* Astronomy

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.

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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.

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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.

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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.

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