On The Habitability of Gliese 581g: Review

The Gliese 581 system has been making headlines recently for the most newly announced planet that may lie in the habitable zone. Hopes were somewhat dashed when we were reminded that the certainty level of its discovery was only 3 sigma (95%, whereas most astronomical discoveries are at or above the 99% confidence level before major announcements), but the Gliese 581 system may yet have more surprises.

When the second planet, Gliese 581d, was first discovered, it was placed outside of the expected habitable zone. But in 2009, reanalysis of the data refined the orbital parameters and moved the planet in, just to the edge of the habitable zone. Several authors have suggested that, with sufficient greenhouse gases, this may push Gliese 581d into the habitable zone. A new paper to be published in an upcoming issue of Astronomy & Astrophysics simulates a wide range of conditions to explore just what characteristics would be required.

Artist impression of Gliese 581 g, which is thought to have three times the mass of Earth. Credit: Lynette Cook

The team, led by Robin Wordsworth at the University of Paris, varied properties of the planet including surface gravity, albedo, and the composition of potential atmospheres. Additionally, the simulations were also run for a planet in a similar orbit around the sun (Gliese581 is an M dwarf) to understand how the different distribution of energy could effect the atmosphere.

The team discovered that, for atmospheres comprised primarily of CO2, the redder stars would warm the planet more than a solar type star due to the CO2 not being able to scatter the redder light as well, thus allowing more to reach the ground.

One of the potential roadblocks to warming the team considered was the formation of clouds. The team first considered CO2 clouds which would be likely towards the outer edges of the habitable zone and form on Mars. Since clouds tend to be reflective, they would counteract warming effects from incoming starlight and cool the planet. Again, due to the nature of the star, the redder light would mitigate this somewhat allowing more to penetrate a potential cloud deck.

Should some H2O be present its effects are mixed. While clouds and ice are both very reflective, which would decrease the amount of energy captured by a planet, water also absorbs well in the infrared region. As such, clouds of water vapor can trap heat radiating from the surface back into space, trapping it and resulting in an overall increase. The problem is getting clouds to form in the first place.

Astronomers have discovered many planets orbiting the star Gliese 581. This artist’s representation shows Gliese 581 e (foreground), which is only about twice the mass of our Earth. Other confirmed planets in the system are 16 (planet b, nearest to the star), 5 (planet c, center), and 7 Earth-masses (planet d, with the bluish color). Credit:ESO

The inclusion of nitrogen gas (common in the atmospheres of planets in the solar system) had little effect on the simulations. The primary reason was the lack of absorption of redder light. In general, the inclusion only slightly changed the specific heat of the atmosphere and a broadening of the absorption lines of other gasses, allowing for a very minor ability to trap more heat. Given the team was looking for conservative estimates, they ultimately discounted nitrogen from their final considerations.

With the combination of all these considerations, the team found that even given the most unfavorable conditions of most variables, should the atmospheric pressure be sufficiently high, this would allow for the presence of liquid water on the surface of the planet, a key requirement for what scientists maintain is critical for abiogenesis. The favorable merging of characteristics other than pressure were also able to produce liquid water with pressures as low as 5 bars. The team also notes that other greenhouse gasses, such as methane, were excluded due to their rarity, but should the exist, the ability for liquid water would be improved further.

Ultimately, the simulation was only done as a one dimensional model which essentially considered a thin column of the atmosphere on the day side of the planet. The team suggests that, for a better understanding, three dimensional models would need to be created.

In the future, they plan to use just such modeling which would allow for a better understanding of what was happening elsewhere on the planet. For example, should temperatures fall too quickly on the night side, this could lead to the condensation of the gasses necessary and put the atmosphere in an unstable state.

Additionally, as we discover more transiting exoplanets and determine their atmospheric properties from transmission spectra, astronomers will better be able to constrain what typical atmospheres really look like.

Atmospheric Circulation Simulation and Habitability

Ever since the planet has been discovered, a lot of rumors came to existence like detection of so called and long awaited ‘alien signal’. I tend to agree with preamble that there could indeed be life but human life, not exactly good claim without any compelling evidence. The next frontier in extrasolar planet-hunting is the discovery and characterization of Earth-sized exoplanets — “exo-Earths”. A particularly promising route is to search for such planets around nearby M stars. M dwarf stars have several unique attributes that are driving exoplanet studies and astrobiology, as well as next-generation interferometry and direct imaging missions; they constitute at least 72% of nearby stars. As the least massive stars, they have the greatest reflex motion due to an orbiting exoplanet. Furthermore, the classical habitable (liquid water) zone around M dwarfs is typically located in the range  0.1–0.2 AU, corresponding to orbital periods of  20 to 50 days — well matched to the capabilities of ground based precision-Doppler surveys. With such short periods, hundreds of cycles of a few-Earth-mass planet can be obtained within a decade, realizing factors of at least 10 in increased sensitivity for strictly periodic Keplerian signals and enabling Doppler reflex barycentric signals as small as 1 m s−1to be recovered even in the presence of similar-amplitude stellar jitter and Poisson noise. Although these attributes have only recently become widely recognized by the astronomical community, many of the nearest M stars have been prime targets for scrutiny by leading precision-radial-velocity surveys for over a decade now.

