordination network
2 A. L´ eger et al.: CoRoT-7 b: the first Super-Earth with measured radius
The space mission CoRoT is performing wide-field stellar pho-
tometry at ultra-high precision (Rouan et al. 1998; Baglin et al.
2006). During an observing run, up to 12,000 stars can be mon-
itored simultaneously and continuously over 150 days of obser-
vation. CoRoT is thus particularly well-suited to detecting plan-
ets with orbital periods shorter than 50 days. Because the transit
signal is proportional to the planet’s projected surface, the first
published CoRoT results (Barge et al. 2008; Alonso et al. 2008;
Deleuil et al. 2008; Aigrain et al. 2008; Moutou et al. 2008)
were focused on the population of rather massive planets, one
of which has even been quoted as “the first inhabitant of the
brown-dwarf desert”, with a well-defined mass (20 ± 1 MJup)
and a well-defined radius (1.0 ± 0.1 RJup) (Deleuil et al. 2008).
However, CoRoT has the capability of detecting significantly
smaller planets, and analysis of the noise on the light curves (LC
hereafter) indeed shows that in many cases it is not far from the
photon noise limit (see Aigrain et al., this volume). In the same
line, blind tests performedby different teams of the CoRoT con-
the performances of CoRoT are such that ≈ 2 REarthhot Super-
Earth planets1are within reach. Such planets on close-in orbits
should be accessible for stars brighter than mV≈ 13 (Auvergne
The study of small hot planets is becoming a major question
(Mayor & Udry 2008; Mayor et al. 2009; Bouchy et al. 2009)
that has a direct link with planetary system formationand evolu-
tion.Inthis paper,we reportthe discoveryofthe smallest transit-
ing object detected so far arounda main-sequencestar, an object
that deserves the name of Super-Earth.
In transit surveys, ground-based follow-up is mandatory for
confirming a transiting planet candidate. In the case of CoRoT-
7b an intensive follow-up campaign has been set up, including
programmes of photometry, imaging, spectroscopy, and radial
velocity (RV), using different ground-based facilities over the
world. The results of this campaign allow us to exclude almost
all the possible false positive cases that could mimic a transiting
planet. Preliminary results of RV measurements are consistent
with the presence of a low mass planet and exclude any giant
planet or stellar companion.
We present the photometric analysis of the CoRoT data
where we discovered this shallow transit candidate (Sects. 2 and
4), as well as the photometric and imaging follow-up, includ-
ing adaptiveoptics (Sect. 3), infraredspectroscopy(Sect. 5), and
order to secure the planetary nature of the transiting body. The
stellar parameters are presented in Sect. 8 and planetary ones
in Sect. 9. Such a small and hot planet raises several questions
about its composition,structure, and surface temperature,as dis-
cussed in Sect. 10.
2. Photometric observations with CoRoT
The star CoRoT-7 was observed during the first long run of
CoRoT towards the Monoceros constellation (anti-centre run
(OPTICON), a major international collaboration supported by the
Research Infrastructures Programme of the European Commissions
Sixth Framework Programme; Radial-velocity observations were ob-
tained with the SOPHIE spectrograph at the 1.93m telescope of
Observatoire de Haute Provence, France.
a significant hydrogen envelope, e.g. < 10−3times the Earthmass. It can
be either rocky or water-rich (L´ eger et al. 2004; Grasset et al. 2009).
Table 1: CoRoT-7 IDs, coordinates, and magnitudes.
102708694, LRa01 E2 0165
(a) Provided by Exo-Dat, based on observations taken at the INT
(b) From TYCHO catalogue.
(c) From 2-MASS catalogue.
LRa01, the letter a indicating that the field is close to the
Galactic anti-centre). Its ID is given in Table 1, based on the
Exo-Dat database (Deleuil et al. 2009). Because it is one of the
brightest stars monitored in this field, it was a member of the
oversampled(32sec) target list fromthe beginningof the LRa01
run. After the first 40 days of data acquisition, the Alarm Mode
pipeline (Quentin et al. 2006; Surace et al. 2008) detected the
first series of transits in the star LC.
As illustrated by the whole CoRoT LC (Fig 1), CoRoT-7 is
effect of stellar spots driven by the stellar rotation and crossing
the disk. A period of ≈ 23 days is inferred.
Several teams of the CoRoT exoplanet consortium have
searched for transits. Spurious spikes and stellar variations at
frequencies outside the range expected for planetary transits
were removed with low- and high-pass filters. Then, different
detection algorithms were used and 153 individual transits were
eventually detected. In agreement with the CoRoT consortium
rules, the CoRoT-7 LC, with the same data pre-processing as in
the present paper (v1 of data pipeline), will be accessible from
30 July 2009 at http://idoc-corot.ias.u-psud.fr/index.jsp (select:
CoRoT Public N2 data / Run LRa01 / object with Corot ID
102708694), so that the reader can make his or her own reduc-
tion and analysis of the data.
In this section, we seek to estimate the main transit parame-
ters, i.e., period, central date, ingress/ egress duration, total du-
ration, and relative depth, using a simple trapezoidal model. We
proceedas follows:(1) outliers(mainlydueto the satellite cross-
ing of the South Atlantic anomaly) are filtered out from the LC
using a 7-sample running median; (2) long-term stellar activ-
ity is removed by subtracting a 0.854-day running median; (3)
individual transits are corrected from a local linear trend com-
puted on 3.75-hr windows centred on the transits but excluding
sit signal is performed using only data inside 3.75- hr windows
centred on each transit.
Errors on the transit parameters were estimated using a
procedure analogous to the bootstrap method described by
Press et al. (1992), although slightly modified in order to pre-
serve the correlation properties of the noise: (1) we compute a
A. L´ eger et al.: CoRoT-7 b: the first Super-Earth with measured radius3
Fig.1: LC of the target CoRoT-7 without low-frequency filtering. The stellar rotation period, Protof 23 days can be inferred from
spot induced dips, as pointed out by arrows.
Fig.2: Upper panel: superimposition of 153 individual segments of the LC divided according to the transit period determined by
a detection algorithm after high-pass (3 times the transit period = 2.56 days) and low-pass (3 times the time resolution = 3 × 512
s) filtering. Individual transits are clearly seen when superimposed. Lower panel: mean value of the upper curves but with a shorter
time resolution (64s) and a different low pass filtering (3 times the time resolution = 3 × 64 s) in order to better preserve the transit
transit-free LC by subtracting our best-fit trapezoidal model to
the data; (2) we re-insert the same transit signal at a randomly
chosen phase; (3) we fit a trapezoidal model to the data and
record the best-fit parameters; (4) steps 1-3 are repeated 20,000
times to build histograms used as estimators of the probability
distributions for every transit parameter (Fig. 4). Finally, the er-
ror on a given parameter is computed as the median absolute
deviation of its distribution.
Because the period error yielded by the bootstrap seemed
fairly small, we decided to check this result by carrying out a
different calculation: (1) we produce 4 sets of n = 5, 10, 18,
and 25 sub-LCs, each LC having the total duration of the initial
LC, by keeping just one complete transit period out of 5,10,18,
4 A. L´ eger et al.: CoRoT-7 b: the first Super-Earth with measured radius
Fig.3: Averaged folded LCs in the three colours provided by the CoRoT instrument, after normalization. Red, green and blue
signals are represented with the corresponding colours, and the white signal, summation of the three bands prior to normalization,
is in black.
Fig.4: Transit parameter distributions obtained from a bootstrap method for a trapezoidal transit signal.
and 25 consecutive transits respectively; within one set, the first
transit of a sub-LC is shifted of one with respect to the previ-
ous sub-LC; (2) for each of the 4 sets, we measure the period
for everysub-LC with a trapezoidal least-square fit, compute the
median period, the period standard deviation σP, and the error
on the standard deviation (Fig. 5); (3) we estimate the period
standard deviation for the full LC by extrapolating σPfor n = 1.
