Lovis, C. et al, 2010, A&A

Lovis, C. et al, 2010, A&A
c ESO 2010
Astronomy & Astrophysics manuscript no. HD10180
August 13, 2010
The HARPS search for southern extra-solar planets⋆
XXVII. Up to seven planets orbiting HD 10180:
probing the architecture of low-mass planetary systems
C. Lovis1 , D. Ségransan1 , M. Mayor1 , S. Udry1 , W. Benz2 , J.-L. Bertaux3 , F. Bouchy4 , A. C. M. Correia5 ,
J. Laskar6 , G. Lo Curto7 , C. Mordasini8,2 , F. Pepe1 , D. Queloz1 , and N. C. Santos9,1
Observatoire de Genève, Université de Genève, 51 ch. des Maillettes, CH-1290 Versoix, Switzerland
e-mail: [email protected]
Physikalisches Institut, Universität Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland
Université Versailles Saint-Quentin, LATMOS-IPSL, 11 Boulevard d’Alembert, F-78280 Guyancourt, France
Institut d’Astrophysique de Paris, UMR 7095 CNRS, Université Pierre & Marie Curie, 98bis Bd Arago, F-75014 Paris, France
Departamento de Fı́sica da Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
ASD, IMCCE-CNRS UMR8028, Observatoire de Paris, UPMC, 77 Av. Denfert-Rochereau, F-75014 Paris, France
European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching bei München, Germany
Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
Departamento de Fı́sica e Astronomia, Faculdade de Ciências, Universidade do Porto, 4150-762 Porto, Portugal
Received 12 August 2010 / Accepted ...
Context. Low-mass extrasolar planets are presently being discovered at an increased pace by radial velocity and transit surveys,
opening a new window on planetary systems.
Aims. We are conducting a high-precision radial velocity survey with the HARPS spectrograph which aims at characterizing the
population of ice giants and super-Earths around nearby solar-type stars. This will lead to a better understanding of their formation
and evolution, and yield a global picture of planetary systems from gas giants down to telluric planets.
Methods. Progress has been possible in this field thanks in particular to the sub-m s−1 radial velocity precision achieved by HARPS.
We present here new high-quality radial velocities from this instrument.
Results. We report the discovery of a planetary system comprising at least five Neptune-like planets with minimum masses ranging
from 12 to 25 M⊕ , orbiting the solar-type star HD 10180 at separations between 0.06 and 1.4 AU. A sixth radial velocity signal is
present at a longer period, probably due to a 65-M⊕ object. Moreover, another body with minimum mass as low as 1.4 M⊕ may be
present at 0.02 AU from the star. This is the most populated exoplanetary system known to date. The planets are in a dense but
still well-separated configuration, with significant secular interactions. Some of the orbital period ratios are fairly close to integer
or half-integer values, but the system does not exhibit any mean-motion resonances. General relativity effects and tidal dissipation
play an important role to stabilize the innermost planet and the system as a whole. Numerical integrations show long-term dynamical
stability provided true masses are within a factor ∼3 from minimum masses. We further note that several low-mass planetary systems
exhibit a rather ”packed” orbital architecture with little or no space left for additional planets. In several cases, semi-major axes are
fairly regularly spaced on a logarithmic scale, giving rise to approximate Titius-Bode-like (i.e. exponential) laws. These dynamical
architectures can be interpreted as the signature of formation scenarios where type I migration and interactions between protoplanets
play a major role. However, it remains challenging to explain the presence of so many Neptunes and super-Earths on non-resonant,
well-ordered orbits within ∼1-2 AU of the central star. Finally, we also confirm the marked dependence of planet formation on both
metallicity and stellar mass. Very massive systems are all found around metal-rich stars more massive than the Sun, while low-mass
systems are only found around metal-deficient stars less massive than the Sun.
Key words. Planetary systems – Stars: individual: HD 10180 – Techniques: radial velocities – Techniques: spectroscopic
1. Introduction
Over the past 15 years, the field of extrasolar planets has been
witnessing uninterrupted developments and several major milestones. Among these one can mention: the initial proof of existence of extrasolar gas giants (Mayor & Queloz 1995), the detection of a large sample of gas giants with a variety of masses and
orbital properties, the characterization of bulk properties and atmospheres of transiting gas giants, and the detection of objects in
Based on observations made with the HARPS instrument on the
ESO 3.6-m telescope at La Silla Observatory (Chile), under program
IDs 072.C-0488 and 183.C-0972.
the Neptune-mass and super-Earth range. Recently, two transiting planets with masses and radii close to those of Earth have
been discovered: CoRoT-7b (Léger et al. 2009; Queloz et al.
2009) and GJ 1214 b (Charbonneau et al. 2009). High-precision
radial velocity surveys are now able to find planets with minimum masses as low as 1.9 M⊕ (Mayor et al. 2009a). Preliminary
results from the HARPS survey are hinting at a large population of Neptune-like objects and super-Earths within ∼0.5 AU of
solar-type stars (Lovis et al. 2009). Moreover, hundreds of smallradius candidate planets have been announced by the Kepler
Team (Borucki & the Kepler Team 2010). Clearly, the exploration of the low-mass planet population has now fully started,
C. Lovis et al.: The HARPS search for southern extra-solar planets
and will become the main focus of the field in the coming years.
It is expected that the characterization of planetary system architectures, taking into account all objects from gas giants to
Earth-like planets, will greatly improve our understanding of
their formation and evolution. It will also allow us to eventually put our Solar System into a broader context and determine
how typical it is in the vastly diverse world of planetary systems. The characterization of a significant sample of low-mass
objects, through their mean density and some basic atmospheric
properties, is also at hand and will bring much desired insights
into their composition and the physical processes at play during
planet formation.
As part of this broad effort to explore the low-mass exoplanet
population, we are conducting a high-precision radial velocity
survey of about 400 bright FGK stars in the solar neighbourhood
using the HARPS instrument (Mayor et al. 2003). Observations
of this sample were obtained during HARPS GTO time from
2004 to 2009 (PI: M. Mayor), and then were continued as an
ESO Large Program (PI: S. Udry) until today. Several stars from
this survey have already revealed orbiting low-mass objects:
HD 160691 (Santos et al. 2004; Pepe et al. 2007), HD 69830
(Lovis et al. 2006a), HD 4308 (Udry et al. 2006), HD 40307
(Mayor et al. 2009b), HD 47186, HD 181433 (Bouchy et al.
2009), and HD 90156 (Mordasini et al., in prep.). More and more
candidates are detected as measurements accumulate, and many
new systems are about to be published (Queloz et al., Udry et
al., Ségransan et al., Benz et al., Dumusque et al., Pepe et al.).
Following 400 stars to search for radial velocity signals at the
m s−1 level requires a lot of telescope time, and this survey is
by construction a long-term project. Over the years, we chose
to focus on a smaller sample of stars showing a low level of
chromospheric activity to minimize the impact of stellar noise
on our planet detection limits. Based on measured Ca II activity
levels log R′HK , we kept about 300 stars which we are monitoring regularly. Once a sufficient number of observations has been
gathered for each star, we will be able to derive important statistical properties of the low-mass planet population (Mayor et al.,
in prep.).
In this paper we present the discovery of a new low-mass
planetary system comprising at least 5 Neptune-mass planets
and, probably, a longer-period object and a close-in Earth-mass
planet. The parent star is the G1V dwarf HD 10180, located 39
pc away from the Sun towards the southern constellation Hydrus.
2. Observations and data reduction
The data presented in this paper have been obtained with the
HARPS spectrograph at the ESO 3.6-m telescope at La Silla
Observatory (Chile). This instrument has demonstrated a subm s−1 radial velocity precision over more than 6 years (e.g. Lovis
et al. 2006b; Mayor et al. 2009b) and has led to the detection of
the majority of the low-mass planets known to date.
