Search

International audience; Context. Transiting ultra-hot Jupiters are ideal candidates for studying the exoplanet atmospheres and their dynamics, particularly by means of high-resolution spectra with high signal-to-noise ratios. One such object is KELT-20b. It orbits the fast-rotating A2-type star KELT-20. Many atomic species have been found in its atmosphere, with blueshifted signals that indicate a day- to night-side wind. Aims. We observe the atmospheric Rossiter-McLaughlin effect in the ultra-hot Jupiter KELT-20b and study any variation of the atmospheric signal during the transit. For this purpose, we analysed five nights of HARPS-N spectra covering five transits of KELT-20b. Methods. We computed the mean line profiles of the spectra with a least-squares deconvolution using a stellar mask obtained from the Vienna Atomic Line Database ( T eff = 10 000 K, log g = 4.3), and then we extracted the stellar radial velocities by fitting them with a rotational broadening profile in order to obtain the radial velocity time-series. We used the mean line profile residuals tomography to analyse the planetary atmospheric signal and its variations. We also used the cross-correlation method to study a previously reported double-peak feature in the FeI planetary signal. Results. We observed both the classical and the atmospheric Rossiter-McLaughlin effect in the radial velocity time-series. The latter gave us an estimate of the radius of the planetary atmosphere that correlates with the stellar mask used in our work ( R p+atmo ∕ R p = 1.13 ± 0.02). We isolated the planetary atmospheric trace in the tomography, and we found radial velocity variations of the planetary atmospheric signal during transit with an overall blueshift of ≈10 km s −1 , along with small variations in the signal depth, and less significant, in the full width at half maximum (FWHM). We also find a possible variation in the structure and position of the FeI signal in different transits. Conclusions. We confirm the previously detected blueshift of the atmospheric signal during the transit. The FWHM variations of the atmospheric signal, if confirmed, may be caused by more turbulent condition at the beginning of the transit, by a variable contribution of the elements present in the stellar mask to the overall planetary atmospheric signal, or by iron condensation. The FeI signal show indications of variability from one transit to the next.

International audience; Context. Radial-velocity (RV) signals arising from stellar photospheric phenomena are the main limitation for precise RV measurements. Those signals induce RV variations an order of magnitude larger than the signal created by the orbit of Earth-twins, thus preventing their detection. Aims: Different methods have been developed to mitigate the impact of stellar RV signals. The goal of this paper is to compare the efficiency of these different methods to recover extremely low-mass planets despite stellar RV signals. However, because observed RV variations at the meter-per-second precision level or below is a combination of signals induced by unresolved orbiting planets, by the star, and by the instrument, performing such a comparison using real data is extremely challenging. Methods: To circumvent this problem, we generated simulated RV measurements including realistic stellar and planetary signals. Different teams analyzed blindly those simulated RV measurements, using their own method to recover planetary signals despite stellar RV signals. By comparing the results obtained by the different teams with the planetary and stellar parameters used to generate the simulated RVs, it is therefore possible to compare the efficiency of these different methods. Results: The most efficient methods to recover planetary signals take into account the different activity indicators, use red-noise models to account for stellar RV signals and a Bayesian framework to provide model comparison in a robust statistical approach. Using the most efficient methodology, planets can be found down to K/N= Kpl/RV_{rms×&surd;{Nobs}=5} with a threshold of K/N = 7.5 at the level of 80-90% recovery rate found for a number of methods. These recovery rates drop dramatically for K/N smaller than this threshold. In addition, for the best teams, no false positives with K/N > 7.5 were detected, while a non-negligible fraction of them appear for smaller K/N. A limit of K/N = 7.5 seems therefore a safe threshold to attest the veracity of planetary signals for RV measurements with similar properties to those of the different RV fitting challenge systems.

We analysed the public data of the CoRoT Long Run LRc10 looking for planetary candidates. In a first step we removed outliers and trends caused by stellar activity and instrumental problems. Then we applied the Box-fitting Least Squares (BLS) algorithm to detect periodic decreases in luminosity. From all the significative transit detections, we selected the ones that passed different checks. For the 7 planetary candidates found, we applied a new ``ad hoc'' normalization and fitted the orbital parameters, to verify their reliability. Using the stellar temperature information, we could estimate the dimensions that the candidates should have if confirmed. Our research method demonstrated to be sensitive to candidates with hypothetic dimension up to ∼3.5 REarth on stars of mag V≃14.