For this purpose, we performed a least-square fit of a power law
ofthe formσP(n)= σP(1)× nαandgot α = 0.57(closeto the 0.5
exponent expected for uncorrelated measurements) and σP(1) =
2.1 s. We note that this error is a factor of 2 larger than the one
obtained with the bootstrap method, so we conservatively chose
to keep this higher value as our final estimate of the period error
We finally find a period of 0.853585± 24 10−6day.Figure 2,
where all transits are superimposed, shows that even individual
transits can be tracked down despite the low S/N. The fit by a
A. L´ eger et al.: CoRoT-7 b: the first Super-Earth with measured radius5
Fig.5: Perioderrorcalculationforthe full LC based ona extrap-
olation of period error estimates from chopped LCs containing
one transit out of n and spanning the LC total duration. Left: pe-
riod errors as a function of n. Right: deviation from the period
value Pmedianyielded by the bootstrap.
modelled with a trapezoid.
Transit parameters and associated uncertainties, as
central date (1sttransit)
total duration (trapezoid)
0.853 585 ± 0.000 024 day
2 454 398.0767 ± 0.0015 HJD
15.8 ± 2.9 min
75.1 ± 3.2 min
3.35 10−4± 0.12 10−4
h, τ14= 1.253 h for the short and long bases of the trapezium2,
and ∆F/F = 3.35 10−4± 0.12 10−4. This is the faintest relative
flux change that has been detected in transit search photometry,
up to now.
The CoRoT camera is equipped with a low-dispersion de-
vice (a bi-prism) before the exoplanet CCDs (Rouan et al. 2000,
1998; Auvergne2006) that provides LCs in three colours, called
red, green and blue, even if the band pass does not correspond
to classical photometric filters. The corresponding phase-folded
and averaged LCs are shown in Fig.3. The transit is observed
in the three colours with similar relative depths, a behaviour ex-
2We use τijfor the parameters related to the trapezoidal fit and Tij
for the parameters related to the more realistic transit modelling (see
Fig.6: Shift-and-add image of the stack of 100 exposures taken
at CFHT with MEGACAM. The arrows indicate the two faint
stars located at ≈ 5 arcsec from CoRoT-7 (at the centre of the
image) and about ∆m = 10 mag fainter. The size of the field
shown is 1 arcmin (see scale); north is to the top and east to the
left. The insert to the left shows the shape, size, and orientation
of the photometric mask applied on the star CoRoT-7 onboard
CoRoT and the X marks the position of the star on the mask.
The grey levels corresponds to the measurement by CoRoT on
pected for the transit of a planet in front of its star that will be
used to assess the planet hypothesis (Sect. 4).
3. Photometric and imaging follow-up
Whenever transits are detected in a CoRoT LC and when the
candidate survives the set of tests performed to rule out obvi-
ous stellar systems (see Carpano et al, this volume) a ground
basedfollow-upprogrammeis initiated.Thegoalis to checkfur-
ther for possible contaminating eclipsing binaries (EBs) whose
point spread function (PSF) could fall within the CoRoT photo-
metric mask. In the specific case of CoRoT-7b with its shallow
transits, we performed a rigorous complementary observational
campaign to check all conceivable blend scenarios. These in-
cludedi) a search for photometricvariationson nearbystars dur-
ing the assumed transit; ii) deep imaging, with good-to-highan-
gular resolution, searching for the presence of fainter and closer
contaminating stars; iii) spectroscopic observations of the tar-
get at high resolution and high S/N; iv) infrared spectroscopy,
searching for faint low-mass companions; v) examination of X-
rayflux fromputativeclose binarysystems; andvi) RV measure-
ments. In addition, we took advantage of CoRoT’s capability to
provide colour information on transit events.
3.1. Time-series photometric followup
The CoRoT exoplanet channel has a large PSF. In the case of
the CoRoT-7 target, the FWHM is 8.6 arcsec along the disper-
sion axis and 6.0 arcsec perpendicular to it. Ninety-nine percent
of the flux is extended over a larger and roughly ellispoidal area
of 60 arcsec × 32 arcsec (Fig. 6 - left). This large area implies
a significant probability that candidates detected in the CoRoT
data arise from nearby background eclipsing binaries (BEBs).
A photometric follow- up programme of CoRoT candidates in-
tendsto identifysuchBEBs, comparingobservationsduringpre-
6 A. L´ eger et al.: CoRoT-7 b: the first Super-Earth with measured radius
Fig.7: Final NACO image in J-band after substraction of a me-
dian PSF and de-rotationof the field. The circles locate the three
faint stars that are detected in the field, the two at east identified
with those marked by arrows in Fig. 6. The scale is given by a
line of 5 arcsec length, and the central circle of 0.5 arcsec radius
gives an idea of the angular distance at which the presence of
a faint star could not be detected close to CoRoT-7. North is at
top, and east to the left, as in Fig. 6.
dicted transit-times with observationsout of transit. This follow-
up programme, as well as the time- series follow-up performed
on CoRoT-7, are described in more detail in Deeg et al. (2009).
Here we only give a summary.
For any catalogued nearby star around the CoRoT-7 target,
we calculated the expected eclipse amplitude if this star was
the source of the observed dips. Calculation of this amplitude is
based on a model of the stellar PSF, the shape of the photomet-
ric aperture, and the position and magnitudes of the target and
the contaminating stars, respectively. Several stellar candidates
for false alarms were identified that way, with expected eclipse
amplitudes between 0.2 and 1 mag.
Observations to identify such alarms were then done on two
telescopes: IAC-80 and CFHT. Images with the IAC-80 were
taken on several occasions of the observed periodic loss of flux
between February and April 2008; CFHT observations with
MEGACAM (Boulade et al. 2003) were performed during the
ingress of a transit on 7 March 2008. From both data sets, pho-
tometric LCs were extracted through classical differential pho-
tometric techniques and the stars on- and off-transit brightness
are compared. The observations from IAC-80 showed that none
of the known contaminating stars could have been the source of
an alarm, with all of them varying several times less than the
amount required for a false positive. The CFHT time-series im-
ages (Fig 6) also shows a faint contaminator of about V = 19.5
some10 arcsecnorthofthe target.However,this faint star would
have to show strong variations of its brightness by a factor of 5
– 8 to become a false alarm source, something that can clearly
be excluded. Follow-up from photometric time-series imaging
thereforeallowed any false alarm to be excludedfrom sources at
distances over about 4 arcsec from the target.
Fig.8:CentralpartoftheNACO image,afterasimulatedstar 6.5
magnitudes fainter than CoRoT-7 was added (within square).
The circle of 400 mas radius defines the only region in which
such a star could be confused with residual speckles.
3.2. High angular resolution imaging followup
The next step is to search for additional faint stars closer to the
target that might be potential sources for false alarms. This test
employs high-resolution imaging with three different kinds of
observations: construction of the best image from the CFHT set,
sharp short exposures images taken with FASTCAM at the 1.5m
CST and finally adaptive optics imaging with NACO at VLT.
In the first case we made a sub-pixel recentring of all 100
MEGACAM images and took the median image. The result is
shown in Fig. 6. Two very faint stars, invisible in single images,
become apparent at angular distances of 4.5 and 5.5 arcsec from
CoRoT-7 (indicatedwith arrows in Fig. 6). By adding simulated
stars with known brightnesses at similar angular distances, we
estimated them to be about 10 mag fainter than CoRoT-7, which
is too faint to be potential alarms, even if they were to totally
vanish during the transit.
FASTCAM is a lucky imaging camera (Oscoz et al. 2008).
where 12,000 images, each with 50ms exposure time, were ob-
tained on 24 October 2008 in I band, with a pixel resolution of
32 mas. The best result was obtained from a selection of the best
10% of images followed by their centring and co-adding. Based
on the absenceof signals with an S/N higherthan 5, the presence
of relatively bright nearby objects with I ≤ 15 could be excluded
beyond 0.18 arcsec, I ≤ 16 beyond 0.3 arcsec and I ≤ 17 beyond
0.8 arcsec. However,significantly fainter objects would not have
been detected at any larger distance.