We have obtained a total of 190 data points on HD 10180,
spread over more than 6 years. This star is part of the highprecision planet-search sample of about 400 stars that we have
been following closely since 2004. Exposure times were set to
15 min to average out stellar oscillations. The achieved SNR at
550 nm ranges from 120 to 270, depending on weather conditions. The estimated photon noise level ranges from 80 to
30 cm s−1 , respectively. Including other measurable instrumental
errors (wavelength calibration, noise on instrumental drift measurement), we obtain error bars between 0.4 and 1.3 m s−1 . This
does not include other instrumental systematics like telescope
guiding (light injection) errors, which are expected to be small
Table 1. Fundamental properties of HD 10180.
Spectral type
T eff
log g
v sin i
log R′HK
Prot (log R′HK )
Age (log R′HK )
[L⊙ ]
[M⊙ ]
[km s−1 ]
HD 10180
25.39 ± 0.62
5911 ± 19
4.39 ± 0.03
0.08 ± 0.01
1.49 ± 0.02
1.06 ± 0.05
24 ± 3
4.3 ± 0.5
but difficult to estimate. Data reduction was performed with the
latest version of the HARPS pipeline (see Lovis et al. 2010, in
prep. for a more detailed description).
The end products of the reduction are barycentric radial velocities with internal error bars, bisector span measurements,
parameters of the cross-correlation functions (FWHM and contrast), and Ca II activity indices S and log R′HK . Being a stabilized, well-controlled instrument, HARPS does not only provide
precise radial velocities, but also precise spectroscopic indicators in general, which is very useful to better understand the stars
under consideration (see e.g. the case of the active star CoRoT-7,
Queloz et al. 2009). The whole set of radial velocities and spectroscopic measurements of HD 10180 can be found in electronic
form at CDS.
3. Stellar properties
The fundamental properties of HD 10180 (G1V, V=7.33) are
taken from the Hipparcos catalogue (ESA 1997) and the spectroscopic analysis by Sousa et al. (2008). They are listed in Table 1.
HD 10180 is a solar-type star with a mass M = 1.06 ± 0.05 M⊙
and metallicity [Fe/H] = 0.08 ± 0.01 dex. With a mean activity
index log R′HK = -5.00, measured on the HARPS spectra presented here, it is clearly an inactive star. Furthermore, it does not
show any well-defined activity cycle such as the solar one (the
rms dispersion of the log R′HK measurements is only 0.012 dex).
Given this activity level and the early-G spectral type, we expect
a stellar RV jitter at the level of ∼1 m s−1 based on comparisons
with similar stars in the HARPS sample (see Dumusque et al.
2010, in prep., Lovis et al. 2010, in prep.). Among inactive stars,
early-G dwarfs appear to have slightly more jitter than late-G
and early-K dwarfs, possibly due to more vigorous convection
and thus increased granulation noise. We thus adopt a value of
1.0 m s−1 for the stellar jitter in this paper and add this number
quadratically to the instrumental error bars. The main purpose
for doing this is to avoid large, unjustified differences in the individual weights (wi = 1/σ2i ) used in the χ2 -minimization process.
4. Analysis of the radial velocity data
4.1. Detection of 5 strong signals
The raw rms dispersion of the radial velocities is 6.42 m s−1 , well
above instrumental errors and the expected stellar jitter, hinting
at the presence of planets orbiting HD 10180. We proceed to an
analysis of the data using the Yorbit software, an analysis pack-
C. Lovis et al.: The HARPS search for southern extra-solar planets
Fig. 3. Peak power distributions for the successive sets of random permutations of the residuals. The peak power of the actual data is shown as a
full vertical line, while 10%, 1% and 0.1% thresholds are shown as dashed lines. The first 5 signals have extremely high significance, while the
6th and 7th signals have 0.6% and 1.4% FAP, respectively. Nothing significant remains after subtracting the 7-Keplerian model (last panel).
(GLS) periodogram as described in Zechmeister & Kürster
(2009). False-alarm probabilities (FAPs) are computed by performing random permutations of the residuals, recording the
peak power for each trial, and comparing the power of the real
signal to the peak power distribution of the permuted datasets.
Each time a signal is considered significant, we include it in the
multi-Keplerian model and proceed further, assuming the radial
velocity signals are due to orbiting planets. To do so, a genetic algorithm is run to efficiently explore the parameter space around
suspected orbital periods. Once the population of solutions has
converged towards the single best-fitting region of parameter
space, a final Levenberg-Marquardt minimization is performed
to reach the deepest χ2 minimum found.
Fig. 1. Close-up views of the window function of the radial velocity
measurements, centered on the regions that have significant peaks and
that may induce aliases in the RV data.
age for radial velocity, astrometric and photometric data featuring a genetic algorithm and various tools for exoplanet search
(Ségransan et al. 2010). We perform successive Lomb-Scargle
periodograms to search for periodic signals of significant amplitude in the RV data. We use the generalized Lomb-Scargle
Identification of signals in periodograms is sometimes ambiguous due to the presence of aliases, induced by the nonrandom time sampling of the observations. Aliases occur at frequencies separated from the true peak by a frequency difference
at which the window function shows significant power. Fig. 1
shows 3 close-up views of the window function of the HD 10180
measurements, centered around f = 0, f = 1 d−1 and f = 2 d−1 .
These are the regions which contain significant peaks and which
could contribute aliases in the frequency range of interest. As
expected, the most prominent features are found at f = 0.0027
d−1 (1 year), f = 1.0027 d−1 (1 sidereal day), f = 1.0000 d−1 (1
day), f = 2.0027 d−1 and f = 2.0055 d−1 . In the following we
will pay particular attention to possible aliases induced by these
Fig. 2 shows the successive GLS periodograms of the radial
velocity data. At each step, the main peak is identified and considered significant if its false-alarm probability (FAP) is smaller
than 10−2 . FAP thresholds of 10−1 , 10−2 and 10−3 are indicated
by horizontal lines in Fig. 2. The peak power distributions of the
shuffled datasets are shown in Fig. 3. As can be seen from both
figures, 5 very significant signals can be successively fitted to the
data. All the corresponding peaks are already clearly apparent in
the periodogram of the raw data. A 5-Keplerian fit to the data
C. Lovis et al.: The HARPS search for southern extra-solar planets
Table 2. Characteristics of the successive multi-Keplerian models fitted
to the data.
k6 (e5 =0)
k7 (e1 =e6 =0)
yields periods of 5.760 d, 16.35 d, 49.74 d, 122.4 d and 2231 d
for these 5 signals. Each of them has a FAP far below 10−3 , as
can be seen from the peak power distributions.
Various aliases of these signals are present in the periodograms. It turns out that the 5 successive highest peaks are
always located at periods larger than 2 d, and in each case two
other peaks potentially corresponding to their 1-d aliases are
seen. Given a highest-peak frequency f0 , these are found
at f0 ± 1.0027 d−1 and show amplitudes similar to each other
but lower than the main peak. Given the properties of the window function, this is the expected pattern if the true signal is the
lower-frequency peak at P > 2 d. On the contrary, if the true
signal was one of the two peaks near 1 d, the peak at P > 2 d
would be a 1-d alias and the other high-frequency peak would
be a 0.5-d alias (2.0055 d−1 ). In this case, one would expect a
large difference in amplitude between the two peaks near 1 d,
and the low-frequency peak should be of intermediate strength.
We checked the global pattern of aliases on the periodogram of
the raw data for the 5 strong signals, extending the computation
to frequencies around 2 d−1 . For each signal, we verified that
the amplitude envelope outlined by the relevant peaks (the main
peak plus its supposed 1-d and 0.5-d aliases) corresponds to the
amplitude envelope in the window function. In each case we find
a symmetric amplitude pattern centered on the lower-frequency
peak and thus we conclude that, for the 5 strong signals seen
in the raw data, the correct periods are the ”long” ones, and the
forest of peaks around P = 1 d are aliases.