[abridged] We analyse four transits of WASP-33b observed with the optical high-resolution HARPS-N spectrograph to confirm its nodal precession, study its atmosphere and investigate the presence of star-planet interactions.We extract the mean line profiles of the spectra by using the LSD method, and analyse the Doppler shadow and the RVs. We also derive the transmission spectrum of the planet, correcting it for the stellar contamination due to rotation, CLV and pulsations. We confirm the previously discovered nodal precession of WASP-33b, almost doubling the time coverage of the inclination and projected spin-orbit angle variation. We find that the projected obliquity reached a minimum in 2011 and use this constraint to derive the geometry of the system, in particular its obliquity at that epoch ($\epsilon=113.99^{\circ}\pm 0.22^{\circ}$) and the inclination of the stellar spin axis ($i_{\rm s}=90.11^{\circ}\pm 0.12^{\circ}$), as well as the gravitational quadrupole moment of the star $J_2=(6.73\pm 0.22)\times 10^{-5}$. We present detections of H$\alpha$ and H$\beta$ absorption in the atmosphere of the planet with a contrast almost twice smaller than previously detected in the literature. We also find evidence for the presence of a pre-transit signal, which repeats in all four analysed transits. The most likely explanation lies in a possible excitation of a stellar pulsation mode by the presence of the planetary companion. Future common analysis of all available datasets in the literature will help shedding light on the possibility that the observed Balmer lines transit depth variations are related to stellar activity and/or pulsation, and to set constraints on the energetics possibly driving atmospheric escape. A complete orbital phase coverage of WASP-33b with high-resolution spectroscopic (spectro-polarimetric) observations could help understanding the nature of the pre-transit signal., Comment: Accepted for publication in A&A

