The HARPS search for southern extra-solar planets. X. A m sin i = 11
  Mearth planet around the nearby spotted M dwarf GJ 674
Astronomy & Astrophysics manuscript no. Gl674 c© ESO 2018
October 30, 2018
The HARPS search for southern extra-solar planets?
X. A m sin i = 11 M⊕ planet around the nearby spotted M dwarf GJ 674
X. Bonfils1, M. Mayor2, X. Delfosse3, T. Forveille3, M. Gillon2, C. Perrier3, S. Udry2, F. Bouchy4, C. Lovis2, F. Pepe2,
D. Queloz2, N. C. Santos1,2,5, and J.-L. Bertaux6
1 Centro de Astronomia e Astrofı́sica da Universidade de Lisboa, Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018
Lisboa, Portugal e-mail: xavier.bonfils@oal.ul.pt
2 Observatoire de Genève, 51 ch. des Maillettes, CH-1290 Sauverny, Switzerland
3 Laboratoire d’Astrophysique, Observatoire de Grenoble, BP 53, F-38041 Grenoble, Cedex 9, France
4 Institut d’Astrophysique de Paris, CNRS, Université Pierre et Marie Curie, 98bis Bd Arago, 75014 Paris, France
5 Centro de Geofisica de Évora, Rua Romão Ramalho 59, 7002-554 Évora, Portugal
6 Service d’Aéronomie du CNRS, BP 3, 91371 Verrières-le-Buisson, France
Received January 09, 2007; accepted xxxx xx, 2007
ABSTRACT
Context. How planet properties depend on stellar mass is a key diagnostic of planetary formation mechanisms.
Aims. This motivates planet searches around stars which are significantly more massive or less massive than the Sun, and in particular
our radial velocity search for planets around very-low mass stars.
Methods. As part of that program, we obtained measurements of GJ 674, an M2.5 dwarf at d=4.5 pc, which have a dispersion much
in excess of their internal errors. An intensive observing campaign demonstrates that the excess dispersion is due to two superimposed
coherent signals, with periods of 4.69 and 35 days.
Results. These data are well described by a 2-planet Keplerian model where each planet has a ∼11 M⊕ minimum mass. A careful
analysis of the (low level) magnetic activity of GJ 674 however demonstrates that the 35-day period coincides with the stellar rotation
period. This signal therefore originates in a spot inhomogeneity modulated by stellar rotation. The 4.69-day signal on the other hand
is caused by a bona-fide planet, GJ 674b.
Conclusions. Its detection adds to the growing number of Neptune-mass planets around M-dwarfs, and reinforces the emerging
conclusion that this mass domain is much more populated than the jovian mass range. We discuss the metallicity distributions of M
dwarf with and without planets and find a low 11% probability that they are drawn from the same parent distribution. Moreover, we
find tentative evidence that the host star metallicity correlates with the total mass of their planetary system.
Key words. stars: individual: GJ 674 – stars: planetary systems – stars: late-type – technique: radial-velocity
1. Introduction
M dwarfs, the most common stars in our Galaxy, were added to
the target lists of planet-search programs soon after the first exo-
planet discoveries. Compared to Sun-like stars, they suffer from
some drawbacks: they are faint and photon noise therefore of-
ten limits measurements of their radial velocity, and many are
at least moderately active and thus prone to so-called “radial-
velocity jitter” (Saar & Donahue 1997). On the other hand, the
smaller masses of M dwarfs result in a higher wobble ampli-
tude for a given planetary mass, and their p-mode oscillations
have both smaller amplitudes and shorter periods than those of
solar type stars. These oscillations therefore average out much
faster. As a result, the detection of an Earth-like planet in the –
closer – habitable zone of an M dwarf is actually within reach
of today’s best spectrographes. Perhaps most importantly, how-
ever, M dwarfs represent unique targets to probe the dependance
on stellar mass of planetary formation, thanks to the wide mass
range (0.1 to 0.6 M�) spanned by that spectral class alone.
Send offprint requests to: X. Bonfils
? Based on observations made with the HARPS instrument on the
ESO 3.6 m telescope under the GTO program ID 072.C-0488 at Cerro
La Silla (Chile).