In summary, we obtain a 5-Keplerian fit with periods 5.760 d,
16.35 d, 49.74 d, 122.4 d and 2231 d. Table 2 lists the statistical
characteristics of the successive multi-Keplerian models.
4.2. The 600-d signal
Fig. 2. Successive GLS periodograms of the HD 10180 radial velocity
time series, where the main signal is subtracted at each step by adding
one more planet in the multi-Keplerian model. FAP thresholds of 10%,
1% and 0.1% are indicated as dashed lines.
After subtraction of these 5 signals, the periodogram of the residuals still contains appreciable power, with peaks at P = 600
d, 227 d and around 1 d. One immediately sees that the first
two peaks are aliases of each other with the 1-year frequency
(1/600 + 1/365 ≃ 1/227). The FAP of the highest peak (P = 600
d) is 0.6%, and we thus consider it as significant. Given that the
spectral window has relatively high sidelobes at the 1-year frequency, it is not surprising that a fraction of the power is leaking
into the 1-year aliases. Here we assume that the correct period is
not one of the peaks around 1 d because it is highly unlikely to
find a planet with an orbital period so close to 1 d (closer than
0.005 d). To determine which period (600 d or 227 d) is the most
likely one, we proceed in the following way. We perform simulations in which we take the residuals from the full 6-Keplerian
model (with either P = 600 d or P = 227 d as the 6th signal),
shuffle them using permutations, inject an artificial signal at either P = 600 d or P = 227 d, and compare the overall pattern
C. Lovis et al.: The HARPS search for southern extra-solar planets
of peaks in the resulting periodograms to the observed one. The
periods, amplitudes and phases of the injected signals are drawn
from Gaussian distributions centered on the fitted values in the
6-Keplerian model. As a quantitative measure of the similarity
of periodograms, we check whether the 3 highest peaks are the
same as in the actual data. As actual data to compare to, we take
the residuals to the 6-Keplerian model with the fitted 6th signal added (either at P = 600 d or P = 227 d). In this way we
compare the simulated data to the 5-Keplerian residuals that are
closest to reality under both assumptions (the 600-d or the 227-d
peak is the correct one). We take all these precautions because
we are dealing with data on which 5 signals have already been
subtracted, and the exact choice of parameters for these 5 signals
has a significant impact on the alias pattern of the 6th signal.
The results of the simulations are as follows: when injecting
a 600-d signal, the 3 highest peaks seen in the actual data (600
d, 0.9956 d and 0.9989 d) are correctly reproduced in 53% of
the simulated periodograms. When injecting a 227-d signal, the
3 highest peaks (227 d, 1.0017 d and 0.9956 d) are recovered in
only 1.3% of the cases. We thus conclude that a 600-d signal is
much more likely to correctly reproduce the data than a 227-d
We pproceed to fit a 6-Keplerian model to the data, which
yields χ2r = 1.31 and σ(O-C) = 1.39 m s−1 , compared to χ2r
= 1.47 and σ(O-C) = 1.57 m s−1 for the 5-Keplerian model. We
check whether the eccentricity for the 6th, lowest-amplitude signal is constrained by the data byp fixing it to zero, refitting and
looking at the reduced χ2 . With χ2r = 1.31, as before, the addition of more free parameters in the model is not really justified
and we adopt the zero-eccentricity solution. Finally, it is also
worth mentioning that
p the 6-Keplerian solution with P = 600 d
has a slightly better χ2r (1.31) than the corresponding one with
P = 227 d (1.35), reinforcing the case for the longer period.
4.3. A potential 7th signal
Continuing the process one step further, we notice two more
fairly strong peaks in the periodogram of the residuals to the
6-Keplerian model (see Fig. 2). These are located at periods P
= 6.51 d and P = 1.178 d. Again, one of them is clearly the
alias of the other one, this time with the 1 sidereal day period
(|1/6.51 − 1.0027| ≃ 1/1.178). After 10,000 random permutations of the residuals, we obtain a FAP of 1.4% for the higher
peak (P = 6.51 d). This is slightly too high to confidently claim
a detection, but it is nevertheless intriguing. We proceed to fit
this possible 7th signal, but first the correct period has to be determined. As before, we perform simulations by injecting artificial signals into the 7-Keplerian residuals at both periods. When
signals are added at P = 1.178 d, the 3 highest peaks found in
the actual data (6.51 d, 1.178 d, 1.182 d) are reproduced in only
0.2% of the cases. With signals injected at P = 6.51 d, the 3 highest peaks (6.51 d, 1.178 d and 1.182 d, as before) are recovered
in just 0.7% of the simulated periodograms. This slightly favors
the 6.51-d period, but only marginally so. Above all, the very
low ”success” rate of the simulations seems to indicate that this
method cannot reliably decide which peak is the correct one. We
also note that as much as 11% of the simulated periodograms
with an injected signal at P = 1.178 d yield a highest peak at P =
6.51 d. It would therefore be very speculative to draw any conclusion from the fact that the observed highest peak is at P = 6.51
d. In summary, it is likely that the present data are simply not sufficient to distinguish between aliases. The strong 1-d aliases in
the window function (86% of the main peak) are the main obsta-
Fig. 5. Long-period radial velocity signal (P = 2229 d) as obtained from
the 7-Keplerian model, as a function of activity index log R′HK . A weak
correlation is present between both quantities.
cles to overcome, which means that several data points spread
within nights will be required in the future to resolve this issue.
In the meantime, we use another kind of argument: if this
7th signal is caused by an orbiting planet, then it is very unlikely
that the system would be dynamically stable with two objects
at P = 5.76 d and P = 6.51 d, especially considering the 13M⊕ minimum mass of the former planet (see Sect. 6 for further
discussion on this point). We thus adopt P = 1.178 d as the only
viable period from a dynamical point of view. The 7-Keplerian
model, with eccentricities
of the two lowest-amplitudes signals
fixed to zero, has χ2r = 1.23 and σ(O-C) = 1.27 m s−1 .
As a last step, an inspection of the periodogram of the residuals to the 7-Keplerian model does not reveal any peak with a
FAP below 10%, thus ending the search for signals in the RV
In summary, we firmly detect 6 signals with periods 5.760 d,
16.36 d, 49.75 d, 122.7 d, 601 d and 2222 d. A 7th signal with
P = 1.178 d has FAP 1.4% and requires confirmation. The 6Keplerian
model has 29 free parameters for 190 data points, and
χr = 1.31. This value goes down to 1.23 for the 7-Keplerian
model. The rms dispersion of the residuals is 1.39 m s−1 , down
to 1.27 m s−1 in the 7-Keplerian model. We thus have a good
fit to the data, confirming that the adopted stellar jitter value
(1.0 m s−1 ) is reasonable.
5. Origin of the radial velocity signals
So far we have assumed that all RV signals in the data are caused
by orbiting planets. Obviously, with the small semi-amplitudes
in play (between 0.82 and 4.5 m s−1 ), this assumption should be
further verified. For this purpose we study the behaviour of several spectroscopic indicators: bisector velocity span, FWHM of
the cross-correlation function and CaII activity index log R′HK .
Time series of these indicators and the corresponding GLS periodograms are shown in Fig. 4.
C. Lovis et al.: The HARPS search for southern extra-solar planets
Fig. 4. Time series (left) and GLS periodogram (right) for the bisector velocity span, CCF FWHM and activity index log R′HK (from top to bottom).
