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Gliese 581d
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Exoplanet near Gliese 581 star 'could host life'

A red dwarf star 20 light-years away is again providing hints that it hosts the first definitively habitable planet outside our Solar System.
The planet Gliese 581d is at the colder outer edge of the "Goldilocks zone" in which liquid water can be sustained.
Now a study in Astrophysical Journal Letters suggests its atmosphere may keep things warm enough for water.

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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: Gliese 581d is the first discovered terrestrial-mass exoplanet in the habitable zone
Authors: Robin Wordsworth, François Forget, Franck Selsis, Ehouarn Millour, Benjamin Charnay, Jean-Baptiste Madeleine

It has been suggested that the recently discovered exoplanet GJ581d might be able to support liquid water due to its relatively low mass and orbital distance. However, GJ581d receives 35% less stellar energy than Mars and is probably locked in tidal resonance, with extremely low insolation at the poles and possibly a permanent night side. Under such conditions, it is unknown whether any habitable climate on the planet would be able to withstand global glaciation and / or atmospheric collapse. Here we present three-dimensional climate simulations that demonstrate GJ581d will have a stable atmosphere and surface liquid water for a wide range of plausible cases, making it the first confirmed super-Earth (exoplanet of 2-10 Earth masses) in the habitable zone. We find that atmospheres with over 10 bar CO2 and varying amounts of background gas (e.g., N2) yield global mean temperatures above 0 degrees Celsius for both land and ocean-covered surfaces. Based on the emitted IR radiation calculated by the model, we propose observational tests that will allow these cases to be distinguished from other possible scenarios in the future.

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Gliese 581
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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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Gliese 581 g
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Raymond Pierrehumbert at the University of Chicago examined the range of climates that Gliese 581 g might have and found one that would have a pool of water on one side, making it look like an eyeball. Even if further observations disprove the existence of Gliese 581 g, the work could help determine the habitability of exo-Earths still to be discovered.
First spotted in September via wobbles in the light emitted by its host star, Gliese 581 g is likely to be rocky. That, combined with the fact that it orbits the star at just the right, "Goldilocks" distance to provide the warmth needed for liquid water, made it the first planet discovered outside our solar system with the potential to host life.

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Gliese 581
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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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Title: Gliese 581g as a scaled-up version of Earth: atmospheric circulation simulations
Authors: Kevin Heng, Steven S. Vogt

We use three-dimensional simulations to study the atmospheric circulation on the first Earth-sized exoplanet discovered in the habitable zone of an M star. We treat Gliese 581g as a scaled-up version of Earth and examine the long-term, global temperature and wind maps near the surface of the exoplanet --- the climate. The specific locations for habitability on Gliese 581g depend on whether the exoplanet is tidally-locked and how fast radiative cooling occurs on a global scale. Independent of whether the existence of Gliese 581g is confirmed, our study anticipates the use of meteorological solvers to quantify the atmospheric circulation on potentially habitable, Earth-sized exoplanets, which will be the prime targets of exoplanet discovery and characterization campaigns in the next decade.

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Zarmina's World
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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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RE: Gl581 planet
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Last month, astronomers announced the discovery of the first potentially habitable extrasolar planet. But this week at an International Astronomical Union meeting, doubts were raised about the existence of this exciting new planet said to be orbiting the star Gliese 581.
Called 'Gliese 581 g,' the planet was determined to be about 3 times the mass of Earth, meaning it was a rocky world, not a gas giant like Jupiter. Rocky extrasolar planets have been found before, but the unique trait about this planet was that it orbited within the red dwarf star's habitable zone, that region of space where temperatures are sufficient for water to remain as a liquid on a planetary surface.

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Posts: 114800
Date:
Gliese 581g
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The new planet Gliese 581g has at least three times the mass of Earth which is large enough to hold on to a atmosphere; and it orbits at a distance from its star that would allow water to remain liquid.
However, until it is verified by other observations the discovery may only be an artefact observed by  the astronomers.

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