Transiting exoplanets from the CoRoT space mission

Abstract

Aims. We report the discovery of very shallow ($\Delta F/F \approx 3.4\times 10^{-4}$), 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 optics imaging, near-infrared spectroscopy, and preliminary results from radial velocity measurements, to test the diluted eclipsing binary scenarios. The parameters of the host star were derived from optical spectra, which were then combined with the CoRoT  light curve to derive parameters of the companion.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 $8 \times 10^{-4}$ risk 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 $59 \pm 3 \times 10^{-5}$ day and a radius of $R_{\rm p} = 1.68 \pm 0.09$ $R_{\rm Earth}$. Analysis of preliminary radial velocity data yields an upper limit of 21 $M_{\rm Earth}$  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, $\approx$1800–2600 K at the substellar point, and a very low one, $\approx$50 K, on its dark face assuming no atmosphere, have been derived.

Full text

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A&A 506, 287–302 (2009)
DOI: 10.1051/0004-6361/200911933
c ? ESO 2009
Astronomy
&
Astrophysics
Special feature
The CoRoT space mission: early results
Transiting exoplanets from the CoRoT space mission
VIII. CoRoT-7b: the first super-Earth with measured radius?
A. Léger1, 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. Bordé1, 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. Hébrard20, L. Jorda4, H. Lammer14, A. Llebaria4, B. Loeillet1,4, M. Mayor12, T. Mazeh17,
C. Moutou4, M. Pätzold18, F. Pont8, D. Queloz12, H. Rauer9,22, S. Renner9,24, R. Samadi2, A. Shporer17, Ch. Sotin19,
B. Tingley6, G. Wuchterl5, M. Adda2, P. Agogu16, T. Appourchaux1, H. Ballans1, P. Baron2, T. Beaufort11,
R. Bellenger2, R. Berlin25, P. Bernardi2, D. Blouin4, F. Baudin1, P. Bodin16, L. Boisnard16, L. Boit4, F. Bonneau16,
S. Borzeix2, R. Briet16, J.-T. Buey2, B. Butler11, D. Cailleau2, R. Cautain4, P.-Y. Chabaud4, S. Chaintreuil2,
F. Chiavassa16, V. Costes16, V. Cuna Parrho2, F. De Oliveira Fialho2, M. Decaudin1, J.-M. Defise15, S. Djalal16,
G. Epstein2, G.-E. Exil2, C. Fauré16, T. Fenouillet4, A. Gaboriaud16, A. Gallic2, P. Gamet16, P. Gavalda16, E. Grolleau2,
R. Gruneisen2, L. Gueguen2, V. Guis4, V. Guivarc’h2, P. Guterman4, D. Hallouard16, J. Hasiba14, F. Heuripeau2,
G. Huntzinger2, H. Hustaix16, C. Imad2, C. Imbert16, B. Johlander11, M. Jouret16, P. Journoud2, F. Karioty2,
L. Kerjean16, V. Lafaille16, L. Lafond16, T. Lam-Trong16, P. Landiech16, V. Lapeyrere2, T. Larqué2,16, P. Laudet16,
N. Lautier2, H. Lecann4, L. Lefevre2, B. Leruyet2, P. Levacher4, A. Magnan4, E. Mazy15, F. Mertens2,
J.-M. Mesnager16, J.-C. Meunier4, J.-P. Michel2, W. Monjoin2, D. Naudet2, K. Nguyen-Kim1, J.-L. Orcesi1,
H. Ottacher14, R. Perez16, G. Peter25, P. Plasson2, J.-Y. Plesseria15, B. Pontet16, A. Pradines16, C. Quentin4,
J.-L. Reynaud4, G. Rolland16, F. Rollenhagen25, R. Romagnan2, N. Russ25, R. Schmidt2, N. Schwartz2, I. Sebbag16,
G. Sedes2, H. Smit11, M. B. Steller14, W. Sunter11, C. Surace4, M. Tello16, D. Tiphène2, P. Toulouse16, B. Ulmer21,
O. Vandermarcq16, E. Vergnault16, A. Vuillemin4, and P. Zanatta2
(Affiliations can be found after the references)
Received 23 February 2009 / Accepted 28 July 2009
ABSTRACT
Aims. We report the discovery of very shallow (ΔF/F ≈ 3.4×10−4), 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, andpreliminaryresultsfromradialvelocitymeasurements, 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
the companion.
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. techniques: photometric – techniques: spectroscopic – planetary systems – techniques: high angular resolution –
techniques: radial velocities
?The CoRoT space mission, launched on27 December 2006, has been
developed and is operated by CNES, with the contribution of Austria,
Belgium, Brazil, ESA,Germany, and Spain. First CoRoT data are avail-
able to the public from the CoRoT archive:
http://idoc-corot.ias.u-psud.fr. The complementary obser-
vations 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),
Article published by EDP Sciences