5.1. Bisector velocity span and CCF FWHM
The global rms dispersion of the bisector span is only 1.33 m s−1 ,
a remarkable stability over more than 6 years. An analysis of
the bisector periodogram reveals no significant power at any frequency, indicating a very quiet star. In particular, no power is
seen around the estimated rotation period (∼24 d) or at any of
the 7 frequencies seen in the radial velocities. The same is true
for the CCF FWHM, although some more structure seems to
be present in the data. The FWHM periodogram shows no peak
with a FAP below 3%.
5.2. CaII activity index
We now turn to the activity index log R′HK . We see that a longperiod modulation is present in these data, although of very low
amplitude (a few 0.01 dex) and at the same level as the shortterm scatter. This behaviour is not typical of Sun-like magnetic
cycles, which have long-term amplitudes an order of magnitude
larger. The peak-to-peak variations here are only 0.06 dex, compared to 0.2-0.3 dex for typical activity cycles in solar-type stars.
Also, the data seem to show a modulation only in the second
half of the observing period. Thus, HD 10180 does not presently
show an activity cycle like the Sun and, with an average log R′HK
of -5.00 over 6 years, seems to be in a very quiet state. Its fundamental parameters do not indicate a subgiant status, which
could have explained the low and stable activity level. More
likely, HD 10180 either has a magnetic cycle with a period much
longer than 10 years, or it is in a relatively quiet phase of its
main-sequence lifetime, with sporadic, weak variations in activity level. If the latter explanation is true, we might be witnessing
an activity state similar to the Maunder minimum of the Sun in
the XVIIth century.
As shown by Lovis et al. (2010, in prep.), magnetic cycles do
induce RV variations in solar-type stars, at a level that depends
on spectral type. Studying a large sample of stars observed with
HARPS, they were able to measure the degree of correlation between activity level and radial velocities. It turns out that the
sensitivity of RVs to activity variations increases with increasing temperature, early-G dwarfs being much more affected than
K dwarfs, which are almost immune to this phenomenon (Santos
et al. 2010). In the case of HD 10180, the log R′HK periodogram
exhibits a very significant peak around 2500 d, as can already
be guessed by eye from the time series (see Fig. 4). A comparison with the raw radial velocity curve shows that the long-period
RV signal at P ∼ 2200 d might be correlated to the log R′HK signal. Fig. 5 shows this long-period RV signal (plus residuals), as
obtained from the 7-Keplerian model, as a function of log R′HK .
The Pearson’s weighted correlation coefficient R is 0.44, indicating a weak positive correlation between both quantities. The
measured slope of the RV-log R′HK relation is 0.92 ± 0.13 m s−1
per 0.01 dex. This number is in good agreement with the predicted sensitivity to activity of HD 10180. Indeed, for T eff =
5911 K, the activity-RV relation of Lovis et al. (2010, in prep.)
gives 0.99 m s−1 per 0.01 dex. It is thus possible that the longperiod RV signal is not of planetary origin, but the result of the
C. Lovis et al.: The HARPS search for southern extra-solar planets
varying fraction of the stellar surface covered by magnetic regions over time. However, HD 10180 does not show a typical
solar-type magnetic cycle, and the quality of the correlation is
quite poor. For example, the activity data around JD=53700 exhibit a large scatter instead of a uniformly low value, as would be
expected from the RV values in the case of an activity-induced
signal. Moreover, the CCF FWHM does not show a clear correlation with log R′HK , in contrast to the stars studied by Lovis et al.
(2010, in prep.). Also, the fitted semi-amplitude of the RV signal is 3.11 m s−1 while the fitted semi-amplitude on the log R′HK
data is 0.011 dex, which appears to be a factor ∼3 too low to
account for the RV signal considering a sensitivity of ∼1 m s−1
per 0.01 dex. Such a factor is large compared to the scatter in the
Lovis et al. (2010, in prep.) relation and it thus seems unlikely
that HD 10180 could be so sensitive to activity.
In conclusion, we still favor the planetary interpretation for
this long-period RV signal, although some doubt remains on its
origin. Future observations will likely solve this issue. In particular, it will be interesting to see whether radial velocities follow
a downward trend in the near future, as would be expected in the
planetary case, or whether they follow the more chaotic variations of the activity index.
There is no indication that the 6 other RV signals might be
due to stellar activity. None of them are related to the expected
stellar rotation period (24 ± 3 d) or to its harmonics. We tried
to detect the rotation period in the log R′HK data, but found no
convincing signal. This confirms that HD 10180 is a very quiet
star. Another strong argument in favor of the planetary interpretation is the very high significance of the signals in the periodograms. It is a clear indication that the signals are coherent
in time, as expected from the clock-like signature of an orbiting
planet. Activity-related phenomena like spots and plages have
short coherence times in inactive stars (of the order of the rotation period), and are not able to produce such well-defined,
high-significance peaks in a dataset spanning more than 6 years.
We thus conclude that a planetary origin is the only viable interpretation for the first 6 RV signals.
6. The HD 10180 planetary system
From now on we will assume that all 7 RV signals are real and
of planetary origin. We thus keep the 7-Keplerian model - and
the fit - as they were obtained in Sect. 4. Table 3 gives the orbital
parameters of all planets in the system. Fig. 6 shows the full radial velocity curve as a function of time, while the phased RV
curves for all signals are shown in Fig. 7. Uncertainties on the
fitted parameters have been obtained with Monte Carlo simulations, where actual data points are modified by drawing from a
Gaussian distribution with a standard deviation equal to the error
bar on each point. The modified datasets are then re-fitted, initializing the non-linear minimization with the nominal solution,
and confidence intervals are derived from the obtained distributions of orbital elements.
The orbital solution given in Table 3 can be further improved by using N-body integrations and adding constraints on
the orbital elements obtained from dynamical considerations
(see Sect. 7). To decouple things, we first show here the solution obtained with a multi-Keplerian model and based on the RV
data alone. Its main advantage is its much higher computational
simplicity, which allows us to derive reliable error bars through
Monte Carlo simulations.
The HD 10180 planetary system is unique in several respects. First of all, it presents five Neptune-like planets, orbiting
between 0.06 and 1.4 AU from the central star. With minimum
Fig. 6. Radial velocity time series with the 7-Keplerian model overplotted. The lower panel shows the residuals to the model.
masses between 12 and 25 M⊕ , this represents a relatively large
total planetary mass within the inner region of the system, and
multi-body migration processes are likely needed to explain this.
Besides these Neptune-mass objects, the system has no massive
gas giant. At most, it contains a small Saturn (m sin i = 65 M⊕ )
at 3.4 AU. In fact, the present data show no detectable long-term
drift and allow us to exclude any Jupiter-mass planet within ∼10
AU for an edge-on system. At the other extreme of the mass
and semi-major axis scales, the system also probably contains
an Earth-mass object (m sin i = 1.40 M⊕ ) orbiting at only 0.022
AU. This is the planet with the lowest minimum mass found to
date, and may represent another member of a hot rocky planet
population that is starting to emerge (e.g. CoRoT-7b, GJ 581 e,
HD 40307 b, etc). The discovery of the HD 10180 planets highlights once again how diverse the outcome of planet formation
can be.
7. Dynamical analysis
With such a complex system at hand, various dynamical studies are in order. For the first time, an extrasolar planetary system comes close to the Solar System as far as the number of
bodies involved is concerned. An obvious aspect to be checked
is whether the fitted orbital solution is dynamically stable over
Gyr timescales (the age of the star). The stability of such a system is not straightforward, in particular taking into account that
the minimum masses of the planets are of the same order as
Neptune’s mass and the fitted eccentricities are relatively high
when compared with the eccentricities of the planets in the Solar
System. As a consequence, mutual gravitational interactions between planets in the HD 10180 system cannot be neglected and
may give rise to instability. This said, it must be noted that the
fitted eccentricities of all planets are different from zero by less
than 2.5σ (their probability distributions are close to Gaussians),
thus making it difficult to discuss this issue based on the RV data
only. Considering the low radial velocity amplitudes induced by
these objects, it will be challenging to better constrain these eccentricities in the future with the RV method.
