Gl 581d Research Articles

Adiabatic lapse rate of nonideal gases: The role of molecular interactions and vibrations.

We report a formula for the dry adiabatic lapse rate that depends on the compressibility factor and the adiabatic curves. Then, to take into account the nonideal behavior of the gases, we consider molecules that can move, rotate, and vibrate and the information of molecular interactions through the virial coefficients. We deduce the compressibility factor in its virial expansion form and the adiabatic curves within the virial expansion up to any order. With this information and to illustrate the mentioned formula, we write the lapse rate for the ideal gas, and the virial expansion up to the second and third coefficient cases. To figure out the role of the virial coefficients and vibrations, under different atmospheric conditions, we calculate the lapse rate for Earth, Mars, Venus, Titan, and the exoplanet Gl 581d. Furthermore, for each one we consider three models in the virial expansion: van der Waals, square-well, and hard-sphere. Also, when possible, we compare our results to the experimental data. Finally, we remark that for Venus and Titan, which are under extreme conditions of pressure or temperature, our calculations are in good agreement with the observed values, in some instances.

Periodic Hαvariations in GL 581: Further evidence for an activity origin to GL 581d

Radial velocity measurements showed evidence that the M dwarf GL 581 might host a planet, GL 581d, in the so-called "habitable zone" of the star. A study of Halpha in GL 581 demonstrated that changes in this activity indicator correlated with radial velocity variations attributed to GL 581d. An exopplanet that was important for studies of planet habitability may be an artifact of stellar activity. Previous investigations analyzing the same activity data have reached different conclusions regarding the existence of GL 581d. We therfore investigated the Halpha variations for GL 581 to assess the nature of the radial velocity variations attributed to the possible planet GL 581d. We performed a Fourier analysis of the published Halpha measurements for GL 581d using a so-called pre-whitening process to isolate the variations at the orbital frequency of GL 581d. The frequency analysis yields five significant frequencies, one of which is associated with the 66.7 d orbital period of the presumed planet Gl 581d. The Halpha variations at this period show sine-like variations that are 180 degrees out-of-phase with the radial velocity variations of GL 581d. This is seen in thefull data set that spans almost 7 years, as well as a subset of the data that had good temporal sampling over 230 days. Furthermore, No significant temporal variations are found in the ratio of the amplitudes of the Halpha index and radial velocity variations. This provides additional evidence that the radial velocity signal attributed to GL 581d is in fact due to stellar activity.

THE INFLUENCE OF THERMAL EVOLUTION IN THE MAGNETIC PROTECTION OF TERRESTRIAL PLANETS

Magnetic protection of potentially habitable planets plays a central role in determining their actual habitability and/or the chances of detecting atmospheric biosignatures. We develop here a thermal evolution model of potentially habitable Earth-like planets and super-Earths. Using up-to-date dynamo scaling laws we predict the properties of core dynamo magnetic fields and study the influence of thermal evolution on their properties. The level of magnetic protection of tidally locked and unlocked planets is estimated by combining simplified models of the planetary magnetosphere and a phenomenological description of the stellar wind. Thermal evolution introduces a strong dependence of magnetic protection on planetary mass and rotation rate. Tidally locked terrestrial planets with an Earth-like composition would have early dayside magnetospause distances between 1.5 and 4.0 Rp, larger than previously estimated. Unlocked planets with periods of rotation ~1 day are protected by magnetospheres extending between 3 and 8 Rp. Our results are robust against variations in planetary bulk composition and uncertainties in other critical model parameters. For illustration purposes the thermal evolution and magnetic protection of the potentially habitable super-Earths GL 581d, GJ 667Cc and HD 40307g were also studied. Assuming an Earth-like composition we found that the dynamos of these planets are already extinct or close to being shut down. While GL 581d is the best protected, the protection of HD 40307g cannot be reliably estimated. GJ 667Cc, even under optimistic conditions, seems to be severely exposed to the stellar wind and, under the conditions of our model, has probably suffered massive atmospheric losses.

Habitability of the Goldilocks planet Gliese 581g: results from geodynamic models

Aims: In 2010, detailed observations have been published that seem to indicate another super-Earth planet in the system of Gliese 581 located in the midst of the stellar climatological habitable zone. The mass of the planet, known as Gl 581g, has been estimated to be between 3.1 and 4.3 Earth masses. In this study, we investigate the habitability of Gl 581g based on a previously used concept that explores its long-term possibility of photosynthetic biomass production, which has already been used to gauge the principal possibility of life regarding the super-Earths Gl 581c and Gl 581d. Methods: A thermal evolution model for super-Earths is used to calculate the sources and sinks of atmospheric carbon dioxide. The habitable zone is determined by the limits of photosynthetic biological productivity on the planetary surface. Models with different ratios of land / ocean coverage are pursued. Results: The maximum time span for habitable conditions is attained for water worlds at a position of about 0.14+/-0.015 AU, which deviates by just a few percent (depending on the adopted stellar luminosity) from the actual position of Gl 581g, an estimate that does however not reflect systematic uncertainties inherent in our model. Therefore, in the framework of our model an almost perfect Goldilock position is realized. The existence of habitability is found to critically depend on the relative planetary continental area, lending a considerable advantage to the possibility of life if Gl 581g's ocean coverage is relatively high. Conclusions: Our results are a further step toward identifying the possibility of life beyond the Solar System, especially concerning super-Earth planets, which appear to be more abundant than previously surmised.