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288A. Léger et al.: Transiting exoplanets from the CoRoT space mission. VIII.
1. Introduction
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 12000 stars can be mon-
itored simultaneously and continuously over 150 days of ob-
servation. CoRoT is thus particularly well-suited to detecting
planets 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;Aigrainet al. 2008;Moutouet 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 (21.7 ± 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. 2009). In the same line,
blind tests performed by different teams of the CoRoT consor-
tium on actual LCs where transits were added did confirm that
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
2006).
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 formation and evolu-
tion.In this paper,wereportthediscoveryofthesmallest 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, in-
cludingadaptiveoptics (Sect. 3), infraredspectroscopy(Sect. 5),
and preliminary results of the RV measurements (Sect. 7), all
done in order to secure the planetary nature of the transiting
body. The stellar parameters are presented in Sect. 8 and plane-
tary ones in Sect. 9. Such a small and hot planet raises several
questions about its composition, structure, and surface tempera-
ture, as discussed in Sect. 10.
DDT 282.C-5015; the IAC80 telescope operated by the Instituto
de Astrofísica de Tenerife at the Observatorio del Teide; the Isaac
Newton Telescope (INT), operated 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 (OPTICON), a major international col-
laboration supported by the Research Infrastructures Programme of the
European Commissions Sixth Framework Programme; Radial-velocity
observations wereobtained with theSOPHIEspectrograph at the 1.93m
telescope of Observatoire de Haute Provence, France.
1We define a Super-Earth as a planet larger than the Earth but without
a significant hydrogen envelope, e.g. <10−3times the Earth mass. It can
be either rocky or water-rich (Léger et al. 2004; Grasset et al. 2009).
Table 1. CoRoT-7 IDs, coordinates, and magnitudes.
CoRoT ID
USNO-A2
2-MASS
TYCHO
RA (2000)
Dec (2000)
B-mag(a)
V-mag(a)
r?-mag(a)
i?-mag(a)
102708694, LRa01 E2 0165
0825-03049717
06434947-0103468
4799-1733-1
06:43:49.0
−01:03:46.0
12.524
11.668
11.378
10.924
10.301
9.880
9.806
12.9 mas/yr
–4.0 mas/yr
±0.018
±0.008
±0.008
±0.017
±0.021
±0.022
±0.019
1.4
1.5
J(c)
H(c)
Ks
μα(b)
μδ(b)
(c)
(a)Provided by Exo-Dat, based on observations taken at the INT tele-
scope;(b)from TYCHO catalogue;(c)from 2-MASS catalogue.
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
LRa01,theletteraindicatingthatthefieldisclosetotheGalactic
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 oversam-
pled (32 s) target list from the 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
an active star. Its LC shows ≈2% modulations, interpreted as the
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 / ob-
ject with Corot ID 102708694), so that the reader can make his
or her own reduction 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
usinga7-samplerunningmedian;(2)long-termstellaractivityis
removedbysubtractinga0.854-dayrunningmedian;(3)individ-
ual transits are corrected from a local linear trend computed on
3.75-h windows centred on the transits but excluding the transits
themselves; (4) a least-square fit of a trapezoidal transit signal
is performed using only data inside 3.75-h windows centred on
each transit.
Errors on the transit parameters were estimated using a pro-
cedure analogous to the bootstrap method described by Press
et al. (1992), although slightly modified in order to preserve the

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A. Léger et al.: Transiting exoplanets from the CoRoT space mission. VIII.289
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.
Table 2. Transit parameters and associated uncertainties, as modelled
with a trapezoid.
period
central date (1st transit)
ingress/egress duration
total duration (trapezoid)
depth (trapezoid)
0.853585 ± 0.000024 day
2454398.0767 ± 0.0015 HJD
15.8 ± 2.9 min
75.1 ± 3.2 min
3.35 × 10−4± 0.12 × 10−4
correlation properties of the noise: (1) we compute a 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 20000 times to
build histograms used as estimators of the probability distribu-
tions for every transit parameter (Fig. 4). Finally, the error on a
givenparameteris computedas the median absolutedeviationof
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,
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 every sub-LC with a trapezoidal least-square fit, computethe
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
of the form σP(n) = σP(1) × nαand got α = 0.57 (close to
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 conserva-
tively chose to keep this higher value as our final estimate of the
period error (Table 2).
We finallyfindaperiodof0.853585±24×10−6day.Figure2,
where all transits are superimposed, shows that even individ-
ual transits can be tracked down despite the low S/N. The
fit by a trapezoid on the average curve yields the parameters:
τ23 = 0.808 h, τ14 = 1.253 h for the short and long bases of
the trapezium2, and ΔF/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-
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 obvious
stellar systems (see Carpano et al. 2009) a ground based follow-
up programme is initiated. The goal is to check further for pos-
sible contaminatingeclipsing binaries (EBs) whose point spread
function (PSF) could fall within the CoRoT photometric 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 included: i) a
search for photometric variations on nearby stars during the as-
sumed transit; ii) deep imaging, with good-to-highangular reso-
lution, searching for the presence of fainter and closer contami-
nating stars; iii) spectroscopic observations of the target at high
resolution and high S/N; iv) infrared spectroscopy, searching
for faint low-mass companions; v) examination of X-ray flux
from putative close binary systems; and vi) RV measurements.
In addition, we took advantageof CoRoT’s capability to provide
colour information on transit events.
3.1. Time-series photometric follow-up
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
2We use τijfor the parameters related to the trapezoidal fit and Tij
for the parameters related to the more realistic transit modelling (see
Sect. 9).