C. Lovis et al.: The HARPS search for southern extra-solar planets
Table 3. Orbital and physical parameters of the planets orbiting HD 10180, as obtained from a 7-Keplerian fit to the data. Error bars are derived
using Monte Carlo simulations. λ is the mean longitude (λ = M + ω) at the given epoch.
[km s−1 ]
[m s−1 ]
m sin i
[M⊕ ]
HD 10180 b
HD 10180 c
−141 )
[m s−1 ]
HD 10180 d HD 10180 e HD 10180 f
2,454,477.878676 (fixed)
90 (fixed)
−209 )
−10 )
7.1. The secular planetary equations
HD 10180 h
Table 5. Fundamental secular period and frequencies.
Over long times, and in absence of strong mean motion resonances, the variations of the planetary elliptical elements are
well described by the secular equations, that is the equations obtained after averaging over the longitudinal motion of the planets
(see Laskar 1990). The secular system can even be limited to its
first order and linear part, which is usually called the LaplaceLagrange system (see Laskar 1990) which can be written using
the classical complex notation zk = ek eiωk for k = b, c, ..., h
 
 
 zb 
 zb 
d  . 
 ..  .
 =i
 . 
dt  . 
This linear equation is classically solved by diagonalizing
the real matrix (L) through the linear transformation to the
proper modes
 
 
 zb 
 u1 
 . 
 . 
 ..  = (S )  ..  .
 
 
Using the initial conditions of the fit in Table 6, we have computed analytically the (real) Laplace-Lagrange matrix (L), and
derived the (real) matrix (S ) of its eigenvectors which gives the
relation from the original eccentricity variables zk to the proper
modes uk . After the transformation (2), the linear system (1) becomes diagonal
(uk ) = Diag(g1, . . . g7 )(uk )
where g1 , . . . , g7 are the eigenvalues of the Laplace-Lagrange
matrix (L). The solution is then trivially given for all k = 1, . . . , 7
uk = uk (0)ei gk t ,
HD 10180 g
gk (num)
gk (ana)
-3.68 ×10−07
-3.40 ×10−09
-1.11 ×10−09
-1.43 ×10−10
-5.16 ×10−12
-3.93 ×10−15
-8.14 ×10−17
Note. The period and gk (num) are computed numerically from the integrated solution of Table 6 through frequency analysis. gk (ana) are the
corresponding frequencies computed from the Laplace-Lagrange linear
approximation (Eq.2), and g̃k is the value of the damping term in the
proper mode amplitudes resulting from the tidal dissipation of planet b.
while the secular solution is obtained through (2). It can be noted
that as the matrix (L), and thus (S ), only depend on the masses
and semi-major axis of the planets, they do not change much for
different fits to the data because the periods and masses are well
constrained (for a given inclination of the system).
The secular frequencies gk that are responsible for the precession of the orbits and for most of the eccentricity variations
are given in Table 5.
7.2. Stability of the short-period planet b
Despite the proximity of planet b to the star, it undergoes strong
gravitational perturbations from the remaining bodies, due to
secular interactions. Even for a model where the initial eccentricity of the planet b is initially set at zero (Table 3), its eccentricity shows a rapid increase, which can reach up to 0.8 in only
1 Myr (Fig. 8a). The inclusion of general relativity in the model
calms down the eccentricity evolution, but still did not prevent
C. Lovis et al.: The HARPS search for southern extra-solar planets
Table 4. The orthogonal matrix (S ) of transformation to the proper modes (Eq. 2).
the eccentricity of planet b to attain values above 0.4, and planet
c to reach values around 0.3, which then destabilizes the whole
system (Fig. 8b).
Most of the variations observed in Fig. 8 are well described
by the secular system (1,2,3). In particular, the effect of general
relativity will be to largely increase the first diagonal terms in
the Laplace-Lagrange matrix (L), which will increase in z1 the
contribution of the proper mode u1 and thus decrease the long
time oscillations due to the contribution of the modes.
At this stage, one may question the existence of the innermost short-period planet. However, since the mass of this planet
should be in the Earth-mass regime, we may assume that it is
mainly a rocky planet. As a consequence, due to the proximity
of the star, this planet will undergo intense tidal dissipation, that
may continuously damp its orbital eccentricity.
7.3. Tidal contributions
Using a simplified model (Correia 2009) the tidal variation of
the eccentricity is
ė = −Kn f6 (e)(1 − e2 )e ,
where f6 (e) = (1 + 45e2 /14 + 8e4 + 685e6 /224 + 255e8 /448 +
25e10 /1792)(1 − e2 )−15/2 /(1 + 3e2 + 3e4 /8), and
21 M⋆ R 5 k2
2 m
a Q
M⋆ is the mass of the star, m and R are the mass and the radius
of the planet respectively, k2 is the second Love number, and Q
the dissipation factor.
As for the Laplace-Lagrange linear system, we can linearize
the tidal contribution from expression (5), and we obtain for each
planet k a contribution
ėk = −γk ek
γk = Kk nk ,
In order to obtain a realistic solution, that is the result of the
tidal evolution of the system, it is thus not sufficient to impose
that the innermost planets have small eccentricity, as this may
only be realized at the origin of time (Fig. 8). It is also necessary
that the amplitude of the first proper modes, and particularly u1
are set to small values. This will be the way to obtain a solution
with moderate variations of the eccentricities on the innermost
planets, which will then increase its stability.
7.4. Orbital solution with dissipative constraint
As the proper modes of the two innermost planets, u1 and u2
are damped after about 1 Gyr, we expect them to be small for
the present observations (the age of the star is estimated to be
4 Gyr, Table 1). The initial conditions for the HD 10180 planetary system should take into account the tidal damping. We have
thus chosen to modify our fitting procedure in order to include a
constraint for the tidal damping of the proper modes u1 and u2 .
For that purpose, we added to the χ2 minimization, an additional
term, corresponding to these proper modes:
χ2R = R u21 + u22 ,
where R is a positive constant, that is chosen arbitrarily in order
to obtain a small value for u1 and u2 simultaneously.
Using R =p 350 we get u1 ∼ 0.0017 and u2 ∼ 0.044 and
to the results
obtain a final χ2 = 1.24, which is nearly identical p
obtained without this additional constraint (R = 0, χ2 = 1.22).
The best-fit solution obtained by this method is listed in Table 6.
We believe that this solution is a more realistic representation of
the true system than the one listed in Table 3, and we will adopt it
henceforward for the remaining of the paper. Actually, with this
constraint, the variation of the eccentricity of the two innermost
planets are now smaller (Fig.8c).
that is, an additional real contribution on each diagonal term of
the Laplace-Lagrange matrix i (L)
żk = −γk zk ,
which thus adds an imaginary part iγk to the diagonal terms of
(L). In fact, since apart from planet b, all planetary masses are
relatively large (Table 6), the dissipation in these planets may be
small, and we will uniquely consider the tidal dissipation in the
innermost planet b. It should nevertheless be stressed that all the
eigenvalues of the matrix (L) will be modified, and will present
an imaginary part that will induce an exponential term in the
amplitude of the proper modes (Table 5).
Adopting a value similar to Mars k2 /Q = 0.0015, we can see
in Fig. 9 that the amplitude of the proper mode u1 will be rapidly
damped in a few tens of Myr, whatever its original value. Over
more than 1 Gyr, the amplitude of the second proper mode u2 is
also most probably damped to a small value.