The first planet found to orbit an M dwarf, GJ 876b (Delfosse
et al. 1998; Marcy et al. 1998), was only the 9th exoplanet
discovered around a main sequence star. Besides showing that
Jupiter-mass planets can form at all around very-low-mass stars,
its discovery suggested that they might be common, since it was
found amongst the few dozen M dwarfs that were observed at
that time. Against these early expectations, no other M dwarf
was reported to host a planet until 2004, though a second planet
(GJ 876c, mp sin i = 0.56 MJup – Marcy et al. 2001) was soon
found around GJ 876 itself.
In 2004, the continuous improvement of the radial-velocity
techniques resulted in the quasi-simultaneous discovery of three
Neptune-mass planets, around µ Ara (mp sin i = 14 M⊕ – Santos
et al. 2004), ρ Cnc (mp sin i = 14 M⊕ – McArthur et al. 2004)
and GJ 436 (mp sin i = 23 M⊕ – Butler et al. 2004; Maness et al.
2006). Of those three, GJ 436b, orbits an M dwarf, and put that
spectral class back on the discovery forefront. It was soon fol-
lowed by another two, a single planet around GJ 581 (mp sin i =
17 M⊕ – Bonfils et al. 2005b) and a very light (mp = 7.5 M⊕)
third planet in the GJ 876 system (Rivera et al. 2005). As a re-
sult, planets around M dwarfs today represent a substantial frac-
tion (30%) of all known planets with m sin i <∼ 30 M⊕.
2 X. Bonfils et al.: An 11 M⊕ planet around the nearby M dwarf GJ 674
Even with GJ 849b (mp sin i = 0.82 MJup – Butler et al. 2006)
now completing the inventory of M-dwarf planets found with
radial-velocity techniques, the upper-range of planet masses re-
mains scarcely populated. This contrasts both with the (still very
incompletely known) Neptune-mass planets orbiting M dwarfs
and with the jovian planets around Sun-like stars. At larger sep-
arations, microlensing surveys similarly probe the frequency of
planets as a function of their mass. That technique has detected
four putative planets that likely orbit M dwarfs: OGLE235-
MOA53b (mp ∼ 1.5 − 2.5 MJup – Bond et al. 2004), OGLE-05-
071Lb (mp = 0.9 MJup – Udalski et al. 2005), OGLE-05-390Lb
(mp = 0.017 MJup – Beaulieu et al. 2006) and OGLE-05-169Lb
(mp = 0.04 MJup – Gould et al. 2006). Two of these four plan-
ets have likely masses below 0.1 MJup . Given the detection bias
of that technique towards massive companions, this again sug-
gests that Neptune-mass planets are much more common than
Jupiter-mass ones around very-low-mass stars.
Here we report the discovery of a 11 M⊕ planet orbiting
GJ 674 every 4.69 days. GJ 674b has the 5th lowest mass of
the known planets, and coincidentally is also the 5th planetary
system centered on a M dwarf. Its detection adds to the small in-
ventory of both very-low mass planets and planets around very-
low mass stars. After reviewing the properties of the GJ 674 star
(§2), we briefly present our radial velocity measurements (§3)
and their Keplerian analysis (§4). A careful analysis of the mag-
netic activity of GJ 674 (§5) assigns one of the two periodicities
to rotational modulation of a stellar spot signal, and the other
one to a bona fide planet. We conclude with a brief discussion of
the properties of the detected planet.
2. The properties of GJ 674
GJ 674 (HIP 85523, LHS 449) is a M2.5 dwarf (Hawley et al.
1997) in the Altar constellation. At 4.5 pc (π = 220.43±1.63 mas
– ESA 1997), it is the 37th closest stellar system, the 54th clos-
est star (taking stellar multiplicity into account)1, and only the
2nd closest known planetary system (after � Eridani, and slightly
closer than GJ 876).
Its photometry (V = 9.382 ± 0.012; K = 4.855 ± 0.018 –
Turon et al. 1993; Cutri et al. 2003) and parallax imply absolute
magnitudes of MV = 11.09 ± 0.04 and MK = 6.57 ± 0.04. GJ
674’s J − K color (= 0.86 – Cutri et al. 2003) and the Leggett
et al. (2001) colour-bolometric relation result in a K-band bolo-
metric correction of BCK = 2.67, and in a 0.016 L� luminosity.