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290A. Léger et al.: Transiting exoplanets from the CoRoT space mission. VIII.
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 (64 s) and a different low
pass filtering (3 times the time resolution = 3 × 64 s) in order to better preserve the transit shape.
Fig.3. Averaged folded LCs in the three colours provided by the CoRoT instrument, after normalization. Red, green and blue signals are repre-
sented with the corresponding colours, and the white signal, summation of the three bands prior to normalization, is in black.
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-
tends toidentifysuchBEBs, comparingobservationsduringpre-
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 (Bouladeet al. 2003)were performedduringthe

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A. Léger et al.: Transiting exoplanets from the CoRoT space mission. VIII.291
Fig.4. Transit parameter distributions obtained from a bootstrap method for a trapezoidal transit signal.
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
some 10arcsec northof thetarget.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
therefore allowed any false alarm to be excludedfrom sources at
distances over about 4 arcsec from the target.
3.2. High angular resolution imaging follow-up
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,
sharpshortexposuresimagestakenwithFASTCAM 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 re-
sult is shown in Fig. 6. Two very faint stars, invisible in sin-
gle images, become apparent at angular distances of 4.5 and
5.5 arcsec from CoRoT-7 (indicated with arrows in Fig. 6). By
adding simulated stars with known brightnesses at similar angu-
lar 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).
HereweonlyreportonthedeeperobservationstakenattheNOT,
where 12000 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
ontheabsenceofsignalswithanS/N higherthan5,thepresence
of relativelybrightnearbyobjects with I ≤ 15 couldbe excluded
beyond0.18arcsec, I ≤ 16 beyond0.3arcsec and I ≤ 17 beyond
0.8 arcsec. However,significantly fainter objects would not have
been detected at any larger distance.
The VLT/NACO observationswere performedthanks to dis-
cretionary time granted by ESO (DDT 282.C-5015). A set of
J-band images with a pixel size of 13 mas was taken at different
anglesoftheNACOrotator(15◦step),injittermode.Theimages
are recentred at sub-pixel level and median- stacked for each ro-
tatorangle.Themedianofthese stackedimagesgivesessentially
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 arcsec, whereas no other star could be indentified between
0.5 and 5 arcsec. Clearly two of those stars are the counterpart
of the two stars detected on the CFHT stacked image: the small
difference in astrometry can be explainedby our using of the av-
erage pixel size of Megacam, which is not constant throughout
its very wide field of view. Photometry – cross-checked against
added simulated stars – shows that the brightness difference

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292A. Léger et al.: Transiting exoplanets from the CoRoT space mission. VIII.
Fig.5. Period error calculation for the full LC based on a extrapolation
of period error estimates from chopped LCs containing one transit out
of n and spanning the LC total duration. Left: period errors as a func-
tion of n. Right: deviation from the period value Pmedianyielded by the
bootstrap.
between these stars and CoRoT-7 is ΔmJ= 8.1–8.4. If we take
mJ(CoRoT-7) = 10.3, this translates to mJ(faint stars) = 18.4–
18.7.On average,stars in the neighbourhoodexhibitV−J = 1.7,
so that, if we applythis reddeningto 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 massstars andthusarefairlyred,ortheyareverydistantand
thus suffer a large interstellar extinction. Their brightness differ-
ence to CoRoT-7 in V of ΔmV≈ 10, is thus consistent with what
is foundon the CFHT stackedimage.Evenif these stars undergo
a 50% decrease in brightness, they would produce only an am-
plitude variation ΔF/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 contaminatorstill
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 in-
stance it could be a star instrinsically bluer than CoRoT-7 but
distant enough to be both reddened and faint enough to provide
the observed signal. In that case, the star should be at maximum
6.5 mag fainter than CoRoT-7 in J, taking the reddening into
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
Fig.6. Shift-and-add image of the stack of 100 exposures taken at
CFHT with MEGACAM. The arrows indicate the two faint stars lo-
cated at ≈5 arcsec from CoRoT-7 (at the centre of the image) and about
Δm = 10mag fainter. The sizeof thefieldshown is1 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 an imagette.
Fig.7. Final NACO image in J-band after substraction of a median PSF
and de-rotation of 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.
6.5 mag fainter than CoRoT-7 as shown in Fig. 8. The simu-
lated star shows up clearly, brighter than any residual speckles
farther than 400 mas from CoRoT-7. We can conclude that if it
is a background binary system that mimics the observed tran-
sit 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