Radiative constraints on the habitability of exoplanets Gliese 581c and Gliese 581d

Aims. The M-type star Gliese 581 is likely to have two super-Earth planets, i.e., Gl 581c and Gl 581d. The present study is to investigate their habitability constrained by radiative properties of their atmospheres and the threshold of carbon-dioxide (CO2), assuming that the two exoplanets are terrestrial, and that they have similar outgassing processes to those of the terrestrial planets in our own solar system. Methods. Radiative-convective atmospheric models are used. Different values of CO2 concentrations and water-vapor mixing ratios are tested. Results. Our simulation results suggest that Gl 581d is probably a habitable planet. However, at least 6.7 bars of CO2 are required to raise its surface temperature (T s) above the freezing point of water. In contrast, Gl 581c might have experienced runaway greenhouse, like Venus, because of its too high surface temperature and the lack of an effective cold trap for water vapor. We compare our results with other independent studies.

The extrasolar planet Gliese 581d: a potentially habitable planet?

Aims. The planetary system around the M star Gliese 581 contains at least three close-in potentially low-mass planets, Gl 581c, d, and e. In order to address the question of the habitability of Gl 581d, we performed detailed atmospheric modeling studies for several planetary scenarios.

The habitability of super-Earths

The unexpected diversity of exoplanets includes a growing number of super Earth-planets, i.e. exoplanets with masses smaller than 10 Earth masses and a similar chemical and mineralogical composition like Earth. We present a thermal evolution model for a super-Earth to calculate the sources and sinks of atmospheric carbon dioxide. The photosynthesis-sustaining habitable zone (pHZ) is determined by the limits of biological productivity on the planetary surface. We apply the model to calculate the habitability of super-Earths in Gliese 581. The super-Earth Gl 581c is clearly outside the pHZ while Gl 581d at the outer edge of the pHZ could at least harbor some primitive life forms.

Habitable planets around the star Gliese 581?

Radial velocity surveys are now able to detect terrestrial planets at habitable distance from M-type stars. Recently, two planets with minimum masses below 10 Earth masses were reported in a triple system around the M-type star Gliese 581. Using results from atmospheric models and constraints from the evolution of Venus and Mars, we assess the habitability of planets Gl 581c and Gl 581d and we discuss the uncertainties affecting the habitable zone (HZ) boundaries determination. We provide simplified formulae to estimate the HZ limits that may be used to evaluate the astrobiological potential of terrestrial exoplanets that will hopefully be discovered in the near future. Planets Gl 581c and 'd' are near, but outside, what can be considered as the conservative HZ. Planet 'c' receives 30% more energy from its star than Venus from the Sun, with an increased radiative forcing caused by the spectral energy distribution of Gl 581. Its habitability cannot however be positively ruled out by theoretical models due to uncertainties affecting cloud properties. Irradiation conditions of planet 'd' are comparable with those of early Mars. Thanks to the warming effect of CO2-ice clouds planet 'd' might be a better candidate for the first exoplanet known to be potentially habitable. A mixture of various greenhouse gases could also maintain habitable conditions on this planet.

The habitability of super-Earths in Gliese 581

Aims: The planetary system around the M star Gliese 581 consists of a hot Neptune (Gl 581b) and two super-Earths (Gl 581c and Gl 581d). The habitability of this system with respect to the super-Earths is investigated following a concept that studies the long-term possibility of photosynthetic biomass production on a dynamically active planet. Methods: A thermal evolution model for a super-Earth is used to calculate the sources and sinks of atmospheric carbon dioxide. The habitable zone is determined by the limits of biological productivity on the planetary surface. Models with different ratios of land / ocean coverage are investigated. Results: The super-Earth Gl 581c is clearly outside the habitable zone, since it is too close to the star. In contrast, Gl 581d is a tidally locked habitable super-Earth near the outer edge of the habitable zone. Despite the adverse conditions on this planet, at least some primitive forms of life may be able to exist on its surface.Therefore, Gl 581d is an interesting target for the planned TPF/Darwin missions to search for biomarkers in planetary atmospheres.

Habitability of super-Earths: Gliese 581c & 581d

AbstractThe unexpected diversity of exoplanets includes a growing number of super-Earth planets, i.e., exoplanets with masses smaller than 10 Earth masses. Unlike the larger exoplanets previously found, these smaller planets are more likely to have similar chemical and mineralogical composition to the Earth. We present a thermal evolution model for super-Earth planets to identify the sources and sinks of atmospheric carbon dioxide. The photosynthesis-sustaining habitable zone (pHZ) is determined by the limits of biological productivity on the planetary surface. We apply our model to calculate the habitability of the two super-Earths in the Gliese 581 system. The super-Earth Gl 581c is clearly outside the pHZ, while Gl 581d is at the outer edge of the pHZ. Therefore, it could at least harbor some primitive forms of life.