The K-band mass-luminosity relation of Delfosse et al.
(2000) gives a 0.35 M� mass and the Bonfils et al. (2005a)
photometric calibration of the metallicity results in [Fe/H] =
−0.28 ± 0.2.
The moderate X-ray luminosity (Lx/Lbol ' 5.10−5 – Hünsch
et al. 1999) and Ca ii H & K emission depict a modestly active
M dwarf (Fig. 1). Its UVW galactic velocities place GJ 674 be-
tween the young and old disk populations (Leggett 1992), sug-
gesting an age of ∼ 108−9yr.
Last but not least, since we are concerned with radial veloc-
ities, the high proper motion of GJ 674 (1.05 arcsec yr−1 – ESA
1997) changes the orientation of its velocity vector along the
line-of-sight (e.g. Kürster et al. 2003) to result in an apparent
secular acceleration of 0.115 m s−1 yr−1. At our current precision
this acceleration will not be detectable before another decade.
1 on Mar. 1st 2007 (http://www.chara.gsu.edu/RECONS/TOP100.htm)
3962.5 3965 3967.5 3970 3972.5 3975
Wavelength [Å]Wavelength [Å]
Fig. 1. Emission reversal in the Ca ii H line of GJ 674 (top) and GJ
581 (bottom). Within our sample GJ 581 has one of the weakest Ca ii
emission and illustrates a very quiet M dwarf. GJ 674 has much stronger
emission and is moderately active.
Table 1. Observed and inferred stellar parameters for GJ 674
Parameter GJ 674
Spectral Type M2.5
V 9.382 ± 0.012
π [mas] 220.43 ± 1.63
Distance [pc] 4.54 ± 0.03
MV 11.09 ± 0.04
K 4.855 ± 0.018
MK 6.57 ± 0.04
L? [L�] 0.016
Lx/Lbol 5.10−5
v sin i [km s−1] . 1
dvr/dt [m s−1yr−1] 0.115
[Fe/H] −0.28
M? [ M�] 0.35
age [Gyr] 0.1-1
Teff [K] 3500-3700
3. Radial-velocity data
We observed GJ 674 with the HARPS echelle spectrograph
(Mayor et al. 2003) mounted on the ESO 3.6-m telescope at La
Silla Observatory (Chile). After demonstrating impressive planet
finding capabilities right after its commissioning (Pepe et al.
2004), this spectrograph now defines the state of the art in radial-
velocity measurements, delivering a significantly better preci-
sion than its ambitious 1 m s−1 specification. As one recent pub-
lished example, Lovis et al. (2006a) obtained a 0.64 m s−1 disper-
sion for the residuals of their orbital solution of the 3 Neptune-
mass planets of HD 69830.
We observed GJ 674 without interlaced Thorium-Argon light
to obtain cleaner spectra for spectroscopic analysis, at some
small cost in the ultimate Doppler precision. Since June 2004
we have gathered 32 exposures of 900 s each with a median
S/N ratio of ∼ 90. Their Doppler information content, evalu-
ated according to the prescriptions of Bouchy et al. (2001), is
mostly below 1 m s−1. Our internal errors additionally include,
in quadrature sum, an “instrumental” uncertainty of 0.5 m s−1 for
the nightly drift of the spectrograph (since we do not use the
ThAr lamp to monitor it) and the measurements uncertainty of
the daily wavelength zero point calibration. We did benefit of
the recent improvements of the HARPS wavelength calibration,
which is now stable to 0.1 m s−1 (Lovis et al. 2006b).
A constant radial velocity gives a very large reduced chi-
square (χ̄2 = 132) for the time series, which reflects a disper-
X. Bonfils et al.: An 11 M⊕ planet around the nearby M dwarf GJ 674 3
σ = 7.35 m s−1
χ̄2ν = 132
53000 53250 53500 53750 54000
Julian date −2,400,000 [day]Julian date −2,400,000 [day]
1 10 100 1000
Period [day]Period [day]
Fig. 2. Upper panel: Radial-velocity measurements of GJ 674 as a func-
tion of time. The high dispersion (σ = 7.35 m s−1) and chi-square value
(χ̄2 = 132) betray a (coherent or incoherent) signal in the data. Bottom
panel: the Lomb-Scargle periodogram of the velocities has prominent
power excess around P = 4.69 days (downward arrow), which indi-
cates that much of the excess dispersion reflects a coherent signal with
a period close to that value. The second highest peak, at 1.27 day, is a
one-day alias of the 4.69 days period (1.27 ' 1 + 1/4.69).
sion (∼ 7.4 m s−1) well above our internal errors (Fig. 2). This
prompted a search for an orbital (§4) and/or magnetic activity
(§5) signal.
