Title: Dynamical stability of the Gliese 581 exoplanetary system Author: Zsuzsanna Tóth, Imre Nagy

Using numerical methods we investigate the dynamical stability of the Gliese 581 exoplanetary system. The system is known to harbour four planets (b-e). The existence of another planet (g) in the liquid water habitable zone of the star is debated after the latest analyses of the radial velocity (RV) measurements. We integrated the 4 and 5-planet model of Vogt et al. (AN 333, 561-575, 2012) with initial circular orbits. To characterize stability, the maximum eccentricity was used that the planets reached over the time of the integrations and the LCI and RLI to identify chaotic motion. Since circular orbits in the RV fits seem to be a too strong restriction and the true orbits might be elliptic, we investigated the stability of the planets as a function of their eccentricity. The integration of the circular 4-planet model shows that it is stable on a longer timescale for even an inclination i = 5°. A fifth planetary body in the 4-planet model could have a stable orbit between the two super-Earth sized planets c and d, and beyond the orbit of planet d, although another planet would likely only be stable on circular or near-circular orbit in the habitable zone of the star. Gliese 581 g in the 5-planet model would have a dynamically stable orbit, even for a wider range of orbital parameters, but its stability is strongly dependent on the eccentricity of planet d. The low-mass planet e, which quickly became unstable in eccentric models, remains stable in the circular 4-planet model, but the stable region around its initial semi-major axis and eccentricity is rather small. The stability of the inner planets e and c is dependent on the eccentricity of the Neptune-size planet b. The outermost planet d is far away from the adjacent planet c to considerably influence its stability, however, the existence of a planet between the two super-Earth planets c and d constrains its eccentricity.

Title: Dynamical stability of the Gliese 581 exoplanetary system Authors: Zsuzsanna Tóth, Imre Nagy

Using numerical methods we investigate the dynamical stability of the Gliese 581 exoplanetary system. The system is known to harbour four certain planets (b-e). The existence of another planet (g) in the liquid water habitable zone of the star is supported by the latest analysis of the radial velocity (RV) measurements. Vogt et al. (AN 333, 561-575, 2012) announced a 4- and a 5-planet model fitted to the RV curve with forced circular orbits. To characterise stability, we used the maximum eccentricity the planets reached over the time of the integrations and the Lyapunov Characteristic Indicator to identify chaotic motion. The integration of the 4-planet model shows that it is stable even for i = 5°, i. e. high planetary masses, on a longer timescale. The innermost low-mass planet e, which quickly became unstable in earlier eccentric models, remained stable, although the stable region around its initial semi-major axis and eccentricity is rather small. In the 4-planet model, we looked for stable regions for a fifth planetary body. We found extensive stable regions between the two super-Earth sized planets c and d, and beyond planet d. The Titius-Bode law and its revised version, Ragnarsson's formula applied to the Gliese 581 planetary system both predict distances of additional planets in these stable regions. The planet Gliese 581 g would have a dynamically stable orbit, even for a wider range of orbital parameters. Since circular orbits in the models seem to be a too strong restriction and the true orbits might be elliptic, we investigated the stability of the planets as a function of their eccentricity, and derived dynamical constraints for the ellipticity of the orbits.

Title: A DEBRIS Disk Around The Planet Hosting M-star GJ581 Spatially Resolved with Herschel Authors: J.-F. Lestrade, B. C. Matthews, B. Sibthorpe, G. M. Kennedy, M. C. Wyatt, G. Bryden, J. S. Greaves, E. Thilliez, Amaya Moro-Martin, M. Booth, W. R. F. Dent, G. Duchene, P. M. Harvey, J. Horner, P. Kalas, J. J. Kavelaars, N. M. Phillips, D. R. Rodriguez, K. Y. L. Su, D. J. Wilner

Debris disks have been found primarily around intermediate and solar mass stars (spectral types A-K) but rarely around low mass M-type stars. We have spatially resolved a debris disk around the remarkable M3-type star GJ581 hosting multiple planets using deep PACS images at 70, 100 and 160 microns as part of the DEBRIS Program on the Herschel Space Observatory. This is the second spatially resolved debris disk found around an M-type star, after the one surrounding the young star AU Mic (12 Myr). However, GJ 581 is much older (2-8 Gyr), and is X-ray quiet in the ROSAT data. We fit an axisymmetric model of the disk to the three PACS images and found that the best fit model is for a disk extending radially from 25±12 AU to more than 60 AU. Such a cold disk is reminiscent of the Kuiper Belt but it surrounds a low mass star (0.3 solar masses) and its fractional dust luminosity L_dust/L_* of ~ 10^-4 is much higher. The inclination limits of the disk found in our analysis make the masses of the planets small enough to ensure the long-term stability of the system according to some dynamical simulations. The disk is collisionally dominated down to submicron-sized grains and the dust cannot be expelled from the system by radiation or wind pressures because of the low luminosity and low X-ray luminosity of GJ581. We suggest that the correlation between low-mass planets and debris disks recently found for G-type stars also applies to M-type stars. Finally, the known planets, of low masses and orbiting within 0.3 AU from the star, cannot dynamically perturb the disk over the age of the star, suggesting that an additional planet exists at larger distance that is stirring the disk to replenish the dust.