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A. Léger et al.: Transiting exoplanets from the CoRoT space mission. VIII.293
Fig.8. Central part of the NACO image, after a simulated star 6.5 mag
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.
of the 400 mas radius circle to the surface of the CoRoT mask:
p = π0.42/640 = 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
value3.
4. CoRoT colours
While the probability that there is a background star closer than
400 mas is low (<8 × 10−4), 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 brown dwarf (tr_3) transiting in front of a fainter
star (star_2) physically associated with the target star (star_1).
Assuming that the transiting object has a Jupiter size radius, the
spectral type of the secondarystar can be estimated thanks to the
required brightness differences between (star_1) and (star_2).
Using theEddington’sapproximationforthe limb-darkening
effect (Fig. 12), the maximum reduction of star_2 flux is
(ΔF2/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 Sect. 9), the maximum flux reduction as measured by
CoRoT is
(ΔF/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
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 located
at a distance 0.4 arcsec from a given CoRoT target, with an amplitude
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). We obtain an average of 4 × 10−4object in the simulation of
the LRa01 field (≈104stars) that exhibits such a small and detectable
transit, a probability compatible with the upper limit we find here.
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 associ-
ated faint star and tr_3 a dark transiting object, e.g. a hot Jupiter or a
brown dwarf.
browndwarf,has a Jupiterradius,R3= RJup, andusingthe radius
and the flux of a mean sequence star (Drilling & Landolt 2000)
for star _2, and the CoRoT measured (ΔF/F)max= 3.5 × 10−4,
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 targetstar_1,providinga criterionto qualify/ falsifythe
hypothesis.
The details of the argumentationare explainedin a dedicated
paper (Bordé 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 wouldbe indistinguishablewith the CoRoT spatial resolu-
tion. The same boundaries on the CCD for defining the colours
would then apply. This transiting star would lead to photoelec-
tron contents 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).Theredfractionwouldincreaseto94.9%andthegreen
and blue decrease to 2.9% and 2.2%, respectively. These frac-
tions correspond to the expected transit flux variations in that
hypothesis, (ΔF)Red, (ΔF)Green, and (ΔF)Blue.
If the r,g,b quantities are defined as
r =
(ΔF/F)Red
(ΔF/F)White, g =(ΔF/F)Green
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
(ΔF/F)White
and
b =(ΔF/F)Blue
(ΔF/F)White,

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294 A. Léger et al.: Transiting exoplanets from the CoRoT space mission. VIII.
Fig.10. Distribution of the flux of a K0V star (proxy for CoRoT-7, a
G9V star) into the 3 channels, according to the measured relative inten-
sities of the coloured fluxes in photo-electrons.
values and standard deviations. The observed and expected val-
ues are
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 conclude that the observed colours are incompatible with
those of a transit in front of an M5V star, whereas they are com-
patible with a transit in front of the target star (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 criterion. However, this situation would produce a tran-
sit 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 sequence star. The white dwarf possibility is then dis-
carded without ambiguity. We conclude that the observed LC is
not produced by a triple system where a Jupiter size object tran-
sits in front of a secondary star.
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 produceratios g < 0.55 and b < 0.45 that are not compat-
ible with the observations (mean values ±2σ).
4If a circular orbit isassumed, the mere duration of the transit requires
that the star undergoing the transit has a minimum size. The observed
duration of the transit is a fraction f = 0.061 of the period. f also
satisfies f ≤ (R2+ R3)/πa, the equality corresponding to an equatorial
transit. For aJupiter-sizetransiting object, or smaller, thistranslates into
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.
Fig.11. Expected distribution into the 3 colours of the flux drop, ΔF, 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 different. In that triple
system, the flux drop would be significantly redder than observed.
Fig.12. Limbdarkening effect versusfractional radiusof thestar:super-
imposition of the Eddington’s classical law (black dash line) and of the
predicted ones for aK0star(best proxy inClaret’stablesfor aG9V star)
in filter V (blue line), R (green line), I (red line) and their mean value
(thick green, in fact almost coinciding with R). The data for the pre-
dicted laws are from Claret (2000), using Teff= 5250 K, logg = 4.5,
M/H = 0.0.
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 ob-
tained a high-resolutionspectrumof CoRoT-7 usingthe infrared
spectrograph CRIRES mounted on the VLT-UT1 (Antu). The
AO-system was used along with a slit-width of 0.3 arcsec that
resulted in spectral resolution of R = 60000. The wavelength