7.5. Stability of the system
To analyze the stability of the nominal solution in Table 6 we performed a global frequency analysis (Laskar 1993) in the vicinity of the six outermost planets (Fig. 10), in the same way as
achieved for the HD 202206 or the HD 45364 planetary systems
(Correia et al. 2005, 2009). For each planet, the system is integrated on a regular 2D mesh of initial conditions, with varying
semi-major axis and eccentricity, while the other parameters are
retained at their nominal values. The solution is integrated over
10 kyr for each initial condition and a stability indicator is computed to be the variation in the measured mean motion over the
two consecutive 5 kyr intervals of time. For regular motion, there
is no significant variation in the mean motion along the trajectory, while it can vary significantly for chaotic trajectories (for
more details see Couetdic et al. 2009). The result is reported in
color in Fig. 10, where “red” represents the strongly chaotic trajectories, and “dark blue” the extremely stable ones.
C. Lovis et al.: The HARPS search for southern extra-solar planets
Table 6. Orbital parameters for the seven bodies orbiting HD 10180, obtained with a 8-body Newtonian fit to observational data, including the
effect of tidal dissipation. Uncertainties are estimated from the covariance matrix, and λ is the mean longitude at the given epoch (λ = M + ω).
The orbits are assumed co-planar.
[km s−1 ]
[m s−1 ]
m sin i
[M⊕ ]
HD 10180 b
HD 10180 c
[m s−1 ]
HD 10180 d HD 10180 e HD 10180 f
2,454,000.0 (fixed)
90 (fixed)
In all plots (Fig.10), the zones of minimal χ2 obtained in
comparing with the observations appear to belong to stable “dark
blue” areas. This picture thus presents a coherent view of dynamical analysis and radial velocity measurements, and reinforces
the confidence that the sub-system formed by the six outermost
planets given in Table 6 is stable for long timescales.
Nevertheless, due to the large number of planets in the system, many mean motion resonances can be observed, several of
them being unstable. None of the planets determined by the solution in Table 6 are in resonance, but some of them lie in between.
In particular, the pair d and e is close to a 3:1 mean motion resonance, and the pair e and f is close to a 5:2 mean motion resonance (similar to Jupiter and Saturn).
7.6. Long-term orbital evolution
The estimated age of the HD 10180 system is about 4.3 Gyr
(Table 1), indicating that the present planetary system had to
survive during such a long timescale. We tested directly this by
performing a numerical integration of the orbits in Table 6 over
10 Myr using the symplectic integrator SABAC4 of Laskar &
Robutel (2001) with a step size of 10−3 years, including general
The results displayed in Fig. 11 show that the orbits indeed
evolve in a regular way, and remain stable throughout the simulation. We have also integrated the full 7 planet system over 1
Myr with a step size of 10−4 years, without any sign of strong instability, although the frequency analysis of the solutions with 7
planets shows that these solutions are not as well approximated
by quasiperiodic series as the 6 planet solutions. This will have
to be analyzed further as like in our solar system, the presence
of this innermost planet seems to be critical for the long-term
stability of the system (Laskar & Gastineau 2009).
The fact that we are able to find a stable solution compatible with the observational data can still be considered as a good
HD 10180 g
HD 10180 h
Table 7. Semi-major axes and eccentricity minima and maxima observed over 1 Myr in the 7 planet solution of Table 6.
indicator of the reliability of the determination of the HD 10180
planetary system.
Because of the strong gravitational secular interactions between the planets, their orbital eccentricities present significant variations, while their semi-major axes are almost constant
(Table 7), which is also the signature that the system is far from
strong resonances. As the secular frequencies gk (Table 5) are
relatively large, the secular variations of the orbital parameters
are more rapid than in our Solar System, which may allow us
to detect them directly from observations, and hence access the
true masses and mutual inclinations of the planets as it was done
for the system GJ 876 (Correia et al. 2010).
7.7. Additional constraints
The stability analysis summarized in Fig. 10 shows a good agreement between the “dark blue” stable areas and the χ2 contour
curves. We can thus assume that the dynamics of the seven planets is not disturbed much by the presence of an additional body
We then tested the possibility of an additional eighth planet
in the system by varying the semi-major axis and the eccentricity
C. Lovis et al.: The HARPS search for southern extra-solar planets
Fig. 10. Global view of the dynamics of the HD 10180 system for variations of the eccentricity and the semi-major axis of the six outermost planets.
The color scale is the stability index obtained through a frequency analysis of the longitude of the planets over two consecutive time intervals. Red
areas correspond to high orbital diffusion (instability) and the blue ones to low diffusion (stable orbits). Labeled lines give the value of χ2 obtained
for each choice of parameters.
of the periastron over a wide range, and performing a stability
analysis (Fig. 12). The test was done for a fixed K value (K =
0.78 m s−1 ) corresponding to planet b.
From this analysis (Fig. 12), one can see that stable orbits are possible beyond 6 AU (outside the outermost planet’s
orbit). More interestingly, stability appears to be also possible around 1 AU, which corresponds to orbital periods within
300 − 350 days, between the orbits of planets f and g, exactly
at the habitable zone of HD 10180. Among the already known
planets, this is the only zone where additional planetary mass
companions can survive. With the current HARPS precision of
∼1 m s−1 , we estimate that any object with a minimum mass M >
10 M⊕ would already be visible in the data. Since this does not
seem to be the case, if we assume that a planet exists in this
stable zone, it should be at most an Earth-sized object.
We can also try to find constraints on the maximum masses
of the current seven-planet system if we assume co-planarity of
the orbits. Indeed, up to now we have been assuming that the
inclination of the system to the line-of-sight is 90◦ , which gives
minimum values for the planetary masses (Table 6).
By decreasing the inclination of the orbital plane of the system, we increase the mass values of all planets and repeat a stability analysis of the orbits, as in Figure 10. As we decrease the
inclination, the stable “dark-blue” areas become narrower, to a
point that the minimum χ2 of the best fit solution lies outside the
stable zones. At that point, we conclude that the system cannot
be stable anymore. It is not straightforward to find a transition
inclination between the two regimes, but we can infer from our
plots that stability of the whole system is still possible for an
inclination of 30◦ , but becomes impossible for an inclination of
10◦ . Therefore, we conclude that the maximum masses of the
planets are most probably obtained for an inclination around 20◦ ,
corresponding to a scaling factor of about 3 with respect to minimum masses.
8. On the properties of low-mass planetary systems
8.1. Dynamical architecture
The increasing number of multi-planet systems containing at
least three known planets greatly extends the possibilities to
study the orbital architectures of extrasolar planetary systems
and compare them to our Solar System. Although there are already 15 systems with at least three planets as of May 2010, one
should recognize that our knowledge of many of them is still
highly incomplete due to observational biases. The RV technique
finds the most massive, close-in planets first in each system, and
the secure detection of multiple planets requires a large number
of observations, roughly proportional to the number of planets
for RV signals well above the noise floor. Lower-amplitude signals like the ones induced by ice giants and super-Earths require
a noise floor at or below ∼1 m s−1 to keep the number of observations within a reasonable range. Moreover, the phase of each
signal must be sufficiently well covered, which requires a large
enough time span and appropriate sampling. As a consequence,
the planet detection limits in many of the 15 systems mentioned
C. Lovis et al.: The HARPS search for southern extra-solar planets
10 12
time (kyr)
Fig. 8. Evolution of the eccentricities of the planets b (red) and c (green)
during 20 kyr for three different models. In the top picture the initial
eccentricity of planet b is set at zero, but mutual gravitational perturbations increase its value to 0.4 in less than 2 kyr (Table 3). In the middle
figure we included general relativity, which calms down the eccentricity
variations of the innermost planet, but still did not prevent the eccentricity of planet d to reach high values. In the bottom figure we use a model
where the eccentricities of both planets were previously damped by tidal
dissipation (Table 6). This last solution is stable at least for 10 Myr.
time (Gyr)
Fig. 9. Tidal evolution of the amplitude of the proper modes u1 (red), u2
(green), u3 (blue), and u4 (pink) resulting from the tidal dissipation on
planet b with k2 /Q = 0.0015.
above do probably not reach down to the Neptune-mass range
yet, preventing us from having a sufficiently complete picture
of them. Nevertheless, considering the well-observed cases followed at the highest precision, the RV technique shows here its
ability to study the structure of planetary systems, from gas giants to telluric planets.