La scoperta di pianeti che orbitano intorno ad altre stelle è uno degli eventi più importanti nell'astrofisica galattica degli ultimi due decenni. Dalla scoperta del primo esopianeta nel 1995, il numero di esopianeti scoperti è cresciuto sempre più in fretta e attualmente conosciamo più di 4,000 pianeti, molto diversi per dimensioni e distanza dalla stella ospite e anche in fattori come temperatura, massa, densità. La diversità degli esopianeti è un fattore chiave per comprendere la formazione dei sistemi planetari e in particolare la formazione del Sistema Solare e del nostro pianeta, la Terra. Questo è il motivo per cui la scienza osservativa degli esopianeti si sta concentrando su due diversi obiettivi: i) la caratterizzazione degli esopianeti, nel tentativo di determinare il raggio, la massa, la densità e la composizione degli oggetti osservati e ii) la caratterizzazione delle loro atmosfere, stabilendo gli elementi che l'atmosfera di un pianeta può supportare e i meccanismi che guidano i processi atmosferici. Caratterizzazione degli Esopianeti La fotometria, con il metodo dei transiti si è rivelato senza dubbio il metodo di scoperta di esopianeti con il maggior successo. La forza di questo metodo è il numero di parametri che possono essere ottenuti osservando il transito dei pianeti, soprattutto in combinazione con le osservazioni di velocità radiale (VR). In questo contesto, uno dei gruppi più prolifici è il Consorzio GTO di HARPS-N, che sfrutta l'alta risoluzione e l'estrema stabilità dello spettrografo HARPS-N, installato al Telescopio Nazionale Galileo, per caratterizzare e scoprire esopianeti combinando il metodo dei transiti e quello delle velocità radiali. Come collaboratore di questo gruppo, ho studiato un pianeta candidato scoperto dalla Campagna 16 della missione K2, HD 80653 b, una super-Terra che transita davanti alla sua stella con un periodo orbitale molto breve, e ho usato le VR HARPS-N per caratterizzarlo, ottenendo la sua massa e definendone la densità. Il pianeta appartiene ad una particolare classe di esopianeti: i pianeti a periodo ultra-corto, oggetti che orbitano intorno alle loro stelle con periodi estremamente brevi, più piccoli di due raggi terrestri e con composizioni simili a quella terrestre. Caratterizzazione di Atmosfere I gioviani ultra-caldi sono laboratori eccellenti per lo studio delle atmosfere esoplanetarie. Il sodio, per la sua grande sezione d'urto e per il fatto che le sue righe spettrali principali si trovano nel range spettrale della maggior parte degli spettrografi, è l'elemento più studiato, ma lo studio di nuove righe spettrali è cominciato. Righe del ferro, del titanio, del magnesio, ma anche tracce di cromo, scandio e ittrio sono state trovate negli spettri di trasmissione ad alta risoluzione dei pianeti più caldi. I due gioviani ultra-caldi KELT-9 b e KELT-20 b sono stati osservati dal programma atmosfere del gruppo italiano Global architecture of Planetary Systems (GAPS). Come membro del gruppo ho potuto esplorare più in dettaglio il metodo della spettroscopia in transito, creando due diverse routine per la caratterizzazione delle atmosfere. Il primo metodo segue approcci già utilizzati in precedenza, ma è in grado di rilevare righe spettrali deboli come quelle del magnesio, sommandole nello spazio delle velocità. Usando questo approccio ho analizzato gli spettri ad alta risoluzione di KELT-9 b e KELT-20 b e ho ottenuto i loro spettri di trasmissione, rilevando un assorbimento significativo per Na, H, Fe e Mg I. Il secondo metodo estrae gli spettri di trasmissione ad alta risoluzione e li cross-correla con modelli teorici di spettri di trasmissione. Analizzando gli spettri di KELT-20 b e utilizzando la cross-correlazione ho potuto confermare la presenza di Fe I, Fe II e Na I, trovate da analisi precedenti di altri gruppi di ricerca. The discovery of planets orbiting around stars other than the Sun is by far the most relevant event in the galactic astrophysics of the last two decades. Since the discovery of the first exoplanet in 1995, the number of exoplanets discovered grew fast and we currently know more than 4,000 exoplanets, very diverse in dimension and distance from parent stars and also in factors as temperature, mass, density. The diversity of exoplanets is a key factor to understand more about the formation of planetary systems and in particular the formation of the Solar System and our planet, the Earth. This is the reason why observational exoplanetary science is currently focusing on two different fields: i) the characterization of exoplanets, trying to determine the radius, the mass, the density and the bulk composition of the objects observed, and ii) the characterization of their atmospheres, establishing the elements that the atmosphere of a planet supports and the mechanisms that drive the atmospheric processes. Characterization of Exoplanets Photometry with the transit method has arguably been the most successful exoplanet discovery method to date. The method’s strength is the rich set of parameters that can be obtained from transiting planets, in particular in combination with RV observations. In this framework, one of the most prolific groups is the HARPS-N Guaranteed Time Observations (GTO) Consortium, that makes use of the high resolution (R = 115,000) and extreme stability of the HARPS-N spectrograph, installed on the Telescopio Nazionale Galileo (TNG), to characterize and discover exoplanets by combining transits and RV methods. As a collaborator of this group, I studied a candidate planet discovered by K2 Campaign 16, HD 80653 b, a super-Earth planet transiting the star on a short period orbit, and used HARPS-N RV data to characterize it, finding its mass and defining its bulk density. It belongs to a peculiar class of exoplanets: the Ultra-Short Period (USP) planets, objects that orbit their stars with extremely short periods, smaller than about 2 Earth Radii and compositions similar to that of the Earth. Characterization of Atmospheres Ultra-hot Jupiters are excellent laboratories for the study of exoplanetary atmospheres. Sodium, due to its large cross-section and to the fact it is in the wavelength range of most optical spectrographs, is the most studied element, but new interesting features begin to be analyzed. Lines of iron, titanium, magnesium, but also chromium, scandium and yttrium have been found in the high resolution transmission spectra of the hottest planets. The two ultra-hot Jupiters KELT-9 b and KELT-20 b were observed in the framework of the Global architecture of Planetary Systems (GAPS) Atmosphere program. I explored more in detail the transit spectroscopy method, creating two different routines for atmosphere characterization. The first routine follows previous approaches for high-resolution spectroscopy, but is able to detect weak spectral lines such as those of magnesium, by co-adding the lines in the velocities space. Using this procedure, I analyzed the high-resolution spectra of KELT-9 b and KELT-20 b, obtaining their transmission spectra and detecting significant absorption for Na, H, Fe and Mg I. The second routine extracts the high-resolution transmission spectra of exoplanets and cross-correlates them with theoretical transmission spectra models. I analyzed the high-resolution spectra of KELT-20 b and with the cross-correlation technique I confirmed previous detections of Fe I, Fe II, and Na I.