The VLT/NACO observations were performedthanks to dis-
cretionary time granted by ESO (DDT 282.C-5015). A set of J-
gles of the NACO rotator (15ostep), in jitter mode. The images
are recentred at sub-pixel level and median- stacked for each ro-
the PSF of CoRoT-7, as all other objects in the field are removed
by the median operation.Each stacked image is then substracted
from this PSF and is de-rotated before a final median-stacked
image is produced. This resulting image is shown in Fig. 7: it
reveals 3 faint stars (circled) at distances of 4.9, 5.9 and 6.7 arc-
sec, whereas no other star could be indentified between 0.5 and
A. L´ eger et al.: CoRoT-7 b: the first Super-Earth with measured radius7
5 arcsec. Clearly two of those stars are the counterpartof the two
stars detected on the CFHT stacked image: the small difference
in astrometry can be explained by our using of the average pixel
size of Megacam,which is not constant throughoutits very wide
field of view. Photometry – cross-checked against added simu-
lated stars – shows that the brightness difference between these
stars and CoRoT-7 is ∆mJ= 8.1 – 8.4. If we take mJ(CoRoT-7)
= 10.3, this translates to mJ(faint stars) = 18.4 – 18.7. On aver-
age, stars in the neighbourhood exhibit V-J = 1.7, so that, if we
apply this reddening to the three faint stars we find : mV(faint
stars) = 20.1 – 20.4. Indeed the true reddening of those three
very faint stars is likely larger: either those stars are nearby low
mass stars and thus are fairly red, or they are very distant and
thus suffer a large interstellar extinction. Their brightness differ-
ence to CoRoT-7 in V of ∆mV≈ 10, is thus consistent with what
is foundonthe CFHT stacked image.Evenif these stars undergo
a 50 % decrease in brightness, they would produce only an am-
plitude variation ∆F/F = 5 10−5in the CoRoT LC, i.e. much less
than the observed value of 3.35 10−4. We conclude that none of
the detected sources in the field around CoRoT-7could account
for the dips in the CoRoT LC, even if it would vanish totally.
We now have to assess the probability of a contaminator still
closer to the target. This case would correspondto a background
of foreground system of a star and a transiting object (planet or
star), the star having the same colours as CoRoT-7. For instance
it could be a star instrinsically bluer than CoRoT-7 but distant
enough to be both reddened and faint enough to provide the ob-
served signal. In that case, the star should be at maximum 6.5
magnitudes fainter than CoRoT-7 in J, taking the reddeninginto
account and assuming that its flux could be reduced by 50% at
maximum to mimic a transit (case of a fully symmetric EB).
To assess this case, we added on the NACO image a simulated
star 6.5 magnitudes fainter than CoRoT-7 as shown in Fig. 8.
The simulated star shows up clearly, brighter than any residual
speckles farther than 400 mas from CoRoT-7. We can conclude
that if it is a backgroundbinary system that mimics the observed
transit on CoRoT-7, it must be inside a circle of 400 mas radius,
because at any other location it would have been seen. The prob-
ability p that this is the case is simply the ratio of the surface
of the 400 mas radius circle to the surface of the CoRoT mask:
p = π 0.42/ 640 = 8 10−4. The additional condition of similar
colours for the CoRoT-7 and the contaminant makes this prob-
ability even lower, but, conservatively, we keep the preceding
4. CoRoT colours
While the probability that there is a background star closer than
400 mas is low (< 810−4), the probability that CoRoT-7 is a
triple system is significantly higher. A detailed study of stars by
Tokovinin (2008) shows that at least 8% of the solar-type stars
have three or more components.
Illustrated in Fig. 9, the triple system could be a giant planet
or a browndwarf (tr 3)transitingin frontof a fainterstar (star 2)
3An independent estimate of this probability of a false positive can
be made using CoRoTLux 2007 (Fressin et al. 2007). With that model,
we computed the probability that a background eclipsing object is lo-
cated at a distance 0.4 arcsec from a given CoRoT target, with an ampli-
tude that produces an apparent transit depth lower than 5 10−4and with
an SNR above the CoRoT detection threshold (defined in Aigrain et al.
2009, this volume). We obtain an average of 4 10−4object in the sim-
ulation of the LRa01 field (≈104stars) that exhibits such a small and
detectable transit, a probability compatible with the upper limit we find
Fig.9: Scheme of a triple system that could mimic the transit
of a small planet in front of the target star (star 1). Star 2 is a
physically associated faint star and tr 3 a dark transiting object,
e.g. a hot Jupiter or a brown dwarf.
Fig.10: Distributionofthefluxofa K0Vstar (proxyforCoRoT-
7, a G9V star) into the 3 channels, according to the measured
relative intensities of the coloured fluxes in photo-electrons.
physically associated with the target star (star 1). Assuming that
the transiting object has a Jupiter size radius, the spectral type
of the secondary star can be estimated thanks to the required
brightness differences between (star 1) and (star 2).
Usingthe Eddington’sapproximationforthe limb-darkening
effect (Fig. 12), the maximum reduction of star 2 flux is
(∆F2/F)max= 1.25[0.4+ 0.6(1 − z2)1/2](R3/R2)2,
where z is the impact parameter of the transit on star 2.
Assuming a mean value for z = 0.5 (not necessarily that esti-
mated in section 9), the maximum flux reduction as measured
by CoRoT is
(∆F/F)max= 1.15(R3/R2(M))2F2(M)/[F1+ F2(M)],
where the radius and flux of star 2 are a function of its mass
M. Assuming that the transiting object, either a hot Jupiter or a
8 A. L´ eger et al.: CoRoT-7 b: the first Super-Earth with measured radius
Fig.11: Expecteddistributionintothe 3 coloursof the fluxdrop,
∆F, if it was due to the eclipse of a M5V star. Frontiers between
colours are the same as in Fig. 10, but the stellar spectrum is dif-
ferent. In that triple system, the flux drop would be significantly
redder than observed.
Fig.12: Limb darkening effect versus fractional radius of the
star: superimposition of the Eddington’s classical law (black
dash line) and of the predicted ones for a K0 star (best proxy
in Claret’s tables for a G9V star) in filter V (blue line), R (green
line), I (red line) and their mean value (thick green, in fact al-
most coinciding with R). The data for the predicted laws are
from Claret (2000), using Teff= 5250 K, log g = 4.5, M/H =
brown dwarf, has a Jupiter radius, R3= RJup, and using the ra-
dius and the flux of a mean sequence star (Drilling & Landolt
2000) for star
2, and the CoRoT measured (∆F/F)max =
3.510−4, the preceding relation can be solved as an equation
in M. The found star spectral type is M5.1V, approximated as
M5V. The assumption that the transiting object has a Neptune
size would lead star 2 to be a K9V star. Now, such a star would
be redder than the target star 1, providing a criterion to qualify /
falsify the hypothesis.
The details of the argumentationare explainedin a dedicated
paper (Bord´ e et al, in preparation). Only the principles and the
results are reported here. The bi-prism of CoRoT produces a
mini-spectrum for each star, split by a proper selection of pixels
into 3 spectral bands whose fluxes are recorded independently
(Rouan et al. 2000; Auvergne 2006). The boundaries within the
photometric mask that define these colours are chosen so that
the red, green, and blue parts correspond, as much as possible,
to given fractions of the total, but they must also correspond to
an integer numberof columns on the CCD (dispersion is done in
rows). In the case of the G9V target star (star 1) that dominates
the total flux, the actual fractions are 73.3%, 10.9%, and 15.8%,
respectively, as indicated by the number of photoelectronsin the
different channels (Fig. 10).