4. Orbital analysis
A Lomb-Scargle periodogram (Press et al. 1992) of the velocity
measurements shows a narrow peak around 4.69-day (Fig. 2).
Adjustment of a single Keplerian orbit demonstrates that it is
best described by a m2 sin i =12.7 M⊕ planet (0.040 MJup) re-
volving around GJ 674 every P2 = 4.6940 ± 0.0005 days in a
slightly eccentric orbit (e2 = 0.10 ± 0.02). The residuals around
this low-amplitude orbit (K1 = 9.8±0.2 m s−1) have a dispersion
of 3.27 m s−1 (Fig. 3), still well above our measurement errors,
and the reduced chi-square per degree of freedom is χ̄2 = 30.6.
A periodogram of the residuals indicates that much of this ex-
cess dispersion stems from a broad power peak centered around
35 days, prompting us to perform a 2-planet fit.
We searched for 2-planet Keplerian solutions with Stakanof
(Tamuz, in prep.), a program which uses genetic algorithms
to efficiently explore the large parameter space of multi-planet
models. Stakanof quickly converged to a 2-planet solution that
describes our measurements much better than the single planet
fit (σ = 0.82 m s−1, χ̄2 = 2.57 per degree of freedom – Fig. 4).
The orbital parameters of the 4.69-day planet change little from
the 1-planet fit, except for the eccentricity which increases to
e2 = 0.20±0.02. Its mass is revised down to M2 sin i = 11.09 M⊕,
and the period hardly changes, P2 = 4.6938 ± 0.0007 day. The
second planet would have a P3 = 34.8467 ± 0.0324 day pe-
riod, an e3 = 0.20 ± 0.05 eccentricity and a minimum mass
of m3 sin i = 12.58 M⊕. Such periods would correspond to semi-
major axes of 0.04 and 0.15 AU. Those are sufficiently disjoint
σ = 3.27 m s−1
χ2 = 30.6
0 0.25 0.5 0.75 1
53000 53250 53500 53750 54000
Julian date −2,400,000 [day]Julian date −2,400,000 [day]
1 10 100 1000
Period [day]Period [day]
Fig. 3. Upper panel: Radial velocities of GJ 674 (red filled circles)
phase-folded to the 4.6940 days period of the best 1-planet fit (curve).
The dispersion around the fit (σ = 3.27 m s−1) and its reduced chi-
square (χ̄2 = 30.6 per degree of freedom) indicate that a single planet
does not describe the data very well. Middle panel: Radial-velocity
residuals of the 1-planet fit Bottom panel: The Lomb-Scargle peri-
odogram of the residuals shows a broad peak centered around 35 days.
that mutual interactions can be neglected over observable time
scales, and that the system would be stable over longer time
scales.
The low dispersion around the solution and the lack of any
significant peak in the Lomb-Scargle periodogram of its residu-
als shows that our current radial-velocity measurements contain
no evidence for an additional component.
5. Activity analysis
Apparent Doppler shifts unfortunately do not always originate
in the gravitational pull of a companion: in a rotating star, stellar
surface inhomogeneities such as plages and spots can break the
exact balance between light emitted in the red-shifted and blue-
shifted halves of the star. Observationally, these inhomogeneities
translate into flux variations as well as into changes of both the
shape and the centroid of spectral lines (Saar & Donahue 1997;
Queloz et al. 2001). Spots typically also impact spectral indices,
whether designed to probe the chromosphere (to which photo-
spheric spots have strong magnetic connections), or the photo-
sphere (because spots have cooler spectra). Of the two candidate
periods, the 4.69-day one is unlikely to reflect stellar rotation.