International audience; Context. The analysis of exoplanetary atmospheres by means of high-resolution spectroscopy is an expanding research field which provides information on the chemical composition, thermal structure, atmospheric dynamics, and orbital velocity of exoplanets. Aims: In this work, we aim to detect the light reflected by the exoplanet 51 Peg b by employing optical high-resolution spectroscopy. Methods: To detect the light reflected by the planetary dayside, we used optical High Accuracy Radial velocity Planet Searcher and High Accuracy Radial velocity Planet Searcher for the Northern hemisphere spectra taken near the superior conjunction of the planet, when the flux contrast between the planet and the star is maximum. To search for the weak planetary signal, we cross-correlated the observed spectra with a high signal-to-noise ratio stellar spectrum. Results: We homogeneously analyze the available datasets and derive a 10-5 upper limit on the planet-to-star flux contrast in the optical. Conclusions: The upper limit on the planet-to-star flux contrast of 10-5 translates into a low albedo of the planetary atmosphere (Ag ≲ 0.05-0.15 for an assumed planetary radius in the range of 1.5-0.9 RJup, as estimated from the planet's mass). Full Table 2 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/646/A159 Based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated by the Fundación Galileo Galilei (FGG) of the Istituto Nazionale di Astrofisica (INAF) at the Observatorio del Roque de los Muchachos (La Palma, Canary Islands, Spain).

Context. Young stars and multi-planet systems are two types of primary objects that allow us to study, understand, and constrain planetary formation and evolution theories. Aims. We validate the physical nature of two Neptune-sized planets transiting TOI-942 (TYC 5909-319-1), a previously unacknowledged young star (50−20+30 Myr) observed by the TESS space mission in Sector 5. Methods. Thanks to a comprehensive stellar characterization, TESS light curve modeling and precise radial-velocity measurements, we validated the planetary nature of the TESS candidate and detected an additional transiting planet in the system on a larger orbit. Results. From photometric and spectroscopic observations we performed an exhaustive stellar characterization and derived the main stellar parameters. TOI-942 is a relatively active K2.5V star (log R′HK = −4.17 ± 0.01) with rotation period Prot = 3.39 ± 0.01 days, a projected rotation velocity v sin i⋆ = 13.8 ± 0.5 km s−1, and a radius of ~0.9 R⊙. We found that the inner planet, TOI-942 b, has an orbital period Pb = 4.3263 ± 0.0011 days, a radius Rb = 4.242−0.313+0.376 R⊕, and a mass upper limit of 16 M⊕ at 1σ confidence level. The outer planet, TOI-942 c, has an orbital period Pc = 10.1605−0.0053+0.0056 days, a radius Rc = 4.793−0.351+0.410 R⊕, and a mass upper limit of 37 M⊕ at 1σ confidence level.

Ultra-short period (USP) planets are a class of exoplanets with periods shorter than one day. The origin of this sub-population of planets is still unclear, with different formation scenarios highly dependent on the composition of the USP planets. A better understanding of this class of exoplanets will, therefore, require an increase in the sample of such planets that have accurate and precise masses and radii, which also includes estimates of the level of irradiation and information about possible companions. Here we report a detailed characterization of a USP planet around the solar-type star HD 80653 $\equiv$ EP 251279430 using the K2 light curve and 108 precise radial velocities obtained with the HARPS-N spectrograph, installed on the Telescopio Nazionale Galileo. From the K2 C16 data, we found one super-Earth planet ($R_{b}=1.613\pm0.071 R_{\oplus}$) transiting the star on a short-period orbit ($P_{\rm b}=0.719573\pm0.000021$ d). From our radial velocity measurements, we constrained the mass of HD 80653 b to $M_{b}=5.60\pm0.43 M_{\oplus}$. We also detected a clear long-term trend in the radial velocity data. We derived the fundamental stellar parameters and determined a radius of $R_{\star}=1.22\pm0.01 R_{\odot}$ and mass of $M_{\star}=1.18\pm0.04 M_{\odot}$, suggesting that HD 80653, has an age of $2.7\pm1.2$ Gyr. The bulk density ($\rho_{b} = 7.4 \pm 1.1$ g cm$^{-3}$) of the planet is consistent with an Earth-like composition of rock and iron with no thick atmosphere. Our analysis of the K2 photometry also suggests hints of a shallow secondary eclipse with a depth of 8.1$\pm$3.7 ppm. Flux variations along the orbital phase are consistent with zero. The most important contribution might come from the day-side thermal emission from the surface of the planet at $T\sim3480$ K., Comment: 15 pages, 12 figures; accepted by A&A