Assuming that the M5V star 2 is part of the triple system, it
would be at an angular position so close to the target star that it
would be indistinguishable with the CoRoT spatial resolution.
The same boundarieson the CCD for definingthe colours would
then apply. This transiting star would lead to photoelectron con-
tents in the 3 bands that are different from those of the target.
They can be estimated from the spectrum of an M5V star (Fig.
11). The red fraction would increase to 94.9% and the green and
blue decrease to 2.9% and 2.2%, respectively. These fractions
correspond to the expected transit flux variations in that hypoth-
esis, (∆F)Red, (∆F)Green, and (∆F)Blue.
If the r, g, b quantities are defined as
their expected and observed values can be compared. To work
with a sufficient S/N, we bin 150 colour transit curves into 15
bins of 10 LCs each, calculate the r, g and b values, make the
corresponding histograms and estimate the observed mean val-
ues and standard deviations. The observed and expected values
robs= 0.88 ± 0.18,rM5= 1.29;
gobs= 1.24 ± 0.30,gM5= 0.27;
bobs= 1.42 ± 0.41,bM5= 0.14.
We concludethat theobservedcoloursare incompatiblewith
those of a transit in front of an M5V star, whereas they are com-
patiblewithatransitinfrontofthetargetstar(rG9= gG9= bG9=
1)4. Another possibility could be that star 2 is a white dwarf
with, by chance, the very temperature of the target star. With
a proper luminosity ratio, this could be indistinguishable from a
small planet in front of the target star, just from the colour crite-
rion. However, this situation would produce a transit duration, 1
mn, much shorter than what is observed, because a white dwarf
has a similar size to a terrestrial planet, not that of a main se-
quence star. The white dwarf possibility is then discarded with-
outambiguity.We concludethattheobservedLC is notproduced
by a triple system where a Jupiter size object transits in front of
a secondary star.
(∆F/F)White, g =(∆F/F)Green
(∆F/F)Whiteand b =
4If a circular orbit is assumed, the mere duration of the transit re-
quires that the star undergoing the transit has a minimum size. The ob-
served duration of thetransit isafraction f = 0.061 of the period. f also
satisfies f ≤ (R2+ R3)/πa, the equality corresponding to an equatorial
transit. Fora Jupiter-sizetransitingobject, or smaller, thistranslatesinto
R2> 0.72 R⊙, corresponding to a star 2 earlier than K6V, if it is a main
sequence star, a statement that is stronger than the one we derive but
that needs the additional assumption of a circular orbit
A. L´ eger et al.: CoRoT-7 b: the first Super-Earth with measured radius9
5. Infrared spectroscopy
The reasoning of Sect. 4 can even tell more about the possible
spectral types of star 2. The observed colours of the transit per-
mit to restrict these to earlier types than M0V because later stars
would produce ratios g < 0.55 and b < 0.45 that are not compat-
ible with the observations (mean values ±2σ).
We can now conclude that the remaining possible star 2 that
could still produce a false positive is rather bright. This could
produce observable signatures in the target spectrum, particu-
larly in the IR where the contrast between a G9V and M or K
type star is lower, so we took these spectra of the CoRoT-7 star.
Thanks to ESO DDT time (DDT 282.C-5015), we obtained
a high-resolution spectrum of CoRoT-7 using the infrared spec-
trograph CRIRES mounted on the VLT-UT1 (Antu). The AO-
system was used along with a slit-width of 0.3 arcsec that re-
sulted in spectral resolution of R = 60,000.The wavelength cov-
erage was 2256 to 2303 nm, a regionthat included the CO- over-
tonelines, whichhavean equivalentwidthabouta factorofthree
larger in an M5V star than in a G9V star. The total integration
time of 1800 s resulted in an S/N = 100 per resolution element.
Standard IRAF routines were used for flat-fielding, sky subtrac-
tion, and wavelength calibration. A spectrum of HD48497 taken
with thesame setupduringthe samenightandat similar airmass
was used to remove the telluric absorption lines.
Figure 13 (top) shows the cross-correlation function of the
CoRoT-7 spectrum with the theoretical spectrum of an M5V
star. Since we could not find an M5V template in the ESO
archives, we simply calculated the theoretical spectrum by using
the spectrum of an M2V star and increased the strength of the
CO lines to the strength of an M5V star. If there is an early M
star associated to CoRoT-7, we would expect a secondary max-
imum exhibiting a difference in RV with respect to CoRoT-7.
This difference would be less than about 100 km/s, since a sys-
tem with a shorter orbital period would not be stable. Figure 13
(bottom) shows the cross-correlation function when a putative
M5V star at the same distance as CoRoT-7 and with an RV dif-
ference of 50 km/s added. With this analysis, we conclude that,
if the separation in RV of the two stars is greater than 8 km/s,
the primary and secondary peaks would be well separated. If the
separation was between 3 and 8 km/s, we would still detect an
asymmetry of the cross-correlation function.
An independent analysis of the CRIRES spectrum using
TODCOR (Zucker & Mazeh 1994) was able to excludethe pres-
ence of an M star with a brightness as low as 7% of CoRoT-7,
thus more than a factor of two lower than needed to rule out
the presence of a bounded M-star. Such a secondary M star is
clearly detected by TODCOR analysis when its spectrum is in-
serted with a proper weight to a sunspot template, a proxy for an
Therefore,the CRIRES spectra can ruleout the presenceofa
secondary star earlier than M5V within 0.3 arcsec (CRIRES slit
width) from CoRoT-7 except if, by chance, observations were
performed when star 1 and star 2 had the same RV within 3
km/s. Assuming that CoRoT-7 is a binary consisting of a G-star
orbited by an M-star that has a eclipsing planet, then the prob-
ability that we observe the system at the very moment of the
conjunction so that the separation is less than 3 km/s is below
Eventually, CoRoT colours allow us to exclude a secondary
companion fainter (redder) than M0V, and CRIRES spectrum a
companionbrighter than M5V, but for a very special case whose
probability is less than 10−6. As the exclusion intervals over-
lap, we can conclude that the observed events are not due to an
associated star (star 2) subjected to a transit. The triple system
hypothesis is essentially rejected.
Fig.13: Top: cross correlation function between the CRIRES
spectrum of CoRoT-7 and a synthetic spectrum of an M5V star.
Bottom: same as above, but an M5V star spectrum with a shift
in RV of -50 km/s was added to the CoRoT-7 spectrum. We
conclude that there is no such companion.
6. Lack of X-ray emission
Thecase ofa triplesystem madeofthe maintargetandtwo graz-
ing eclipsing binaries is already practically excluded by Sects. 4
and 5 and the U shape of the transit. An additional hint is pro-
vided by the lack of X-ray emission. If CoRoT-7 were a triple
system consisting of a G9V star and an eclipsing binary of late
spectral type with an orbital period of ≈ 0.9 day, the binary
would likely be detectable in the X-ray spectral domain. As a
prototype object of this kind, we consider the eclipsing binary
YY Gem consisting of two M1Ve stars with an orbital period of
0.85 days (Haisch et al. 1990; Stelzer et al. 2002). The bright-
ness of YY Gem in the X-ray regime is 2 - 8 1029erg/s, and its
distance is 15.8 ± 0.30 pc (Stelzer et al. 2002). Strassmeier et al.
(1993) in their catalogue of chromospherically active binaries
list at least 7 late-type stars (late G- to early M-type) with rota-
tional (binary)periods of less than1 day.All of these have X-ray
luminosities in the range 0.3 – 0.9 × 1030erg/s. Using the data
obtained in the ROSAT all sky survey (Voges et al. 1999) in the
0.1to2 keV band,we searchedforpossibleX-rayemissionfrom
hypotheticalcompanionsof CoRoT-7. With the lowest X-ray lu-
minosity of such short-period binaries, we would have detected
these companions of CoRoT-7at 4 σ level, if present. Using
d(CoRoT-7) = 150 pc, we can state that LX(CoRoT-7)< 51028
erg/s, that is the level of the sky background above which one
cannot find any evidence of a source at the location of the target.