We measure from our GJ 674 spectra a rotational velocity of
v sin i . 1 km s−1, which would need a rather unprobable stellar
4 X. Bonfils et al.: An 11 M⊕ planet around the nearby M dwarf GJ 674
σ = 0.82 m s−1
χ2 = 2.57
0 0.25 0.5 0.75 1
53000 53250 53500 53750 54000
Julian date −2,400,000 [day]Julian date −2,400,000 [day]
1 10 100 1000
Period [day]Period [day]
Fig. 4. Top two panels: Radial velocity measurements phased to each of
the two periods, after subtraction of the other component of our best 2-
planet Keplerian model. Third panel: Residuals of the best 2-planet fit
as a function of time (O−C, Observed minus Computed). Bottom panel:
Lomb-Scargle periodogram of these residuals.
inclination (i . 15◦) to match such a short period. The moderate
activity level of GJ 674 on the other hand leaves the nature of
the second signal a priori uncertain, and the very small rotation
velocity removes much of the power of the usual bisector test
(Appendix A). We therefore investigated its magnetic activity
through photometric observations (§5.1) and detailed examina-
tion of the chromospheric features in the clean HARPS spectra
(§5.2).
5.1. Photometric variability
We obtained photometric measurements with the CCD cam-
era of the Euler Telescope (La Silla) during 21 nights between
September 2nd and October 19th 2006. GJ 674 was observed
through a VG filter which, amongst the available filters, opti-
∆F = −0.013 sin(2π/35.68 + 54259.93)
53980 53990 54000 54010 54020 54030
Julian date −2,400,000 [day]Julian date −2,400,000 [day]
1 10 100 1000
Period [day]Period [day]
Fig. 5. Upper panel: Differential photometry of GJ 674 as a function
of time. The star clearly varies with a 1.3% amplitude. Bottom panel:
The periodogram of the GJ 674 photometry exhibits significant power
excess peaked at 35 days (small black arrow).
mizes the flux ratio between GJ 674 and its two brightest refer-
ence stars. This relatively blue filters also happens to have good
sensitivity to spots on cool stars such as GJ 674. To minimize at-
mospheric scintillation noise we took advantage of the low stel-
lar density to defocus the images to FWHM ∼ 11′′, so that we
could use longer exposure times. The increased read-out and sky
background noises from the larger synthetic aperture which we
then had to use remain negligible compared to both stellar pho-
ton noise and scintillation.
We gathered 14 to 75 images per night with a median expo-
sure time of 20 seconds. We used the Sept. 24th data, which have
the longest nightly time base, to tune the parameters of the Iraf
Daophot package and optimize the set of reference stars (HD
157931, CD 4611534 and 7 anonymous fainter stars) to min-
imize the dispersion in the GJ 674 photometry for that night.
These parameters were then fixed for the analysis of the full
data set. The nightly light curves for GJ 674 were normalized
by that of the sum of the references, clipped at 3-σ to remove
a small number of outliers, and averaged to one measurement
per night to examine the long term photometric variability of
GJ 674. GJ 674 clearly varies with a ∼1.3% amplitude, and a
(quasi-)period close to 35 days (Fig. 5). To verify that this vari-
ability does not actually originate in one of the reference stars,
we repeated the analysis alternately using as reference star HD
157931 alone and the average of the 8 other references. Both
light curves are very similar to Fig. 5.
The photometric observations are consistent with the signal
of a single spot, within the limitations of their incomplete phase
coverage: the variations are approximately sinusoidal, and their
∼0.2-0.3 radian phase shift from the corresponding radial veloc-
ity signal closely matches the difference expected for a spot. The
spot would cover 2.6% of the stellar surface if completely dark,
corresponding to a ∼ 0.16 R? radius for a circular spot.
X. Bonfils et al.: An 11 M⊕ planet around the nearby M dwarf GJ 674 5
Hα Ca II H+K−5
0 1 2 3 4 5
IndexIndex
Ca II H+K
0 0.2 0.4 0.6 0.8 1
Ca II H+K
1 10 100 1000
Period [day]Period [day]
Fig. 6. Upper panel: Differential radial velocity of GJ 674, corrected for the signature of the 4.69 days planet in our 2-planet Keplerian fit, as
a function of the Hα (red filled circles) and Ca ii H&K (green filled squares) spectral indices defined in the text. Bottom right panels: the Ca ii
H+K and Hα indexes phased to the longer period of the 2-planet Keplerian model. Bottom left panels: Power Density spectra of the spectroscopic
indexes. A clear power excess peaks at 34.8 days (vertical dashed lines).