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A. Léger et al.: Transiting exoplanets from the CoRoT space mission. VIII. 295
coverage was 2256 to 2303 nm, a region that included the CO-
overtone lines, which have an equivalent width about a factor
of three larger in an M5V star than in a G9V star. The to-
tal integration time of 1800 s resulted in an S/N = 100 per
resolution element. Standard IRAF routines were used for flat-
fielding, sky subtraction, and wavelength calibration. A spec-
trum of HD48497 taken with the same setup during the same
night and at similar air mass was used to remove the telluric ab-
sorption 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 us-
ingthespectrumofanM2Vstarandincreasedthestrengthofthe
CO linestothestrengthofanM5Vstar.Ifthereis anearlyMstar
associated to CoRoT-7, we would expect a secondary maximum
exhibiting a difference in RV with respect to CoRoT-7. This dif-
ferencewouldbeless thanabout100kms−1, sincea system 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 difference of
50 kms−1added.With this analysis, we concludethat, if the sep-
aration in RV of the two stars is greater than 8 kms−1, the pri-
mary and secondary peaks would be well separated. If the sep-
aration was between 3 and 8 kms−1, 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 proxyfor an
M-star.
Therefore, the CRIRES spectra can rule out the presence of
a secondary star earlier than M5V within 0.3 arcsec (CRIRES
slit width) from CoRoT-7 except if, by chance, observations
were performedwhen star_1 and star_2 had the same RV within
3 kms−1. Assuming that CoRoT-7 is a binary consisting of a
G-star orbited by an M-star that has a eclipsing planet, then the
probabilitythat we observethe system at the verymomentof the
conjunction so that the separation is less than 3 kms−1is below
10−6.
Eventually, CoRoT colours allow us to exclude a secondary
companion fainter (redder) than M0V, and CRIRES spectrum a
companion brighter than M5V, but for a very special case whose
probability is less than 10−6. As the exclusion intervals overlap,
we can conclude that the observed events are not due to an as-
sociated star (star_2) subjected to a transit. The triple system
hypothesis is essentially rejected.
6. Lack of X-ray emission
The caseof a triplesystemmadeof themaintargetandtwograz-
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 typewith anorbitalperiodof≈0.9day,thebinarywould
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 brightness of YY
Gem in the X-ray regime is 2–8 × 1029erg/s, and its distance is
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 kms−1
was added to the CoRoT-7 spectrum. We conclude that there is no such
companion.
Table 3. SOPHIE RV observations of CoRoT-7.
Julian date
day
2454514.32514
2454517.40230
Orb.
Phase
0.19
0.79
RV
kms−1
31.1444
31.1392
1σ error
kms−1
0.0095
0.0073
15.8 ± 0.30 pc (Stelzer et al. 2002). Strassmeier et al. (1993) in
their catalogueofchromosphericallyactivebinarieslist at least 7
late-type stars (late G- to early M-type) with rotational (binary)
periods of less than 1 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 0.1 to 2 keV
band, we searched for possible X-ray emission from hypothet-
ical companions of CoRoT-7. With the lowest X-ray luminos-
ity of such short-period binaries, we would have detected these
companions of CoRoT-7at 4σ level, if present. Using d(CoRoT-
7) = 150 pc, we can state that LX(CoRoT-7)< 5×1028erg/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
1.93 m 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
observations were assigned high priority because of the shal-
lowness of the transit. The SOPHIE RV measurements are given
in Table 3 and shown in Fig. 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-
sured byCoRoT betweenthe two SOPHIE observationsis 0.2%,
which is 10 times smaller than the largest variation in the total
CoRoT lightcurve,andtwicesmallerthanthemeanstandardde-
viation. The difference in RV between the two measurements is
only 5 ± 10 m/s, although they were obtained at almost extreme

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296 A. Léger et al.: Transiting exoplanets from the CoRoT space mission. VIII.
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 excluded 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 parameter in this simple fit. We conclude that the
planet, if present, has a mass Mpl < 21 MEarth at a 95% confidence
level.
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
(1σ ), it was decided to observe CoRoT-7 with HARPS, which
has ahighervelocityaccuracy,at thenextobservingseasonstart-
ing October 2008. More than 100 measurements were taken and
the results will be described in another paper (Queloz et al., in
preparation).
The following conclusions can be drawn from the RV
SOPHIE measurements5:
(i) there is no Jupiter mass planet or, a fortiori, a stellar com-
panion, bound to CoRoT-7. A Jupiter mass object with a
0.854 day period would produce a 230 m/s RV signal and a
white dwarf companionan amplitude over 100 kms−1. None
of these are observed;
(ii) the data are compatible with a planet with a 0.854 day pe-
riod and a mass of less than 21 MEarth, providedthat no other
change in the RV of the star occurs between these two ob-
servations (due to other companions or stellar activity). A
formal detection and an accurate estimate of the mass is dis-
cussed in Queloz et al. (in preparation).
As a result of Sects. 3 to 7, where we excluded almost all the
cases of false positives, we conclude that a small planet orbits
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
be eliminated because the corresponding transit duration would be sig-
nificantly shorter than observed. In the χ2map resulting from the transit
fit using theorbit inclination and planetary radiusas freeparameters, 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.
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-AustralianObservatory.By comparingthelow-resolution
(R ≈ 1300) AAOmega spectrum of the target with a grid of stel-
lar templates, as described in Frasca et al. (2003) and Gandolfi
et al. (2008),we derivedthe spectraltypeand luminosityclass of
the star(G9V).These observationsallowedus toasses thedwarf
nature of the target, rule out the false positive scenario of a low-
mass star orbiting a giant star, and trigger further and systematic
high-resolution spectroscopic follow-up of the system.
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 ?75000 and the S/N is about 100 per resolution
element at 5500 Å. 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.
Abundances
Using the VWA method (see Bruntt et al. 2008, and ref-
erences 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 Gmain-sequencestarwith nearlysolar-likeabundances.
Mass, radius, and effective temperature
To determinetheatmosphericparametersofthe star,we used
the same approaches as for the other CoRoT host stars (see, e.g.
Deleuil et al. 2008), with different groups carrying independent
analyses usingdifferentmethods.Themass andradiusofthe star
were determinedfrom the photosphericparametersderivedfrom
our spectroscopic analysis combined with evolutionary tracks in
the H-R diagram.
Recent studies have clearly demonstratedthat the luminosity
in transiting systems can be very well constrained by the LC fit-
ting(Pontet al.2007;Sozzettiet al. 2007).However,forCoRoT-
7 the shallow transit and the stellar activity result in a large un-
certainty on the stellar density and further on the planet radius,
so we have abandoned it.
Ontheotherhand,theNaiDandMgilinewingsinthespec-
tra yield goodconstraints on the measuredlogg value, ±0.10, an
accuracy already obtained by other authors, e.g. Sozzetti et al.
(2007), so we used our spectroscopic estimate of the surface
gravity (logg = 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.
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 NH in the
galactic plane of Paresce (1984) indicates a maximum value
of NH = 1020cm−2for the line of sight of CoRoT-7 within