Fig. 7. Phased RV curves for all signals in the 7-Keplerian model. In
each case, the contribution of the other 6 signals has been subtracted.
C. Lovis et al.: The HARPS search for southern extra-solar planets
y (AU)
x (AU)
Fig. 11. Long-term evolution of the HD 10180 planetary system over
10 Myr starting with the orbital solution from Table 6. The panel shows
a face-on view of the system. x and y are spatial coordinates in a frame
centered on the star. Present orbital solutions are traced with solid lines
and each dot corresponds to the position of the planet every 100 kyr.
The semi-major axes (in AU) are almost constant, but the eccentricities
undergo significant variations (Table 7).
Fig. 12. Possible location of an additional eighth planet in the HD 10180
system. The stability of a small-mass particle in the system is analyzed,
for various semi-major axes and eccentricities, and for K = 0.78 m s−1 .
The stable zones where additional planets could be found are the “dark
blue” regions.
The dynamical architecture of planetary systems is likely to
convey extremely useful information on their origins. The dominant planet formation scenario presently includes several physical processes that occur on similar timescales in protoplanetary disks: formation of cores, preferentially beyond the ice line,
through accretion of rocky and icy material; runaway gas accretion on cores having reached a critical mass, rapidly forming giant planets; inward migration of cores (type I) and giant planets
Fig. 13. The 15 planetary systems with at least three known planets as
of May 2010. The numbers give the minimal distance between adjacent
planets expressed in mutual Hill radii. Planet sizes are proportional to
log (m sin i).
(type II) through angular momentum exchange with the gaseous
disk; disk evolution and dissipation within a few Myr; dynamical
interactions between protoplanets leading to eccentricity pumping, collisions or ejections from the system. It is extremely challenging to build models that include all these effects in a consistent manner, given the complicated physics involved and the
scarce observational constraints available on the early stages of
planet formation. In particular, attempts to simultaneously track
the formation, migration and mutual interactions of several protoplanets are still in their infancy. Observational results on the
global architecture of planetary systems may therefore provide
important clues to determine the relative impact of each process.
From the observational point of view, the least that can be
said is that planetary systems display a huge diversity in their
properties, which after all is not surprising given their complex
formation processes. Fig. 13 shows planet semi-major axes on a
logarithmic scale for the 15 systems known to harbour at least
three planets. Systems are shown in increasing order of their
mean planetary mass, while individual planet masses are illustrated by varying dot size. The full range of distances covered
by each planet between periastron and apastron is denoted by
a black line. The minimal distance between each neighbouring
pair of planets is also given, expressed in units of the mutual Hill
radius which is defined as:
a 1 + a 2 m1 + m2
RH,M =
3 M∗
As shown by Chambers et al. (1996), the instability timescale
of a coplanar, low-eccentricity multi-planet system is related to
the distance between planets, expressed in mutual Hill radii, in
a relatively simple manner, and approximate ’life expectancies’
of planetary systems can be estimated based on these separations, the number of planets, and their masses. The Chambers
et al. simulations do not extend to masses above ∼3 M⊕ or to
C. Lovis et al.: The HARPS search for southern extra-solar planets
timescales above 108 years, but a moderate extrapolation of their
results shows that for systems with 3–5 planets and masses between a few M⊕ and a few MJ , separations between adjacent
planets should be of at least 7–9 mutual Hill radii to ensure stability on a 10-Gyr timescale. These numbers should not be taken
too exactly since they were obtained assuming regularly-spaced,
equal-mass bodies. They are also not applicable to eccentric
orbits and dynamical configurations such as mean-motion resonances, where stability ’islands’ do exist at reduced spacings (e.g. GJ 876). However, the global picture emerging from
Fig. 13 shows that many known planetary systems are dynamically ’packed’, with little or no space left for additional planets (e.g. Barnes & Quinn 2004; Barnes & Raymond 2004). This
result was already noted by several authors, giving rise for example to the ’packed planetary system’ hypothesis (Barnes &
Raymond 2004). Here we show that this seems to be also true
for several low-mass systems, i.e. those which do not contain gas
giants (or only distant ones), as illustrated by HD 40307, GJ 581
and HD 10180. Indeed, several planets in these systems are separated from each other by typically less than 15 mutual Hill radii.
There are still a few ’empty’ places, however, and further observations will tell if smaller planets are still hiding between the
known ones.
8.2. Extrasolar Titius-Bode-like laws?
It is intriguing that many gas giant and low-mass systems seem
to share the property of being dynamically packed. An attractive explanation would be that at each moment of their history,
many planetary systems are ’saturated’ with planets and exhibit
dynamical configurations whose lifetime is of the same order of
magnitude as the age of the system. This would point to a major role for dynamical interactions in the shaping of planetary
systems, at least since the dissipation of the gaseous disk. The
observed packing may support the view that close-in low-mass
systems could be primarily the result of strong interactions (collisions and ejections) between several large protoplanets after
these were brought to the inner regions of the disk through type
I migration, i.e. after disk dissipation. These systems would then
naturally evolve towards planets separated from each other by a
roughly constant number of mutual Hill radii (e.g. Laskar 2000;
Raymond et al. 2009). Since Hill radii are proportional to the
semi-major axes, the orbital distances of successive planets with
similar masses will tend to obey an approximate exponential
law, much like the century-long debated and polemical TitiusBode law in the Solar System. Indeed, Hayes & Tremaine (1998)
have shown that any planetary system subject to some ’radiusexclusion’ law such as the Hill criterion is likely to have its
planets distributed according to a geometric progression. Laskar
(2000) presents a simplified model of planetary accretion focusing on the evolution of the angular momentum deficit (AMD) of
the averaged system. Starting from a given density of planetesimals ρ(a), the final state of the system, defined by the end of the
collision phase, can be derived analytically and the spacing between adjacent planets can be predicted for different functional
forms of ρ(a). Interestingly, an exponential law log an = c1 + c2 n
is obtained when the initial density ρ(a) goes as a−3/2 , while a
√ density ρ(a) yields a semi-major axis relation of the
form an = c1 + c2 n.
Looking more closely at Fig. 13, we see that exponential
laws may indeed exist in some planetary systems. However, a
meaningful test requires that all successive planets have been
discovered, especially low-mass ones. This is far from being the
case in the presently-known systems, and we therefore limit our-
Fig. 14. Fit of exponential laws to semi-major axes as a function of
planet number for the inner Solar System (black), HD 40307 (red), GJ
581 (blue), HD 69830 (green) and HD 10180 (magenta).
Table 8. Exponential fits to semi-major axis distributions.
Inner Solar System
HD 40307
GJ 581
HD 69830
HD 10180
Average mass
(M⊕ )
selves to those observed with the HARPS spectrograph, and to
planets within 1 AU, to minimize observational biases. We do
not want to speculate on ’missing’ planets introducing gaps in
the Titius-Bode-like relations, since almost anything can be fitted to the present, limited datasets if more than two free parameters are allowed. As can be seen by eye in Fig. 13, a somewhat
regular spacing between adjacent planets seems to exist in the
low-mass systems HD 40307, HD 69830 and HD 10180, but
less so in GJ 581. Among massive systems, 55 Cnc also shows
a somewhat regular spacing (Poveda & Lara 2008), but the presence of close-in gas giants in this system makes planet-planet
interactions much stronger and hints at a different formation history (e.g. type II instead of type I migration) compared to lowmass systems.