Context. The existence of hot Jupiters is still not well understood. Two main channels are thought to be responsible for their current location: a smooth planet migration through the protoplanetary disk or the circularization of an initial highly eccentric orbit by tidal dissipation leading to a strong decrease in the semimajor axis. Different formation scenarios result in different observable effects, such as orbital parameters (obliquity and eccentricity) or frequency of planets at different stellar ages. Aims. In the context of the GAPS Young Objects project, we are carrying out a radial velocity survey with the aim of searching and characterizing young hot-Jupiter planets. Our purpose is to put constraints on evolutionary models and establish statistical properties, such as the frequency of these planets from a homogeneous sample. Methods. Since young stars are in general magnetically very active, we performed multi-band (visible and near-infrared) spectroscopy with simultaneous GIANO-B + HARPS-N (GIARPS) observing mode at TNG. This helps in dealing with stellar activity and distinguishing the nature of radial velocity variations: stellar activity will introduce a wavelength-dependent radial velocity amplitude, whereas a Keplerian signal is achromatic. As a pilot study, we present here the cases of two known hot Jupiters orbiting young stars: HD 285507 b and AD Leo b. Results. Our analysis of simultaneous high-precision GIARPS spectroscopic data confirms the Keplerian nature of the variation in the HD 285507 radial velocities and refines the orbital parameters of the hot Jupiter, obtaining an eccentricity consistent with a circular orbit. Instead, our analysis does not confirm the signal previously attributed to a planet orbiting AD Leo. This demonstrates the power of the multi-band spectroscopic technique when observing active stars., With funding from the Spanish government through the "María de Maeztu Unit of Excellence" accreditation (MDM-2017-0737)

Exoplanets orbiting very close to their host star are strongly irradiated. This can lead the upper atmospheric layers to expand and evaporate into space. The metastable helium (HeI) triplet at 1083.3nm has recently been shown to be a powerful diagnostic to probe extended and escaping exoplanetary atmosphere. We perform high-resolution transmission spectroscopy of the transiting hot Jupiter HD189733b with the GIARPS (GIANO-B + HARPS-N) observing mode of the Telescopio Nazionale Galileo, taking advantage of the simultaneous optical+near infrared spectral coverage to detect HeI in the planet's extended atmosphere and to gauge the impact of stellar magnetic activity on the planetary absorption signal. Observations were performed during five transit events of HD189733b. By comparison of the in- and out-of-transit GIANO-B observations we compute high-resolution transmission spectra, on which we perform equivalent width measurements and light-curves analyses to gauge the excess in-transit absorption in the HeI triplet. We detect an absorption signal during all five transits. The mean in-transit absorption depth amounts to 0.75+/-0.03%. We detect night-to-night variations in the HeI absorption signal likely due to the transit events occurring in presence of stellar surface inhomogeneities. We evaluate the impact of stellar-activity pseudo-signals on the true planetary absorption using a comparative analysis of the HeI and the H$\alpha$ lines. We interpret the time-series of the HeI absorption lines in the three nights not affected by stellar contamination -exhibiting a mean in-transit absorption depth of 0.77+/-0.04%- using a 3-d atmospheric code. Our simulations suggest that the helium layers only fill part of the Roche lobe. Observations can be explained with a thermosphere heated to $\sim$12000 K, expanding up to $\sim$1.2 planetary radii, and losing $\sim$1 g/s of metastable helium., Comment: 17 pages, 17 figures, accepted for publication in A&A