5.2. Variability of the spectroscopic indices
The emission reversal in the core of the Ca iiH&K resonant lines
results from non-radiative heating of the chromosphere, which
is closely coupled to spots and plages through magnetic con-
nections between the photosphere and chromosphere. The Hα
line is similarly sensitive to chromospheric activity. We mea-
sured these chromospheric spectral features results in the clean
HARPS spectra used to measure the radial velocities, and exam-
ine their variability.
Like the well known Mt. Wilson S index (Baliunas et al.
1995), our Ca ii H+K index is defined as:
Index =
H + K
B + V
. (1)
with H and K sampling the two lines of the Ca ii doublet, and
B and V the continuum on both sides of the doublet. Our H
and K intervals are 31 km s−1 wide and centered on 3933.664
and 3968.47 Å, while B and V are respectively integrated over
[3952.6, 3956 Å] and [3974.8, 3976 Å].
This H+K index varies with a clear period of ∼34.8 days
(Fig. 6). Within the combined errors this is consistent with both
the photometric period and the longer radial velocity period. The
phasing of the chromospheric index and the photometry is such
that lower photometric flux matches higher Ca ii emission, as ex-
pected if active chromospheric regions hover over photospheric
spots.
A plot of the (apparent) radial-velocity as a function of the
H+K spectral index similarly shows the characteristic loop pat-
tern expected for a spot. The radial velocity effect of a spot can-
cels out when it crosses the sub-observer meridian, which oc-
curs twice during a rotation period: once on the hemisphere fac-
ing the observer, and once on the opposite hemisphere. During
the front-facing crossing the spot has maximal projected area,
hence maximal chromospheric emission, while it has a minimal
projected area (and is possibly hidden, depending on its latitude
and the stellar inclination) during the back-facing crossing. As
a result, both extrema of the chromospheric index correspond to
radial-velocity zero-crossings. At intermediate phases the spot
produces intermediate chromospheric emission levels, and it in-
duces positive (respectively negative) radial-velocity shifts when
the masked area is on the rotationally blue- (respectively red-
shifted) half of the star. The net result in a plot of chromospheric
emission as a function of radial velocity is a closed loop.
Chromospheric filling-in of photospheric Hα absorption
has similarly been found a powerful activity diagnostic for M
dwarfs. Kürster et al. (2003) found that in Barnard’s star it corre-
lates linearly with the radial-velocity variations, and interpreted
that finding as evidence that active plage regions inhibit the con-
vective velocity field. The variation pattern in GJ 674 definitely
differs from a linear correlation between Hα and the radial-
velocity residuals, and needs a different explanation.
6 X. Bonfils et al.: An 11 M⊕ planet around the nearby M dwarf GJ 674
Similarly to Kürster et al. (2003) we define our Hα index as:
Index =
F1 + F2
. (2)
with FHα sampling the Hα line, and F1 and F2 the contin-
uum on both sides of the line. Our FHα interval is 31 km s−1
wide and centered on 6562.808 Å, while F1 and F2 are respec-
tively integrated over [6545.495, 6556.245 Å] and [6575.934,
6584.684 Å]. The Hα index behaves similarly to the Ca ii H+K
index.
The chromospheric indices vary by factors of ∼2 and ∼1.3
(for our specific choices of continuum windows), and are thus
much more contrasted than the photometry. They do not how-
ever vary as smoothly with phase as the photometry, perhaps
due to (micro-)flares. This somewhat reduces their value as diag-
nostics of spot-induced radial velocity variations, but these mea-
surements on the other hand require no new observation. They
undoubtedly reinforce the spot interpretation here, and they will
be extremely useful in cases where photometry cannot be imme-
diately obtained.