Concentrating on low-mass systems, Fig. 14 shows an exponential fit an = c1 cn2 to the observed semi-major axes as a function of planet number, starting at n = 1. A reasonable fit is obtained for HD 40307, HD 69830 and HD 10180, with a relative
standard deviation of the residuals of 0.57%, 10.2% and 12.0%,
respectively. The fit to the inner Solar System is also shown, with
a relative standard deviation of 8.0%. The fit to GJ 581 is less
convincing, with a dispersion of 21.0%. It is tempting to conjecture that there exists an additional body between the third and
fourth planets in this system, which would make the exponential
fit significantly better, and at the same time provide an exciting candidate for a habitable world. Table 8 gives the values of
the best-fit parameters c1 and c2 for each system, together with
C. Lovis et al.: The HARPS search for southern extra-solar planets
the average planetary masses. Interestingly, a positive correlation between c2 and mass may be present, possibly illustrating
the fact that more massive planetary systems tend to be more
widely spaced, as would be expected in the context of Hill stability. The c1 values show how ’special’ the inner Solar System
is, with the first planet (Mercury) very distant from the central
star compared to the other systems.
We emphasize that we do not consider these Titius-Bode-like
’laws’ as having any other meaning than a possible signature of
formation processes. As such, we would expect them to hold
only in certain types of planetary systems, e.g. close-in, lowmass, many-body configurations. The presently-known massive
systems, on the other hand, likely experienced a more chaotic
history. Moreover, not all low-mass systems satisfy such exponential relations (e.g. GJ 581) and the physics of planet formation is so diverse and complex that we do not expect any universal rule on planet ordering to exist.
8.3. Formation and evolution
These emerging patterns, if confirmed by further discoveries of
planetary systems, may provide clues on how the observed systems of close-in super-Earths and Neptunes were formed. These
systems appear to be quite common, but their formation history remains a puzzle. On the one hand, it seems unlikely that
they formed in situ given the very high inner disk densities
that would be required. However, little is known about statistical properties of protoplanetary disks and their density profiles, and this possibility can probably not be completely rejected
at this point. On the other hand, such systems may be the result of convergent type I migration of planetary cores formed
at or beyond the ice line (e.g. Terquem & Papaloizou 2007;
Kennedy & Kenyon 2008). But how can several protoplanets
grow to masses in the super-Earth/Neptune range while migrating together during the disk lifetime, and end up in a configuration which is not necessarily close to mean-motion resonances?
Near-commensurability of the orbits would be expected according to Terquem & Papaloizou (2007). Loss of commensurability
could occur through orbital decay due to stellar tides, but this
is probably efficient only for the planets closest to the star. So
this scenario still has difficulties in explaining a system such as
HD 10180.
Nevertheless, as a testable prediction of this type I migration
scenario, Kennedy & Kenyon (2008) suggest that the masses of
close-in planets will increase as stellar mass increases, and will
even reach the gas giant range for stars above ∼1 M⊙ . This is
due the combined effects of increased disk mass, higher isolation
mass, a more distant snow line and a mass-dependent migration
timescale to the inner regions that favors more massive planets
as stellar mass increases. Interestingly, we note that the average
masses of close-in planets do increase between HD 40307 (M
= 0.77 M⊙ , [Fe/H] = -0.31), HD 69830 (M = 0.82 M⊙ , [Fe/H]
= -0.06) and HD 10180 (M = 1.06 M⊙ , [Fe/H] = 0.08), hinting
at some correlation between stellar mass, metallicity and masses
of close-in planetary systems (see also Sect. 8.4 below). In any
case, there is great hope that these systems will allow for a much
better characterization of type I migration in the near future.
Other mechanisms have been proposed to produce closein low-mass planets, many of which involve the influence of
(migrating) gas giant(s) further out in the system (e.g. Fogg &
Nelson 2005; Zhou et al. 2005; Raymond et al. 2008). In this
context, it is interesting to note that the present RV data can
exclude the presence of Jupiter-mass objects within ∼10 AU in
the HD 40307, GJ 581, HD 69830 and HD 10180 systems. It is
therefore unlikely that gas giants played a major role in the shaping of these systems. Their absence may actually be the factor
that favored the formation and survival of many lower-mass objects.
8.4. Correlations with stellar mass and metallicity
Finally, we may also investigate the impact of stellar mass and
metallicity on planet formation by further considering the 15
systems with at least 3 known planets. Fig. 15 shows the total
planetary mass in these systems as a function of stellar mass
alone, stellar metallicity alone, and the total amount of heavy
elements in the star given by M star 10[Fe/H] . We note two obvious facts: 1) all very massive systems are found around massive
and metal-rich stars; 2) the 4 lowest-mass systems are found
around lower-mass and metal-poor stars. It thus appears that
both quantities independently impact the mass of formed planets. When both effects of stellar mass and metallicity are combined (right panel), we obtain an even stronger correlation between total planetary system mass and total metal content in
the star. The latter quantity can be seen as a proxy for the total
amount of heavy elements that was present in the protoplanetary
disk. These findings confirm previous trends already observed
for the whole sample of planet-host stars, and are well explained
by formation scenarios based on the core-accretion model.
9. Conclusion
In this paper we have presented a new, very rich planetary system
with planets ranging from Saturn-like to Earth-like, and comprising 5 Neptune-like objects. Long-term radial velocity monitoring at 1 m s−1 precision was necessary to detect the low RV
amplitudes of these planets. The dynamical architecture of this
system reveals a compact configuration, with planets roughly
equally spaced on a logarithmic scale, and with significant secular interactions. The presence of an Earth-mass body at 0.02
AU has important implications for the dynamics of the system
and highlights the role of tidal dissipation to guarantee stability. Future measurements will allow us to confirm the existence
of this planet. The HD 10180 system shows the ability of the
RV technique to study complex multi-planet systems around
nearby solar-type stars, with detection limits reaching rocky/icy
objects within habitable zones. Future instruments like VLTESPRESSO will build on the successful HARPS experience and
carry out a complete census of these low-mass systems in the solar neighborhood, pushing towards planets of a few Earth masses
at 1 AU.
With the advent of the space observatories CoRoT and
Kepler, low-mass planets have also become accessible to transit
searches. According to early announcements, the Kepler mission
will soon confirm what radial velocity surveys are already starting to find: rocky/icy planets are very common in the Universe.
The combination of both techniques is likely to bring rapid
progress in our understanding of the formation and composition
of this population.
The HD 10180 system represents an interesting example of
the various outcomes of planet formation. No massive gas giant
was formed, but instead a large number of still relatively massive
objects survived, and migrated to the inner regions. Building a
significant sample of such low-mass systems will show what are
the relative influences of the different physical processes at play
during planet formation and evolution.
C. Lovis et al.: The HARPS search for southern extra-solar planets
Fig. 15. Total planetary system mass as a function of stellar mass (left), stellar metallicity (middle) and the overall metal content of the star (right).
Acknowledgements. We are grateful to all technical and scientific collaborators of the HARPS Consortium, ESO Headquarters and ESO La Silla who
have contributed with their extraordinary passion and valuable work to the
success of the HARPS project. We would like to thank the Swiss National
Science Foundation for its continuous support. We acknowledge support from
French PNP and GENCI-CINES supercomputer facilities. NCS would like to
thank the support by the European Research Council/European Community under the FP7 through a Starting Grant, as well from Fundação para a Ciência
e a Tecnologia (FCT), Portugal, through a Ciência 2007 contract funded by
FCT/MCTES (Portugal) and POPH/FSE (EC), and in the form of grants reference PTDC/CTE-AST/098528/2008 and PTDC/CTE-AST/098604/2008.
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