Title: A search for transits of GJ 581 e and characterisation of the host star variability using MOST space telescope photometry Authors: Diana Dragomir, Jaymie M. Matthews, Rainer Kuschnig, Jason F. Rowe, Brett J. Gladman, David B. Guenther, Anthony F. J. Moffat, Slavek M. Rucinski, Dimitar Sasselov, Werner W. Weiss

The GJ 581 system has been amply studied since its discovery in 2005: the number of known planets in the system has increased and their orbital parameters are among the most precisely determined for radial velocity detected exoplanets. We have acquired MOST space-based photometry during 2007 and 2009, with the aims of measuring the stellar variability and searching for transits of GJ 581e, respectively. We quantify our sensitivity to shallow transit signals using Monte Carlo simulations, and perform a transit search within the 3 sigma transit windows corresponding to both the circular and Keplerian orbit ephemerides. Our analysis rules out transits for a planet with an orbital period of 3.15 days (GJ 581 e) having a radius larger than 1.62 solar radii (or a density lower than 2.39 g cm^{-3} for an orbital inclination of 90° to 2 sigma confidence. Thus, if the planet transits, we can exclude hydrogen, helium and water theoretical model compositions. The MOST photometry also allows us to rule out transits of GJ 581b within the Keplerian orbit-derived transit window for impact parameter values smaller than ~0.4 and confirm previous results which exclude transits for this planet within the circular orbit-derived transit window, for all plausible interior compositions. We find that the stellar brightness of GJ 581 is stable to within 1%, a characteristic which is favourable to the development of life in the habitable zone of the system. In the 2009 photometry, we detect a stellar signal with a period of 5.586 ±0.051 days, which is close to the orbital period of GJ 581b (P=5.37 days). However, further monitoring of the system is necessary to verify the nature of this variation.

Title: Counting the number of planets around GJ 581. False positive rate of Bayesian signal detection methods Authors: Mikko Tuomi, James S. Jenkins

The four-planet system around GJ 581 has received attention because it has been claimed that there are possibly two additional low-mass companions as well - one of them being a planet in the middle of the stellar habitable zone. We re-analyse the available HARPS and HIRES Doppler data in an attempt to determine the false positive rate of our Bayesian data analysis techniques and to count the number of Keplerian signals in the GJ 581 data. We apply the common Lomb-Scargle periodograms and posterior sampling techniques in the Bayesian framework to estimate the number of signals in the radial velocities. We also analyse the HARPS velocities sequentially after each full observing period to compare the sensitivities and false positive rates of the two signal detection techniques. By relaxing the assumption that the radial velocity noise is white, we also demonstrate the consequences that noise correlations have on the obtained results and the significances of the signals. According to our analyses, the number of Keplerian signals favoured by the publicly available HARPS and HIRES radial velocity data of GJ 581 is four. This result relies on the sensitivity of the Bayesian statistical analysis techniques but also depends on the assumed noise model. We also show that the radial velocity noise is actually not white and that this feature has to be accounted for when analysing radial velocities in a search for low-amplitude signals corresponding to low-mass planets. ...

Title: The impact of red noise in radial velocity planet searches: Only three planets orbiting GJ581? Authors: Roman V. Baluev

We perform a detailed analysis of the latest HARPS and Keck radial velocity data for the planet-hosting red dwarf GJ581, which attracted a lot of attention in recent time. We show that these data contain important correlated noise component ("red noise") with the correlation timescale of the order of 10 days. This red noise imposes a lot of misleading effects while we work in the traditional white-noise model. To eliminate these misleading effects, we propose a maximum-likelihood algorithm equipped by an extended model of the noise structure. We treat the red noise as a Gaussian random process with exponentially decaying correlation function. Using this method we prove that: (i) planets b and c do exist in this system, since they can be independently detected in the HARPS and Keck data, and regardless of the assumed noise models; (ii) planet e can also be confirmed independently by the both datasets, although to reveal it in the Keck data it is mandatory to take the red noise into account; (iii) the recently announced putative planets f and g are likely just illusions of the red noise; (iv) the reality of the planet candidate GJ581 d is questionable, because it cannot be detected from the Keck data, and its statistical significance in the HARPS data (as well as in the combined dataset) drops to a marginal level of \sim 2\sigma, when the red noise is taken into account. Therefore, the current data for GJ581 really support existence of no more than four (or maybe even only three) orbiting exoplanets. The planet candidate GJ581 d requests serious observational verification.