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A. Léger et al.: Transiting exoplanets from the CoRoT space mission. VIII.297
Table 4. Abundances of 21 elements in Corot-Exo7, from VWA. Last
column indicates the number of lines.
C i
Na i
Mg i +0.07
Al i
Si i
Ca i
Sc i
Sc ii +0.03 ± 0.05
Ti i
+0.06 ± 0.04
Ti ii
+0.00 ± 0.05
V i
+0.17 ± 0.05
Cr i
+0.04 ± 0.04
Cr ii −0.01 ± 0.04
Mn i −0.03 ± 0.05
Fe i
+0.05 ± 0.04 250
Fe ii +0.04 ± 0.05
Co i
+0.04 ± 0.05
Ni i
+0.04 ± 0.04
Cu i
−0.02
Zn i
+0.01
Sr i
+0.14
Y ii
+0.09 ± 0.07
Zr i
+0.01
Ce ii +0.22
Nd ii +0.04 ± 0.07
+0.11 ± 0.36
+0.02 ± 0.08
+0.12
+0.05 ± 0.04
+0.09 ± 0.05
−0.00
3
4
1
2
21
7
2
5
36
11
16
23
3
10
18
15
62
1
1
1
3
1
1
2
Table 5.
analyses.
CoRoT-7 parameters derived from RV and spectroscopic
vrad(kms−1)
vrotsini (kms−1)
Teff
logg
[M/H]
Spectral type
M?
R?
MV
Age
Distance
+31.174
<3.5
5275 K
4.50
+0.03
G9 V
0.93
0.87
5.78
1.2–2.3 Gyr
150 pc
±0.0086
±75
±0.10
±0.06
±0.03
±0.04
±0.10
±20
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.
Distance
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 be-
tween G8 and K2. Given the constraint on the spectroscopic
measurement of Teffour 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,weobtainanestimationofthedistanced = 150±20pc.
Fig.15. Caii H line emission as observed in the co-added HARPS spec-
tra of CoRoT-7.
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 approximated by a Gaussian function (R = 115000),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 kms−1since 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 kms−1. Our
estimate is then vsini < 3.5 kms−1.
Age
The age estimate derived from the H-R diagram is poorly
constrained.Toovercomethislimitation,weuse differentagein-
dicators: Lii abundance,the Caii 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,
logRHK, whichmeasurestheCaiiH &K fluxes,convertedtothe
Mount Wilson system, and corrected for the photospheric flux.
Over the one-yearperiod of our series of HARPS spectra, we es-
timated the mean stellar activity level to be logRHK= −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 chromospheric age es-
timate 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.
A fourth age estimate could be done from the stellar rotation
rate. The rotation-agerelation is often presented as of limited in-
terest; however, Barnes (2007) recently revised the method and
proposes a procedure, called gyrochronology, which provides

