* Astronomy

The first habitable exoplanet : a new candidate revealed by climate scientists

The planetary system around the red dwarf Gliese 581, one of the closest stars to the Sun in the galaxy, has been the subject of several studies aiming to detect the first potentially habitable exoplanet. Two candidates have already been discarded, but a third planet, Gliese 581d, can be considered the first confirmed exoplanet that could support Earth-like life. This is the conclusion of a team of scientists from the Laboratoire de Métrologie Dynamique (CNRS/UPMC/ENS/Ecole Polytechnique) at the Institut Pierre Simon Laplace in Paris, France, whose study is published in "The Astrophysical Journal Letters".
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Title: Bayesian re-analysis of the radial velocities of Gliese 581. Evidence in favour of only four planetary companions
Authors: Mikko Tuomi

The Gliese 581 planetary system has received attention because it has been proposed to host a low-mass planet in its habitable zone. We re-analyse the radial velocity measurements reported to contain six planetary signals to see whether these conclusions remain valid when the analyses are made using Bayesian tools instead of the common periodogram analyses. We analyse the combined radial velocity data set obtained using the HARPS and HIRES spectrographs using posterior sampling techniques and computation of the posterior probabilities of models with differing numbers of Keplerian signals. We do not fix the orbital eccentricities and stellar jitter to certain values but treat these as free parameters of our statistical models. Hence, we can take the uncertainties of these parameters into account when assessing the number of planetary signals present in the data, the point estimates of all of the model parameters, and the uncertainties of these parameters. We conclude that based on the Bayesian model probabilities and the nature of the posterior densities of the different models, there is evidence in favour of four planets orbiting GJ 581. The HARPS and HIRES data do not imply the conclusion that there are two additional companions orbiting GJ 581. We also revise the orbital parameters of the four companions in the system. Especially, according to our results, the eccentricities of all the companions in the system are consistent with zero.

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Title: Bayesian Re-analysis of the Gliese 581 Exoplanet System
Authors: Philip C. Gregory

A re-analysis of Gliese 581 HARPS and HIRES precision radial velocity data was carried out with a Bayesian multi-planet Kepler periodogram (from 1 to 6 planets) based on a fusion Markov chain Monte Carlo algorithm. In all cases the analysis included an unknown parameterised stellar jitter noise term. For the HARPS data set the most probable number of planetary signals detected is 5 with a Bayesian false alarm probability of 0.01. These include the 3.1498±0.0005, 5.3687±0.0002, 12.927_{-0.004}^{+0.006}, and 66.9±0.2d periods reported previously plus a 399_{-16}^{+14}d period. The orbital eccentricities are 0.0_{-0.0}^{+0.2}, 0.00_{-0.00}^{+0.02}, 0.10_{-0.10}^{+0.06}, 0.33_{-0.10}^{+0.09}, and 0.02_{-0.02}^{+0.30}, respectively. The semi-major axis and M sin i of the 5 planets are (0.0285±0.0006 au, 1.9±0.3M_{\earth}), (0.0406±0.0009 au, 15.7±0.7M_{\earth}), (0.073±0.002 au, 5.3±0.4M_{\earth}), (0.218±0.005 au, 6.7±0.8M_{\earth}), and (0.7±0.2 au, 6.6_{-2.7}^{+2.0}M_{\earth}), respectively. The analysis of the HIRES data set yielded a reliable detection of only the strongest 5.37 and 12.9 day periods. The analysis of the combined HIRES/HARPS data again only reliably detected the 5.37 and 12.9d periods. Detection of 4 planetary signals with periods of 3.15, 5.37, 12.9, and 66.9d was only achieved by including an additional unknown but parameterised Gaussian error term added in quadrature to the HIRES quoted errors. The marginal distribution for the sigma of this additional error term has a well defined peak at 1.8 ±0.4m s^{-1}. It is possible that this additional error arises from unidentified systematic effects. We did not find clear evidence for a fifth planetary signal in the combined HIRES/HARPS data set.

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In September 2010, a group of astronomers at the Lick-Carnegie Exoplanet Survey announced the discovery of the first so-called "Goldilocks planet." Gliese 581g (also dubbed "Zarmina's World" by its discoverer, UC Santa Cruz astronomer Steven Vogt, who has romantically named the planet after his wife) is the first rocky planet outside our solar system known to be situated "just right" for life - that is, within the habitable zone of its star.
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