7. Radial velocity campaign
We have obtainedtwo measurementsof CoRoT-7 with the spec-
trograph SOPHIE (Bouchy & The Sophie Team 2006) at the
1.93m telescope in Observatoire de Haute Provence (France).
The RV observations were conducted on 17 and 20 February
2008, just a few weeks after the early discovery of the transiting
candidate and during the CoRoT observing run. RV follow-up
10 A. L´ eger et al.: CoRoT-7 b: the first Super-Earth with measured radius
Table 3: SOPHIE RV observations of CoRoT-7.
Julian date Orb.
Fig.14: SOPHIE RV measurements of CoRoT-7 obtained in
February 2008, versus time. The two data points are shown with
1σ error bars. Superimposed are Keplerian orbital curves of a
0.854 day period planet at the CoRoT ephemeris, which are ex-
cluded by the data at 1σ (dotted line) and 2σ (plain line). They
correspond to semi-amplitudes of 6 and 15 m/s, or planetary
masses of 8.5 and 21 MEarth. The systemic velocity V0is left as
a free parameterin this simple fit. We concludethat the planet, if
present, has a mass Mpl< 21 MEarth at a 95 % confidence level.
observationswere assigned high prioritybecause of the shallow-
ness of the transit. The SOPHIE RV measurements are given in
Table 3 and shown in Figure 14. The two measurements were
obtained on CoRoT dates 514.3 and 517.4 JD-2 454 000 (see
Fig. 1), i.e., at a time when the photometricactivity from the star
is relatively low; consequently, they are not strongly affected by
this variability. Quantitatively, the photometric difference mea-
suredby CoRoT betweenthe two SOPHIE observationsis 0.2%,
which is 10 times smaller than the largest variation in the to-
tal CoRoT light curve, and twice smaller than the mean stan-
dard deviation. The difference in RV between the two measure-
ments is only 5 ± 10 m/s, although they were obtained at al-
most extreme phases (0.19 and 0.79, when the transit occurs at
phase=0). When fitting the systemic velocity and radial-velocity
semi- amplitude of a circular orbit at the CoRoT ephemeris, we
can exclude a planetary mass higher than 21 MEarth at 2σ.
Due to the lack of detected RV signal at the level of 10 m/s
(1 σ ), it was decided to observe CoRoT-7 with HARPS, which
ing October 2008. More than 100 measurements were taken and
the results will be described in another paper (Queloz et al., in
The following conclusions can be drawn from the RV
5It is interesting to note that we do not need this RV information for-
mally. The case of a grazing large object, e.g. a Jupiter or a late star, can
(i) there is no Jupiter mass planet or, a fortiori, a stellar
companion, bound to CoRoT-7. A Jupiter mass object with a
0.854day period would produce a 230 m/s RV signal and a
white dwarf companion an amplitude over 100 km/s. None of
these are observed;
(ii) the data are compatible with a planet with a 0.854day period
and a mass of less than 21 MEarth, providedthat no other change
in the RV of the star occurs between these two observations
(due to other companions or stellar activity). A formal detection
and an accurate estimate of the mass is discussed in Queloz et al
As a result of Sects. 3 to 7, where we excludedalmost all the
cases of false positives, we conclude that a small planet orbits
the star CoRoT-7 with a 0.854 day period, with a risk of false
positives conservatively estimated to be < 8 10−4.
8. Stellar parameters
The central star was first spectroscopically observed in January
2008 with the AAOmega multi-object facility at the Anglo-
Australian Observatory. By comparing the low-resolution
(R/approx1300) AAOmega spectrum of the target with a grid
of stellar templates, as described in Frasca et al. (2003) and
Gandolfi et al. (2008), we derived the spectral type and lumi-
nosity class of the star (G9 V). These observations allowed us
to asses the dwarf nature of the target, rule out the false positive
scenario of a low-mass star orbiting a giant star, and trigger fur-
ther and systematic high-resolution spectroscopic follow-up of
A preliminary photospheric analysis of the central star
was carried out using a UVES spectra registered on October
2008 (Programme 081.C-0413(C)). The resolving power of this
observation is ≃ 75,000 and the S/N is about 100 per resolution
element at 5500Å. Later, we also took advantage of a series of
80 HARPS spectra acquired during the RV monitoring of the
target. Even though the detailed analysis of those more recent
specta is still in progress, the first results we achieved are in
good agreement with those derived from the UVES spectrum
analyses and are also presented in this paper.
Using the VWA method (see Bruntt et al. 2008, and refer-
ences therein) a preliminary abundance analysis was carried
out from non-blended lines. The derived abundances calculated
relative to the solar ones are given in Table 4. They indicate
[M/H] = + 0.03 ± 0.06,i.e. solar-like. This spectral analysis also
shows that CoRoT-7 is a slowly rotating main-sequence star
late G main-sequence star with nearly solar- like abundances.
Mass, radius, and effective temperature
Todeterminethe atmosphericparametersofthestar,we used
the same approaches as for the other CoRoT host stars (see
Deleuil et al. 2008, e.g.), with different groups carrying inde-
pendent analyses using different methods. The mass and radius
of the star were determined from the photospheric parameters
be eliminated because the corresponding transit duration would be sig-
nificantly shorter than observed. In the χ2map resulting from the transit
fitusing the orbit inclination and planetary radius asfree parameters, we
find a minimum at the values given in Sect. 9 but no secondary minima
that would correspond to larger inclinations and radii. For instance, a
grazing Jupiter would give a 22 min long transit – assuming the limb
darkening of Fig. 12 – which is excluded by the CoRoT observations.
A. L´ eger et al.: CoRoT-7 b: the first Super-Earth with measured radius11
Table 4: Abundances of 21 elements in Corot-Exo7,from VWA.
Last column indicates the number of lines.
+0.11 ± 0.36
+0.02 ± 0.08
Mg i +0.07
+0.05 ± 0.04
+0.09 ± 0.05
Sc ii +0.03 ± 0.05
+0.06 ± 0.04
+0.00 ± 0.05
+0.17 ± 0.05
+0.04 ± 0.04
Cr ii −0.01 ± 0.04
Mn i −0.03 ± 0.05
+0.05 ± 0.04 250
Fe ii +0.04 ± 0.05
+0.04 ± 0.05
+0.04 ± 0.04
+0.09 ± 0.07
Ce ii +0.22
Nd ii +0.04 ± 0.07
derived from our spectroscopic analysis combined with evolu-
tionary tracks in the H-R diagram.
Recent studies have clearly demonstrated that the luminos-
ity in transiting systems can be very well constrained by the
LC fitting (Pont et al. 2007; Sozzetti et al. 2007). However, for
CoRoT-7 the shallow transit and the stellar activity result in a
large uncertainty on the stellar density and further on the planet
radius, so we have abandoned it.
Ontheotherhand,theNai DandMgi linewingsinthespec-
tra yield good constraints on the measured log g value, ± 0.10,
anaccuracyalreadyobtainedbyotherauthors,e.g.Sozzetti et al.
(2007), so we used our spectroscopic estimate of the surface
gravity (log g = 4.50) as a proxy for the luminosity.
The grid of the STAREVOL stellar evolution models (Siess
2006) was interpolated within the locus defined by the three ba-
sic parameters (Teff, M⋆, R⋆) and their associated errors. The
resulting stellar parameters are reported in Table 5.
Table 5: CoRoT-7 parameters derived from RV and spectro-
Spectral TypeG9 V
Age1.2 - 2.3 Gyr
We also use the available visible and near infrared photo-
metric data to estimate the effective temperature independently.