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298A. Léger et al.: Transiting exoplanets from the CoRoT space mission. VIII.
Fig.16. Comparison of CoRoT-7 (black line, co-added HARPS spec-
tra) and CoRoT-2b spectra (red line, UVES spectrum) in aspectral win-
dow centered on the Lii λ6707.8 doublet. While the CoRoT-2b spectra
displays a strong Li i feature, only the nearby Fei line at 6707.44 Å is
visible in the co-added HARPS spectra of our target.
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
rotational period of 23 days derived from the LC. These differ-
ent diagnostics all agree. We thus adopt an age between 1.2 and
2.3 Gyr. The measured chromospheric activity, higher than the
solar value, also points to the idea that CoRoT-7 is not an old,
quiet star.
9. Planetary parameters
We derive the planetary parameters using the stellar parameters
of the previous section and the information derived from the
CoRoT LC.
Semi-major axis
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 ± 0.20.
The uncertainties are mainly caused by the stellar mass and ra-
dius uncertainties, because the period is known to a high degree
of accuracy.
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
fitted in the parts surrounding each transit and substracted. The
period is fine-tuned by choosing the one that provides the short-
est duration of the phase-folded transit. The individual measure-
ments are combined in bins of 0.0012 in phase, corresponding
to about 1.5 min, and the error assigned to each binned point is
estimated 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 Giménez (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
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 period (Table 6)
has been used. The green line is the 4-parameter best-fit model, using
Giménez (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 pa-
rameters; the bottom panel shows the residuals of the fit. See the text
for details.
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?us-
ing Eq. (12) of Giménez (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 trans-
lated into a density of 0.17 gcm−3for the host star, a value much
lower than expected for a G9V star (2.0 gcm−3). This apparent
discrepancy probably arises from the transit ingress and egress
appearing less steep than expected for a main sequence star. To
investigate the origin of this problem, we divided the LC in 32
groups, and individuallyfitted each group of transits (containing
between 4 and 5 transits each). The fitted inclinations in groups
of transits is systematically larger than the inclination obtained
from the global phase-folded LC. The mean a/R?= 4.0 is sig-
nificantly different and the resulting density (1.6 gcm−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
this degradation.
The first one is transit timing variations (TTVs) – temporal
shifts tothe centreof eachtransit causedbythe presenceofaddi-
tional bodies in the system (Agol et al. 2005; Holman & Murray
2005). In fact, when each group of transit is shifted accordingly
to the best-fitted center, and the combined transit is built, we ob-
tain a shape with a steeper ingress/egress, thus alleviating the
discrepancy. However, the time scales of the putative TTVs and
their amplitudes are not easily understood in terms of gravita-
tional interactions with other bodies, due to the short distance
between CoRoT-7b and its host star. We thus favour a second
explanation in terms of the stellar activity. Several works (Pont
et al. 2008; Alonso et al. 2009) have shown that the occultations
of active regions and/or spots can induce apparent shifts of the
transit centers that might erroneously be attributed to additional
bodies in planetary systems. Because the host star CoRoT-7 is

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A. Léger et al.: Transiting exoplanets from the CoRoT space mission. VIII.299
Fig.18. Contour of the chi2 of residuals in the space [relative planet
radius, inclination], when a classical fit with 4 free parameters is used.
Contours at 68%, 95%, and 99.7% confidence level for parameter esti-
mates are plotted (red, green, and purple contours). One notes the high
degeneracy of the secure solutions (green and purple contours).
Fig.19. Contour of thechi2 ofresiduals inthetwo-parameter space[rel-
ative 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 val-
ues and uncertainties of the fit can be derived. The uncertainties on the
stellar parameters are not taken into account here.
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 thecombinedtransit (54ppmper1.5min data
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 2σ con-
tour is too broad to be useful; e.g. the resulting ranges for the
planet radius and orbital inclination are Rpl ∈ [1.4–2.3 REarth]
and i ∈ [60–90◦], taking the uncertainty on R?into account and
with a 5% risk of error.
Table 6. Planetary parameters.
Parameter
Period (day)
a (AU)
a/R?
T14(h)
impact parameter z
k = Rpl/ R?
Rpl/ REarth
Mpl/ MEarth
i (deg)
Value
0.853585
0.0172
4.27
1.125
0.61
0.0187
1.68
<21
80.1
Uncertainty
±2.4 10−5
±2.9 × 10−4
±0.20
±0.05
±0.06
±3 × 10−4
±0.09
±0.3
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 approx-
imation. By forcing the stellar radius to be R?= 0.87 ± 0.04
and considering the phase-folded light curve of Fig. 2 (i.e. with-
out 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 dividedthe light curveinto five
phase-foldedsubsetsandcomputedthefittingparametersineach
case. The mean and standard deviation were then computed for
eachofthederivedparameters:planetaryradius,transitduration,
and inclination.
We obtain Rpl = 1.68 ± 0.09 REarth, T14 = 1.125 ± 0.05 h,
and i = 80.1 ± 0.3◦. 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-lengthof 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.
10. Discussion
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
τsynch=
|(n − Ωp|
M?
Mpl(Rpl
3
2
a)3?GM?
a3
?IQp
k2p,
(1)
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