5.3. Planets vs. activity
In §4 we showed that our 32 radial-velocity measurements of
GJ 674 are well described by two Keplerian signals, as illustrated
by the low reduced chi-square of that model. The above analy-
sis (§5) however demonstrates that the rotation period of GJ 674
coincides with the longer of the two Keplerian periods. Both the
stellar flux and the Ca iiH+K emission vary with that period, im-
plying that the surface of GJ 674 has a magnetic spot. This spot
must induce radial-velocity changes, with the observed phase
relative to the photometric signal. As a consequence, some, and
probably all, of the 35-day radial-velocity signal must originate
in the spot. Planet-induced activity through magnetic coupling
(e.g. Shkolnik et al. 2005) would in principle be an alternative
explanation of the correlation, but here it is not a very attrac-
tive one: the inner planet is at least as massive as the hypothet-
ical 35-day planet, and would, at least naively, be expected to
have stronger interactions with the magnetosphere of GJ 674.
The 4.69-day period however is only seen in the radial velocity
signal, and it has no photometric or chromospheric counterpart.
6. Discussion
6.1. Characteristics of GJ 674b
Perhaps the most important result of the above analysis is that
the ∼4.69-day planet of GJ 674 is robust: variability identifies
the stellar rotation period as ∼35 days, and the 4.69-day period
therefore cannot reflect rotation modulation. The short period
signal, in spite of its larger amplitude, also has no counterpart in
either photometry or chromospheric emission, further excluding
a signal caused by magnetic activity.
The 1-planet fit, which effectively treats the activity sig-
nal as white noise, results in a minimum mass for GJ 674b
of m2 sin i = 12.7 M⊕. The 2-planet fit by contrast filters
out this signal. That filtering obviously uses a physical model
which is not completely appropriate, but that remains preferable
to handling a (partly) coherent signal as white noise. We there-
fore adopt the corresponding estimate of the minimum mass,
m2 sin i = 11.09 M⊕.
At 0.039 AU from its parent star, the temperature of GJ 674 b
is ∼450 K. Planets above a few Earth masses planets can, but
need not, accrete a large gas fraction, leaving its composition –
mostly gaseous or mostly rocky – unclear. The orbital eccentric-
ity might shed light on the structure of GJ 674b, if confirmed
by additional measurements: rocky and gaseous planets have
rather different dissipation properties, and significant eccentric-
ity at the short period of GJ 674 b needs a high Q factor, unless it
is pumped by an additional planet at a longer period (e.g. Adams
& Laughlin 2006). For now, the stellar activity leaves the statis-
tical significance of the eccentricity slightly uncertain, and we
therefore prefer to stay clear from overinterpreting it.
6.2. Properties of M-dwarf planets
One important motivation in searching for planets around M
dwarfs is to investigate whether the planet-metallicity correla-
tion found for Jupiter-mass planets around solar-type stars ex-
tends to very-low-mass stars. Our photometric calibration of M
dwarfs metallicity (Bonfils et al. 2005b) gives respective metal-
licities of [Fe/H]=−0.03, −0.25, +0.14, +0.03, and −0.28 for
GJ 436, GJ 581, GJ 849, GJ 876 and GJ 674. M dwarfs with
known planets therefore have an average metallicity of −0.078
and a median of −0.03. By comparison, the 44 M dwarfs of the
Bonfils et al. (2005b) volume limited sample which are not cur-
rently known to host a planet have average and median metal-
licities of −0.181 and −0.160. M dwarfs with planets therefore
appear slightly more metal-rich than M dwarfs without planets.
A Kolmogorov-Smirnov test (Press et al. 1992) of the two sam-
ples gives an 11% probability that they are drawn from the same
distribution. The significance of the discrepancy is therefore still
modest, limited by small-number statistics.
One can additionally note that the two stars which host giant
planets, GJ 876 and GJ 849, occupy the metal-rich tail of the M
dwarf metallicity distribution, with GJ 849 almost as metal-rich
as the most metal-rich star of the comparison sample. The next
most metal-rich of the M dwarfs with planets, GJ 436, has an ad-
ditional long-period companion (P>6 yr) which might well be a
giant planet (Maness et al. 2006) and would then strengthen that
trend. If confirmed by additional data, this would validate the
theoretical predictions (Ida & Lin 2004; Benz et al. 2006) that
only Jovian-mass planets are more likely to form around metal-
rich stars. Current observations are consistent with this predic-
tion, but not yet very conclusively so (Udry et al. 2006).