The map of neutral hydrogen column density NHin the galac-
tic plane of Paresce (1984) indicates a maximum value of
NH = 1020cm−2for the line of sight of CoRoT-7 within 200
pc, which corresponds to E(B−V) ≈ 0.01 − 0.02 mag. The
presence of a small amount of extinction in this direction is also
confirmed by the maps of Lallement et al. (2003). Using Exodat
(Deleuil et al. 2009) and 2MASS photometry, the Masana et al.
(2006) calibration yields Teff= 5300 ± 70 K. This effective
temperature from broad-band colours therefore agrees with the
spectroscopic determination reported in Table 5.
For the distance estimate we have first converted the
2MASS magnitudes into the SAAO system with the relations
of Carpenter (2001). The calculated colours J-H and H-K are
0.474 ± 0.030 and 0.046 ± 0.029, respectively. These colours
are compatible with main sequence stars of spectral types
between G8 and K2. Given the constraint on the spectroscopic
measurement of Teff our best estimate of the spectral type is
G9. Assuming for this spectral type an absolute magnitude of
MV = 5.8 ± 0.1 (Straizys & Kuriliene 1981) and extinction
as already reported, we obtain an estimation of the distance
d = 150 ± 20 pc.
Projected rotational velocity
The projected rotational velocity is determined by fitting
several isolated lines in the HARPS spectrum with synthetic
profiles. The synthetic spectra are convolvedby the instrumental
profile approximatedby a Gaussian function (R = 115,000),and
a broadeningprofile comprisedof macroturbulenceand rotation.
Since macroturbulence and rotation are strongly coupled, the
value of vsini is somewhat uncertain. We therefore explored a
grid of values for macroturbulence. The possible range for the
macroturbulence is 0 – 3 km/s since higher values provide poor
fits for all lines. For this range of increasing macrotubulence,
the best fit of vsini values decreases from 3.5 to 0 km/s. Our
estimate is then vsini < 3.5 km/s.
The age estimate derived from the H-R diagram is poorly
constrained.Toovercomethis limitation,we usedifferentagein-
dicators: Li i abundance, the Ca ii H and K chromosphericemis-
sions (Noyes et al. 1984), and gyrochronology(Barnes 2007).
In the CoRoT-7 spectra, no Li i line is detected (Fig.
16), even in the co-added 53 HARPS exposures. This non-
detection points to an older age than the 0.6 Gyr of the Hyades
(Sestito & Randich 2005).
The activity of CoRoT-7 is apparent not only in the CoRoT
light curve (Fig.1), but also in the broad photospheric Ca ii H
& K absorption lines (Fig. 15), which vary with time. For each
HARPS spectrum, following the prescription of Santos et al.
(2000), we calculated the usual chromospheric flux index, log
RHK, which measures the Ca ii H & K fluxes, converted to the
Mount Wilson system, and corrected for the photospheric flux.
Over the one-year period of our series of HARPS spectra, we
estimated the mean stellar activity level to be log R HK = -
4.601 ± 0.05, with an uncertainty estimated from the range of
observed values. Using the relations in Wright et al. (2004), we
derive a chromospheric age estimate of 1.4 ± 0.40 Gyr and a
rotational period of 23 ± 3 days. We compared this chromo-
spheric age estimate with the new activity-age relation given by
Mamajek & Hillenbrand (2008) that yields an age of 2.0 ± 0.3
Gyr. Both values are consistent within the error bars.
12 A. L´ eger et al.: CoRoT-7 b: the first Super-Earth with measured radius
Fig.15: Ca ii H line emission as observed in the co-added
HARPS spectra of CoRoT-7 .
Fig.16: Comparison of CoRoT-7 (black line, co-added HARPS
spectra) and CoRoT-2b spectra (red line, UVES spectrum) in a
spectral window centered on the Li i λ6707.8doublet. While the
CoRoT-2b spectra displaysa strongLi i feature,onlythe nearby
Fe i line at 6707.44Å is visible in the co-added HARPS spectra
of our target.
A fourth age estimate could be done from the stellar rotation
rate. The rotation-agerelation is often presentedas of limited in-
terest; however, Barnes (2007) recently revised the method and
proposes a procedure, called gyrochronology, which provides
the age of a star as a function of its rotation period and colour.
Using his formalism, we infer an age of 1.7 ± 0.3 Gyr using the
rotationalperiodof 23days derivedfromthe LC. These different
diagnostics all agree. We thus adopt an age between 1.2 and 2.3
Gyr. The measured chromosphericactivity, higher than the solar
value, also points to the idea that CoRoT-7 is not an old, quiet
9. Planetary parameters
We derive the planetary parameters using the stellar parameters
of the previous section and the information derived from the
Fig.17: Phase-folded LC of CoRoT-7b using the ephemeris
given in Table 2, and combined in bins of ∼1.5 min. A fixed pe-
riod (Table 6) has been used. The green line is the 4-parameter
best-fit model, using Gim´ enez (2006), but it leads to a stellar
density in conflict with the one determined by spectroscopy.
The red line corresponds to the finally adopted solution, leaving
only inclination and planet radius as free parameters; the bottom
panel shows the residuals of the fit. See the text for details.
Fig.18: Contour of the chi2 of residuals in the space [relative
planet radius, inclination], when a classical fit with 4 free pa-
rameters is used, fixing the stellar mass and radius at the spec-
troscopically determined values. Contours at 68%, 95%, and
99.7 % confidencelevel for parameterestimates are plotted (red,
green, and purple contours). One notes the high degeneracy of
the secure solutions (green and purple contours).
Applying Newton’s law and using estimates of M⋆and
R⋆from Sect. 8, we find
a = [M⋆(M⊙)P(yr)2]1/3= 0.0172 ± 0.0002 AU, a / R⋆= 4.27 ±
The uncertainties are mainly caused by the stellar mass and ra-
dius uncertainties, because the period is known to a high degree
Radius of the planet and inclination of the orbit.
Following the technique used for the other CoRoT discov-
ered planets (Barge et al. 2008), all the observed transits are
combined after a low-order polynomial (order 2 in this case) is
A. L´ eger et al.: CoRoT-7 b: the first Super-Earth with measured radius13
Fig.19: Contour of the chi2 of residuals in the two-parameter
space [relative planet radius, inclination], fixing the stellar mass
and radius at the spectroscopically determined values. Contours
at 68%, 95% and 99.7 % confidence level are plotted. There is
no more degeneracy, so mean values and uncertainties of the fit
can be derived. The uncertainties on the stellar parameters are
not taken into account here.
fitted in the parts surrounding each transit and substracted. The
period is fine-tuned by choosing the one that provides the short-
est durationof the phase-foldedtransit. The individualmeasure-
ments are combinedin bins of 0.0012in phase, correspondingto
about 1.5 min, and the error assigned to each binned point is es-
timated as the standard deviation of the points inside each bin,
divided by the square root of the number of points .
We first used the formalism of Gim´ enez (2006), combined
with the AMOEBA minimization algorithm (Press et al. 1992)
in order to obtain a first evaluation of the transit parameters.
Because of the moderate S/N of the curve, we did not try to
fit for the quadratic limb-darkening coefficients, but instead we
fixed them to values corresponding to a G9V star (from Claret
2000, with u+= ua+ ub= 0.6, u−= ua− ub= 0.2). The four
fitted variables were the centre of the transit, the ratio k=Rpl /
R⋆, the orbital inclination i, and the phase of transit ingress θ1,
which can be translated into the scale of the system a/R⋆using
Eq. 12 of Gim´ enez (2006). Figure 17 shows a fit of the transit
as the thin green line. Under the assumption of a circular orbit,
the scale of the system, a/R⋆=1.9± 0.1, can be translated into a
density of 0.17 g cm−3for the host star, a value much lower than
expected for a G9V star (2.0 g cm−3). This apparent discrepancy
probablyarises fromthe transit ingressand egressappearingless
steep than expected for a main sequence star. To investigate the
originof this problem,we dividedthe LC in 32 groups,and indi-
transits each). The fitted inclinations in groups of transits is sys-
tematically larger than the inclination obtained from the global
phase-folded LC. The mean a/R⋆= 4.0 is significantly different
and the resulting density (1.6 g cm−3) agrees better with that of
a G9V star.