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300A. Léger et al.: Transiting exoplanets from the CoRoT space mission. VIII.
distributed in its interior. If the body is differentiated (a safe as-
sumptionforabodylargerthan1000kminsize),theheavierma-
terials 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 number
Qp/k2pbetween 30 and 1000 (Yoder1995). The primordialplan-
etary rotation rate Ωpis not known, of course, but using values
for a fast rotator(10 h)and a slow rotator(10days) andusingthe
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 (Pätzold & Rauer 2002; Pätzold et al. 2004)
2(Carone & Pätzold
τ =
2
13[a12/3− a12/3
3(Mpl/M?)R5
Roche]Q?
?(GM?)1/2k2?
,
(2)
with aRoche
(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-
probable to observe this planet today.For Q?/k2?= 106to 106.5,
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 & Pätzold (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
equator prolateness <0.016 and a polar flattening <0.013. If the
non-uniformdistributionof masses in the interiorof the planetis
takenintoaccount,theresultsaresmaller.Ifweusethereduction
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 shortest equator
radius (low tide) and that the polar radius is <120 km 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).
=
2.46 R? as the Roche radius of the star
Fig.20. Planetary radius as a function of mass for different composi-
tions of planets (Grasset et al. 2009). The curves [Fe], [ silicates], [ices],
[H2-He] correspond to planets made of pure Fe, silicates, and metallic
core (analogous to the Earth), pure water ice and pure H2-He gas, re-
spectively. The shaded area corresponds to planets with both silicates
and water. The region between this area and the curves [H2-He] cor-
responds to planets with a water-silicate core and a thick H2-He enve-
lope, 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 up-
per limit for the mass, Mpl < 21 MEarth. A purely iron planet can be
excluded.
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& Seager2008;Valenciaet al. 2007;Grasset et al.
2009).As showninFig. 20,the presentconstraintsarenotstrong
and only exclude CoRoT-7 being a purely metallic planet.
According to Lammer et al. (2009), hydrogen, if present
when the planet was formed, would be driven away by ther-
malandnon-thermalprocesses(Jeansescapefromtheexosphere
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 obtained at the time of submitting the present paper
and announced 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, 28◦in diameter. A high temperature is then
expectedat thesurfaceonthedayside oftheplanet.However,an
estimate of the temperature distribution depends upon different
hypotheses,dependingon whetherornot anefficient mechanism
for transferring the energy from the day side to the night side is
present.

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A. Léger et al.: Transiting exoplanets from the CoRoT space mission. VIII.301
– 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 ∈ [0◦,90◦],
where Φ is the angle between the normal at the surface and the
planet-star axis (Φ = 0 at the substellar point, and Φ = 90◦at
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 ∈ [90◦,180◦], can be surprisingly low because it
mainly faces the cold outerspace. This situation is 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 geothermal flux
of 300 mW m−2is the main heating process, the temperature
would be ≈50 K.
11. Conclusions
The CoRoT satellite has discovered transits around the star
CoRoT-7 thatarecompatiblewiththepresenceofasmallplanet.
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 discoveredthe smallest exoplanet known to date, with a ra-
dius Rpl= 1.68± 0.09 REarth. Taking into account the possibility
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 ΔF/F ≈ 3.35 × 10−4± 0.12
10−4(trapezoidal approximation), as detected by the satellite.
The star is characterized with high-resolution spectroscopy and
is considered as an active star with spectral type G9V. At the
date of this paper’s submission (Feb. 2009), the information
on the planetary mass resulting from RV measurements is only
an upper limit, Mpl < 21 MEarth. The planetary orbital pe-
riod, 0.8536 days, is the shortest one ever detected (http://
exoplanet.eu). The corresponding proximity of the planet to
its star (a = 0.0172 AU = 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 1.68Earth 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.
This situationwill probablyrepeatinthe future,e.g.whenthe re-
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 manyorbits duringa given durationof 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 would be ≈130 times 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
permit precise inferences on its composition, possibly including
between 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 C. Adam, H. Ballans, D. Barbet,
M. Bernard, C. Collin, A. Docclo, O. Dupuis, H. Essasbou, F. Gillard, A-C.
Guriau, M. Joguet, B. Levieuge, A. Oulali, J. Parisot, S. Pau, A. Piacentino, D.
Polizzi, J.-M. Reess, J.-P. Rivet, A. Semery, D. Strul, B. Talureau They are grate-
ful to D. Despois, F. Selsis, B. Zuckerman for stimulating discussions. H.J.D.and
J.M.A. acknowledge support by grants ESP2004-03855-C03-03 and ESP2007-
65480-C02-02 of the Spanish Education and Science Ministry. R.A. acknowl-
edges support by grant CNES-COROT-070879. The German CoRoT Team
(TLS and Univ. Cologne) acknowledges DLR grants 50OW0204, 50OW0603,
50QP07011. The building of the input CoRoT/Exoplanet catalogue was made
possible thanks to observations collected for years at the Isaac Newton Telescope
(INT), operated 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. We thank R. Rebolo and the FastCam teams at IAC
and UPCT for permission to use their camera during technical testing time. The
authors are also grateful to an anonymous referee who helped in improving the
manuscript significantly.
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