Much recent theoretical work has gone into examining how
planet formation depends on stellar mass. Within the “core ac-
cretion” paradigm, Laughlin et al. (2004) and Ida & Lin (2005)
predict that giant planet formation is inhibited around very-
low-mass stars, while Neptune-mass planets should inversely
be common. Within the same paradigm, but assuming that M
dwarfs have denser protoplanetary disks, Kornet et al. (2006)
predict instead that Jupiter-mass planets become more frequent
in inverse proportion to the stellar mass. Finally, Boss (2006) ex-
amines how planet formation depends on stellar mass for plan-
ets formed by disk instability, and concludes that frequency of
Jupiter-mass planet is independent of stellar mass, as long as
disks are massive enough to become unstable.
To date, none of the ∼300 M dwarfs scrutinized for plan-
ets by the various radial-velocity searches (Bonfils et al. 2006;
Endl et al. 2006; Butler et al. 2006) has been found to host a
hot Jupiter. Conversely, GJ 674b is already the 4th hot Neptune.
Though that cannot be established quantitatively yet, these sur-
veys are likely to be almost complete for hot Jupiters, which
are easily detected. Hot Neptune detection, on the other hand, is
definitely highly incomplete. Setting aside this incompleteness
for now, simple binomial statistics shows that the probability of
X. Bonfils et al.: An 11 M⊕ planet around the nearby M dwarf GJ 674 7
−0.75 −0.5 −0.25 0 0.25
[Fe/H][Fe/H]
Fig. 7. Upper panel: Metallicity distributions of 44 M dwarfs without
known planets (gray shading) and of the 5 M dwarfs known to host
planets (black shading). Bottom panel: Corresponding cumulative dis-
tributions (solid and dashed lines, respectively).
Table 2. Keplerian parameterization for GJ 674b and GJ 674’s spot.
Parameter GJ 674b Spot
P [days] 4.6938 ± 0.007 34.8467 ± 0.0324
T [JD] 2453780.085 ± 0.078 2453767.13 ± 0.92
e 0.20 ± 0.02 0.20 ± 0.05
ω [deg] 143 ± 6 113 ± 9
K [m s−1] 8.70 ± 0.19 5.06 ± 0.19
a1 sin i [AU] 3.68 10−6 1.59 10−5
f (m) [M�] 3.0 10−13 4.4 10−13
m2 sin i [MEarth] 11.09 12.58
a [AU] 0.039 0.147
finding no and 4 detections in 300 draws of the same function is
only 3%. There is a thus 97% probability that hot Neptunes are
more frequent than hot Jupiter around M dwarfs. Accounting for
this detection bias in more realistic simulations (Bonfils et al.
in prep.) obviously increases the significance of the difference.
Planet statistics around M dwarfs therefore favor the theoreti-
cal models which, at short periods, predict more Neptune-mass
planets than Jupiter-mass planets.
Acknowledgements. We are grateful to the anonymous referee for construc-
tive comments. XB and NCS acknowledge support from the Fundação para
a Ciência e a Tecnologia (Portugal) in the form of fellowships (references
SFRH/BPD/21710/2005 and SFRH/BPD/8116/2002) and a grant (reference
POCI/CTE-AST/56453/2004). The photometric monitoring has been performed
on the EULER 1.2 meter telescope at La Silla Observatory. We are grateful to
the SNF (Switzerland) for its continuous support. This research has made use of
the SIMBAD database, operated at CDS, Strasbourg, France.
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Appendix A: Bisector analysis
As demonstrated by Saar & Donahue (1997) the bisector anal-
ysis loses much of its diagnostic power when applied to slow
rotators. In simulations of the impact of star spots on radial-
velocity and bisector measurements, they found that, for a given
spot configuration, the radial velocity varies linearly with v sin i
while the bisector span varies as (v sin i)3.3. The bisector signal
therefore decreases faster with decreasing rotational velocities
than the radial-velocity signal, and disappears faster in measure-
ment noise. For GJ 674 we measure a very low rotation velocity
(v sin i < 1km s−1). It is therefore unsurprising that the cor-
8 X. Bonfils et al.: An 11 M⊕ planet around the nearby M dwarf GJ 674
−7.5 −5 −2.5 0 2.5 5 7.5
RV [m/s]RV [m/s]
1 10 100 1000
Period [day]Period [day]
Fig. A.1. Bisector analysis for GJ 674 measurements
relation between the bisector span and radial velocity is weak
(Fig. A.1) and not statistically significant.