Consequently, we assume that the global transit gives
slightly degraded information on the actual ingress and ingress
of the transit. This may result in large errors on the inferred stel-
lar parameters, including the density, if we rely on the analysis
with four free parameters. We consider two possible causes for
The first one is transit timing variations (TTVs) – temporal
shifts to thecentreofeach transitcausedbythe presenceofaddi-
tional bodies in the system (Agol et al. 2005; Holman & Murray
2005). In fact, when each group of transit is shifted accord-
ingly to the best-fitted center, and the combined transit is built,
we obtain a shape with a steeper ingress/egress, thus alleviat-
ing the discrepancy. However, the time scales of the putative
TTVs and their amplitudes are not easily understood in terms
of gravitational interactions with other bodies, due to the short
distance between CoRoT-7b and its host star. We thus favour a
second explanationin terms of the stellar activity. Several works
(Pont et al. 2008; Alonso et al. 2009) have shown that the occul-
tations of active regions and/or spots can induce apparent shifts
ofthe transitcentersthat mighterroneouslybe attributedto addi-
tional bodiesin planetarysystems. Because the host star CoRoT-
7 is clearly an active star, and because of the comparable sizes
of the transiting object and the stellar spots, we suspect that this
effect is important. Unfortunately, to verify this hypothesis, we
would need photometry of individual transits with the same or-
derofprecisionas the combinedtransit(54ppmper1.5mindata
point), which will be difficult to achieve in the next few years.
These limitations mean that we are not able to obtain alto-
gether precise stellar parameters and planetary parameters from
the LC alone, as is the case for giant planets with a high S/N
transit. The solution is too degenerate as illustrated by Fig. 18
where the contour map of chi2 is plotted in the [Rpl, inclination
] frame: clearly the range of possible solutions within the 2 σ
contouris too broad to be useful; e.g. the resulting ranges for the
planet radius and orbital inclination are Rpl ∈ [1.4 – 2.3 REarth]
andi ∈ [60– 90o], takingthe uncertaintyon R⋆into accountand
with a 5% risk of error.
Instead, we rely on the spectroscopic analysis described in
Sect. 7 and make the assumptions of a circular orbit and of a
limb-darkening law following Claret (2000) quadratic approxi-
mation. By forcing the stellar radius to be R⋆= 0.87±0.04 and
considering the phase-folded light curve of Fig. 2 (i.e. without
any correction for putative TTVs), we looked for the best fit
when leaving two free parameters: the radius of the planet and
the inclination of the orbit. To assess the significance of the best
fit and estimate uncertainties,we divided the light curveinto five
case. The mean and standard deviation were then computed for
We obtain Rpl = 1.68 ± 0.09 REarth, T14 = 1.125± 0.05
h, and i = 80.1 ± 0.3o. The error bars are fully dominated by
the uncertainties on the stellar parameters. The resulting transit
curve (Fig. 17) can be compared to the observations. We must
grant that the agreement is not fully satisfying, especially in the
ingress, but we also note equivalent residual structures at phase
values out of the transit that may indicate the effect of stellar
activity, as previously stated.
From the previous analysis, the mean half-length of the tran-
sit projected on the stellar disk, in stellar radius unit, is h = (π
a τ / P)/ R⋆= 0.71 ± 0.06 , and the impact parameter is 0.70 ±
0.06. The final set of adopted planet parameters is summarised
in Table 6.
14A. L´ eger et al.: CoRoT-7 b: the first Super-Earth with measured radius
Table 6: Planetary parameters.
impact parameter z
k = Rpl / R⋆
Rpl / REarth
± 2.4 10−5
± 2.9 10−4
± 3 10−4
10.1. Tidal and centrifugal force effects
The star and the planet are exchanging strong tidal forces.
Tidal forces influence the motion and the evolution the Corot-
7 system. One consequence is the planetary spin-orbit coupling.
According to Murray & Dermott (1999), the star raises tides on
the planet that lead to the synchronization of the planetary rota-
tion with its revolution, in a characteristic time τsynch
where n is the mean motion of the revolution rate of the planet,
Ωpthe primordial rotation rate of the planet, I the normalized
moment of inertia of the planet, Qp the planetary dissipation
constant, and k2pthe Love number of second order. Several of
the stellar and planetary parameters have been determined in
this work. Some planetarycharacteristics are unknownor poorly
known, but can be estimated within reasonable ranges. The nor-
malized moment of inertia I describes how the mass of a body
is distributed in its interior. If the body is differentiated (a safe
assumption for a body larger than 1000 km in size), the heavier
materials are concentrated in the core and I < 0.4. The planets
and even the large moons of the Solar System have 0.2 < I <
0.35. The solar terrestrial planets show values of the Love num-
ber Qp/k2pbetween 30 and 1000 (Yoder 1995). The primordial
planetary rotation rate Ωpis not known, of course, but using val-
ues for a fast rotator (10 hours) and a slow rotator (10 days) and
using the estimated parameters in the ranges above yields a time
constant τsynchin the range one year to decades.
As a consequence, regardless of the poorly known planetary
parameters, the synchronization of the planetary rotation with
its revolution is a fast and efficient process that has already been
completed, given that the age of system age (1 – 2 Gyr) is much
longer than τsynch. Although the planet has a telluric nature and
the planetary to stellar mass ratio is small, the decisive factor for
the efficiency of tidal dissipation is the short distance to the star.
The stability of the planetary orbit under the influence of
tidal forces depends crucially on the ratio of the stellar dissipa-
tion rate and the stellar Love number Q∗/k∗
2007). The time scale for the decay of the planetary orbit from
the currently observed distance toward the Roche zone of the
star is (P¨ atzold & Rauer 2002; P¨ atzold et al. 2004)
2.46R⋆ as the Roche radius of the star
(Chandrasekhar 1969). The dissipation constant and the Love
number of a star are poorly known. Values for Q⋆/k2⋆vary in
wide ranges in the literature. Values from 105to 105.5(Lin et al.
1996; Jackson et al. 2008) would yield unrealistic small time
scales of 70 Myr for the decay because it would be highly im-
probableto observe this planet today. For Q⋆/k2⋆= 106to 106.5,
2(Carone & P¨ atzold
Fig.20: Planetaryradiusas a functionof mass fordifferentcom-
positions of planets (Grasset et al. 2009). The curves [Fe], [ sil-
icates], [ices], [H2-He] correspond to planets made of pure Fe,
silicates, and metallic core (analogous to the Earth), pure wa-
ter ice and pure H2-He gas, respectively. The shaded area cor-
responds to planets with both silicates and water. The region
between this area and the curves [H2-He] corresponds to plan-
ets with a water-silicate core and a thick H2-He envelope, e.g.
Uranus, Neptune or GJ 436. The red band corresponds to the
present determination of the radius, Rpl = 1.68 ± 0.09 REarth,
and upper limit for the mass, Mpl< 21 MEarth. A purely iron
planet can be excluded.
the orbit would decay within 2 Gyr. The orbit may be consid-
ered stable with respect to tidal forces for Q⋆/k2⋆ > 107. The
latter limit has also been derived by Carone & P¨ atzold (2007)
for the case of OGLE-TR-56b and seems to fit observations bet-
ter. These values were computed with the upper planetary mass
limit of 21 Earth masses.