Title: GJ 581 update: Additional Evidence for a Super-Earth in the Habitable Zone Authors: Steven S. Vogt, R. Paul Butler, Nader Haghighipour

We present an analysis of the significantly expanded HARPS 2011 radial velocity data set for GJ 581 that was presented by Forveille et al. (2011). Our analysis reaches substantially different conclusions regarding the evidence for a Super-Earth-mass planet in the star's Habitable Zone. We were able to reproduce their reported \chi_{\nu}˛ and RMS values only after removing some outliers from their models and refitting the trimmed down RV set. A suite of 4000 N-body simulations of their Keplerian model all resulted in unstable systems and revealed that their reported 3.6\sigma detection of e=0.32 for the eccentricity of GJ 581e is manifestly incompatible with the system's dynamical stability. Furthermore, their Keplerian model, when integrated only over the time baseline of the observations, significantly increases the \chi_{\nu}˛ and demonstrates the need for including non-Keplerian orbital precession when modelling this system. We find that a four-planet model with all of the planets on circular or nearly circular orbits provides both an excellent self-consistent fit to their RV data and also results in a very stable configuration. The periodogram of the residuals to a 4-planet all-circular-orbit model reveals significant peaks that suggest one or more additional planets in this system. We conclude that the present 240-point HARPS data set, when analysed in its entirety, and modelled with fully self-consistent stable orbits, by and of itself does offer significant support for a fifth signal in the data with a period near 32 days. This signal has a False Alarm Probability of <4% and is consistent with a planet of minimum mass of 2.2 Earth-masses, orbiting squarely in the star's Habitable Zone at 0.13 AU, where liquid water on planetary surfaces is a distinct possibility

Title: A new analysis of the GJ581 extrasolar planetary system Authors: M. Tadeu dos Santos, G. G. Silva, S. Ferraz-Mello, T. A. Michtchenko

We have done a new analysis of the available observations for the GJ581 exoplanetary system. Today this system is controversial due to choices that can be done in the orbital determination. The main ones are the occurrence of aliases and the additional bodies - the planets f and g - announced in Vogt et al. 2010. Any dynamical study of exoplanets requires the good knowledge of the orbital elements and the investigations involving the planet g are particularly interesting, since this body would lie in the Habitable Zone (HZ) of the star GJ581. This region, for this system, is very attractive of the dynamical point of view due to several resonances of two and three bodies present there. In this work, we investigate the conditions under which the planet g may exist. We stress the fact that the planet g is intimately related with the orbital elements of the planet d; more precisely, we conclude that it is not possible to disconnect its existence from the determination of the eccentricity of the planet d. Concerning the planet f, we have found one solution with period ~ 450 days, but we are judicious about any affirmation concerning this body because its signal is in the threshold of detection and the high period is in a spectral region where the occurrence of aliases is very common. Besides, we outline some dynamical features of the habitable zone with the dynamical map and point out the role played by some resonances laying there.

'A very compelling case for life': Newly discovered planet is just like Earth and could contain liquid water

Scientists have discovered a planet which could have the most Earth-like environment ever found - raising a 'very compelling case' for life there. Gliese 581g, located around 123trillion miles away, orbits a star at a distance that places it squarely in the habitable - or Goldilocks - zone, Nasa said. The research, the product of more than a decade of observations at the WM Keck Observatory in Hawaii, suggests the planet could contain liquid water on its surface. Read more

Title: The HARPS search for southern extra-solar planets XXXII. Only 4 planets in the Gl~581 system Authors: T. Forveille, X. Bonfils, X.Delfosse, R. Alonso, S. Udry, F. Bouchy, M. Gillon, C. Lovis, V. Neves, M. Mayor, F. Pepe, D. Queloz, N.C. Santos, D. Segransan, J.M. Almenara, H. Deeg, M. Rabus

The Gl 581 planetary system has generated wide interest, because its 4 planets include both the lowest mass planet known around a main sequence star other than the Sun and the first super-Earth planet in the habitable zone of its star. A recent paper announced the possible discovery of two additional super-Earth planets in that system, one of which would be in the middle of the habitable zone of Gl 581. The statistical significance of those two discoveries has, however, been questioned. We have obtained 121 new radial velocity measurements of Gl 581 with the HARPS spectrograph on the ESO 3.6 m telescope, and analyse those together with our previous 119 measurements of that star to examine these potential additional planets. We find that neither is likely to exist with their proposed parameters. We also obtained photometric observations with the 2.5 m Isaac Newton Telescope during a potential transit of the inner planet, Gl 581e, which had a 5% geometric transit probability. Those observations exclude transits for planet densities under 4 times the Earth density within -0.2 sigma to +2.7 sigma of the predicted transit center.