We present the first detection of atomic emission lines from the atmosphere of an exoplanet. We detect neutral iron lines from the day-side of KELT-9b (Teq $\sim$ 4, 000 K). We combined thousands of spectrally resolved lines observed during one night with the HARPS-N spectrograph (R $\sim$ 115, 000), mounted at the Telescopio Nazionale Galileo. We introduce a novel statistical approach to extract the planetary parameters from the binary mask cross-correlation analysis. We also adapt the concept of contribution function to the context of high spectral resolution observations, to identify the location in the planetary atmosphere where the detected emission originates. The average planetary line profile intersected by a stellar G2 binary mask was found in emission with a contrast of 84 $\pm$ 14 ppm relative to the planetary plus stellar continuum (40 $\pm$ 5$\%$ relative to the planetary continuum only). This result unambiguously indicates the presence of an atmospheric thermal inversion. Finally, assuming a modelled temperature profile previously published (Lothringer et al. 2018), we show that an iron abundance consistent with a few times the stellar value explains the data well. In this scenario, the iron emission originates at the $10^{-3}$-$10^{-5}$ bar level., Comment: Accepted for publication on ApJL; 19 pages, 4 figures, 3 tables

In the framework of the Global Architecture of Planetary Systems (GAPS) project we collected more than 300 spectra with HARPS-N at the TNG for the bright G9V star HD164922. This target is known to host one gas giant planet in a wide orbit (Pb~1200 days, semi-major axis ~2 au) and a Neptune-mass planet with a period Pc ~76 days. We searched for additional low-mass companions in the inner region of the system. We compared the radial velocities (RV) and the activity indices derived from the HARPS-N time series to measure the rotation period of the star and used a Gaussian process regression to describe the behaviour of the stellar activity. We exploited this information in a combined model of planetary and stellar activity signals in an RV time-series composed of almost 700 high-precision RVs, both from HARPS-N and literature data. We performed a dynamical analysis to evaluate the stability of the system and the allowed regions for additional potential companions. Thanks to the high sensitivity of the HARPS-N dataset, we detect an additional inner super-Earth with an RV semi-amplitude of 1.3+/-0.2 m/s, a minimum mass of ~4+/-1 M_E and a period of 12.458+/-0.003 days. We disentangle the planetary signal from activity and measure a stellar rotation period of ~42 days. The dynamical analysis shows the long term stability of the orbits of the three-planet system and allows us to identify the permitted regions for additional planets in the semi-major axis ranges 0.18-0.21 au and 0.6-1.4 au. The latter partially includes the habitable zone of the system. We did not detect any planet in these regions, down to minimum detectable masses of 5 and 18 M_E, respectively. A larger region of allowed planets is expected beyond the orbit of planet b, where our sampling rules-out bodies with minimum mass > 50 M_E., 23 pages, 16 Figures. Accepted for publication on A&A

Detecting and characterising exoworlds around very young stars (age$, Comment: Accepted for publication on Astronomy and Astrophysics. Abstract slightly modified to fulfill arxiv requirements

Radial-velocity (RV) signals induce RV variations an order of magnitude larger than the signal created by the orbit of Earth-twins, thus preventing their detection. The goal of this paper is to compare the efficiency of the different methods used to deal with stellar signals to recover extremely low-mass planets despite. However, because observed RV variations at the m/s precision level or below is a combination of signals induced by unresolved orbiting planets, by the star, and by the instrument, performing such a comparison using real data is extremely challenging. To circumvent this problem, we generated simulated RV measurements including realistic stellar and planetary signals. Different teams analyzed blindly those simulated RV measurements, using their own method to recover planetary signals despite stellar RV signals. By comparing the results obtained by the different teams with the planetary and stellar parameters used to generate the simulated RVs, it is therefore possible to compare the efficiency of these different methods. The most efficient methods to recover planetary signals {take into account the different activity indicators,} use red-noise models to account for stellar RV signals and a Bayesian framework to provide model comparison in a robust statistical approach. Using the most efficient methodology, planets can be found down to K/N= K_pl/RV_rms*sqrt{N_obs}=5 with a threshold of K/N=7.5 at the level of 80-90% recovery rate found for a number of methods. These recovery rates drop dramatically for K/N smaller than this threshold. In addition, for the best teams, no false positives with K/N > 7.5 were detected, while a non-negligible fraction of them appear for smaller K/N. A limit of K/N = 7.5 seems therefore a safe threshold to attest the veracity of planetary signals for RV measurements with similar properties to those of the different RV fitting challenge systems., 36 pages (including 10 pages of appendix), 23 figures, Accepted in A&A