The shape of the planet is a triaxial Roche ellipsoid
(Chandrasekhar 1969), distorted by the tidal and rotational po-
tentials. The longest of the ellipsoid axes is directed towards the
star while the shortest is directed along the rotation axis of the
planet. In the case of an homogeneous distribution of mass, the
equator prolateness (tidal bulge) of the Roche ellipsoid is given
by (15/4)(M⋆/Mpl)(Rpl/a)3, and the polar flattening, referred to
the mean equatorial radius, by (25/8)(M⋆/Mpl)(Rpl/a)3. Using
the values given in Table 6 and Mpl< 21 MEarth, we obtain an
equatorprolateness < 0.016and a polar flattening < 0.013.If the
non-uniform distribution of masses in the interior of the planet
is taken into account,the results are smaller. If we use the reduc-
tion factor that corresponds to the Earth (≈ 0.78), the expected
values are 0.0125 (≈ 1/80) for the polar flattening and 0.010
(≈ 1/100)for the tidal bulge. This means that the largest equator
radius of the planet (high tide) is < 140 km larger than the short-
est equator radius (low tide) and that the polar radius is <120km
smaller than the mean equatorial radius.
In any case, the corresponding stretching of the planet is
small enough to be neglected in the estimate of its volume be-
cause the corresponding uncertainty is less than on the radius
determination from the transit and spectroscopy (± 570 km).
A. L´ eger et al.: CoRoT-7 b: the first Super-Earth with measured radius15 Download full-text
10.2. Composition of the planet
The estimated radius (1.68 ± 0.09 REarth) and mass limit
(< 21 MEarth) of the planet can be located in the (R, M)
plane and compared with the R(M) relations derived from
the modellings of the internal structures of different planets
(Sotin et al. 2007; Elkins-Tanton & Seager 2008; Valencia et al.
2007; Grasset et al. 2009). As shown in Fig.20, the present con-
straints are not strong and only exclude CoRoT-7 being a purely
According to Lammer et al. (2009), hydrogen, if present
when the planet was formed, would be driven away by ther-
plus sputtering and ion exchange with the stellar wind) in a time
much shorter than the system age ( > 1 Gyr). In the absence of
hydrogen, the main components of the planet can be water, sili-
cates, and metals. If the planetary mass can be determined more
precisely, a better determination of the composition is possible.
The ambiguity between a rocky planet and one containing a sig-
nificant amount of light elements could be overcome.
If the preliminary result, 5 Mearth < Mpl< 11 MEarththat we
have obtainedat the time of submitting the present paperand an-
nounced at the CoRoT Symposium 2-5 February 2009 in Paris,
is confirmed, it would point to, or at least be compatible with, a
rocky planet (Fig. 20).
10.3. Temperature at the planetary surface
The planet is very close to a G9V star (a = 0.0172 AU ± 0.0003
= 4.27 ± 0.20 R⋆), and its spin and orbital rotations are most
likely phase-locked. The stellar disk is seen from the peristellar
point is enormous, 28oin diameter. A high temperature is then
expectedatthe surfaceonthe dayside oftheplanet.However,an
estimate of the temperature distribution depends upon different
hypotheses,dependingonwhetheror notan efficientmechanism
for transferring the energy from the day side to the night side is
– If such a mechanism exists and the temperature is almost uni-
form as on Venus, it would be
(Tpl)1= (1 - A)1/4G(R⋆/2a)1/2T⋆,
where A is the planetary albedo and G stands for the greenhouse
effect. Assuming A = 0 and G = 1 for a rocky planet without
an atmosphere, the temperature reads (Tpl)1= 1810 ± 90 K, the
uncertainty on T reflecting those on T⋆and a/R⋆.
– If there is no such mechanism, the temperature is the result of
the local balance between impinging and emitted powers. In the
(crude) approximation where the incident light beam from the
star is parallel, the temperature is
(Tpl)2≈ (1 - A)1/4G (R⋆/ a)1/2T⋆(cosΦ)1/4, for Φ ∈ [0o,90o],
where Φ is the angle between the normal at the surface and the
planet-star axis (Φ = 0 at the substellar point, and Φ = 90oat
the terminator). At the substellar point, for A = 0 and G = 1, the
temperature reads (Tpl(Φ = 0))2= 2560 ± 125 K.
In the latter hypothesis and in the absence of an atmosphere
that produces a Greenhouse effect, the temperature of the night
side, i.e. Φ ∈ [90o,180o], can be surprisingly low because it
mainlyfaces the coldouter space.This situationis similar to that
of the north and south poles of the Moon (≈ 40 K), and dark face
of Mercury (≈ 90 K) (Vasavada et al. 1999). If a geothermalflux
of 300 mW m−2is the main heating process, the temperature
would be ≈ 50 K.
The CoRoT satellite has discovered transits around the star
Using ground-based follow-up observations and the satellite
colour light-curves, we discarded almost all conceivable cases
of false positives. In so far as we have been exhaustive in listing
the cases of these possible false positives, we conclude that we
have discovered the smallest exoplanet known to date, with a ra-
of a chance alignment of a BEB at less than 400 mas from the
target that is not excluded by our follow-up, the actual presence
of this planet can be considered as established with a risk of a
false positive conservatively estimated to 8 10−4.
The amplitude of transits is ∆F/F ≈ 3.3510−4± 0.12 10−4
(trapezoidalapproximation),as detectedbythesatellite. The star
is characterizedwithhigh-resolutionspectroscopyandis consid-
ered as an active star with spectral type G9V. At the date of this
paper’ssubmission(Feb.2009),the informationon theplanetary
mass resulting from RV measurements is only an upper limit,
Mpl< 21 MEarth. The planetary orbital period, 0.8536 days, is
the shortest one ever detected (http://exoplanet.eu). The corre-
spondingproximityof the planet to its star (a = 0.0172AU = 4.3
R⋆) implies a high temperature at its surface. At the substellar
point, assuming a zero albedo and no Greenhouse effect, it is ≈
1800 K to 2600 K, depending on whether there is an efficient
redistribution of the energy on the planetary surface.
Taking the preceding reserves into account , it should be
noted that it is possible to deduce the presence of a small or-
biting planet with only a small risk of false detection (< 8 10−4)
without a formal RV detection. Even the information that there
is neither a Jupiter mass planet nor a stellar companion around
the main target star is providedby the durationof the transit, e.g.
a grazing Jupiter would give a shorter transit than what is ob-
served. To our present knowledge,a 1.68 Earth radius object can
only be a telluric planet or a white dwarf. Because the latter case
is easily discarded by the RV measurements since it would lead
to a very large signal, we conclude that there is a telluric planet.
sults from the Kepler mission come, as the search for habitable
terrestrial planets becomes a central scientific issue and the con-
firmation by RV very difficult. For RV measurements, CoRoT-
7b is a favourable case because the expected signal is stronger
than for similar planets in the HZ, and its short period allows the
study of many orbits duringa givendurationof the observations,
e.g. over 100 orbits during 4 months. It can be noted that, if this
planet had one Earth mass and was in the HZ of its star (orbital
period of ≈ 220 days), the amplitude of the RV reflex motion of
the star wouldbe ≈ 130times smaller than what can be presently
excluded (k < 15 m/s); the confirmation by RV would then have
probably been impossible in the present state of the art.
If the presently ongoing efforts in RV measurements on
CoRoT-7 are successful, they would be very valuable because
they would allow: (i) an independent detection of the planet, (ii)
a determination of its mass. The latter information would per-
mit precise inferenceson its composition,possibly includingbe-
tween a rocky and a water-rich planet.
Acknowledgements. The authors are grateful to all the people that have worked
on and operated the CoRoT satellite, including Adam C., Ballans H., Barbet D. ,
Bernard M., Collin C., Docclo A., Dupuis O., Essasbou H., Gillard F., Guriau A-
C., Joguet M., Levieuge B., Oulali A., Parisot J., Pau S., Piacentino A., Polizzi
D., Reess J-M., Rivet J-P., Semery A., Strul D., Talureau B. They are grateful
to Despois, D., Selsis F., Zuckerman B. for stimulating discussions. HJD and
JMA acknowledge support by grants ESP2004-03855-C03-03 and ESP2007-
65480-C02-02 of the Spanish Education and Science Ministry. RA acknowl-