Two bodies with a major difference in mass – a star and a planet -- orbit around a common center of mass, or ‘barycenter’ (defined in this animation by the red cross). Astronomers look at the Doppler shift of light as the star moves back and forth, but additional orbiting planets can create a very complicated signal. Credit: Zhatt

One of the most enticing and proximate exoplanet systemsbeing scrutinized is Gliese 581, with at least four exoplanets orbiting a nearby (6.3 pc) M3V star. Two of the exoplanets announced are apparently“super-Earths” that straddle its habitable zone. Recently, Vogt announced two more exoplanet candidates orbiting this star — one with a minimum mass of 3.1 M(Gliese 581g) and an orbital distance of about 0.15 AU, placing it squarely within the habitable zone of its parent star. It is generally accepted that, for stellar masses below 0.6 M, an Earth-mass exoplanet orbiting anywhere in the habitable zone becomes tidally locked or spin-synchronized within the first Gyr of its origin, such that it keeps one face permanently illuminated with the other in perpetual darkness. Such tidal locking will greatly influence the climate across the exoplanet and figures prominently in any discussion of its potential habitability.

Simulation and Results

Figure below shows the Mollweide projection(Pseudo-cylindrical projection of a globe which conserves area but not angle or shape. Also called the “homalographic projection”) of a snapshot from the simulation where Gliese 581g is assumed to be tidally-locked. Since the rotational period of about 37 days is much longer thanthe radiative cooling time (about 4 days), the structure of the flow is sculpted by radiation rather than advection. The relatively fast cooling time implies that the global temperature map relaxes approximately to the input thermal forcing function.

While such visualizations are aesthetically pleasing, more insight is provided by looking at the temporally-averaged temperature and wind maps as functions of  longitude and latitude — the long-term, quasi-stable climate. This is shown in Figure 2, in which they contrast both the tidally-locked and non-tidally-locked cases. For the tidally-locked case, the permanent day side of the exoplanetis just within the classical T = 0◦–100◦C habitable temperature range. In the case where the rotational period is assumed to beequal to one Earth day, the flow is dominated by advection rather than radiation, with temperatures at the equator hovering around afew degrees Celsius. The pair of global temperature maps in Figure(2) makes the point that conclusions on the exact locations for habitabilityon the surface of an exo-Earth depend upon whether the assumption of tidal locking is made. Even on the cold night side, the temperatures are comparable to those experienced in Antarctica where colonies of algae have been discovered and analyzed. All of these statements are made keeping in mind that temperature is a necessary but insufficient condition for habitability.

Figures 3 and 4 show the global zonal and meridional windmaps, respectively. In the case of a tidally-locked Gliese 581g, large-scale circulation cells transport fluid across hemispheric scales at speeds  1 m/s, comparable to typical wind speedson Earth. These cells have a slight asymmetry from west to east due to the rotation of the exoplanet. If the exoplanet instead has a rotational period of one Earth day, there is longitudinal homogenizationof the winds with a counter-rotating jet at the equator andsuper-rotating jets at mid-latitude. The meridional wind map is nowcharacterized by smaller structures. The slightly faster wind speeds recovered from the simulation with a rotational period of one Earthday are artifacts of assuming a higher value of  temperature difference between equator and poles of the planet — nevertheless, the global structure of the wind maps are robust predictions of the simulations.

To further explore the interplay between radiative cooling andadvection, we execute another simulation where the radiative cooling(originally 4 Earth days) and Rayleigh friction (originally  1Earth day) times are set to be 36.562 times their fiducial values— in essence, we are scaling by the ratio of the rotational periodsof (a tidally-locked) Gliese 581g to Earth. Due to the longer cooling time assumed, we now run the simulation for 3000 Earth days and discard the first 2000 days so as to attain quasi-equilibrium. The Mollweide snapshot of the temperature and velocity fields, as well as the long-term wind maps, are shown in Figures 5. Since advection occurs somewhat faster than radiative cooling,zonal winds on the exoplanetary surface develop a stronge reast-west asymmetry and there are hints of energy transport from the permanent day to the night side. The chevron-shaped feature residing around the substellar point is reminiscent of that seenat  0.1 bar in 3D atmospheric circulation simulations of  hot Jupiters. Trailing the featureare large-scale vortices spanning about a third of the hemisphere insize — their large sizes are a consequence of the Rossby deformationlength scale being relatively larger due to the slower rotationof the exoplanet when tidally locked.[ref]

Generally, these studies make the point that anexoplanet found outside of the classical habitable zone may not beuninhabitable — conversely, an exoplanet found within the zonemay not be inhabitable. It simply depends upon life as how it adapts the conditions. Quoting from my previous article “Gliese 581g: Earthlike Exoplanet may Harbor Potentially Rich Alien Life!!

The search for extraterrestrial life is encouraged by a comparison between organisms living in severe environmental conditions on Earth and the physical and chemical conditions that exist on some Solar System bodies. The extremophiles that could tolerate more that one factor of harsh conditions are called poly-extremophiles. There are unicellular and even multicellular organisms that are classified as hyperthermophiles (heat lovers), psychrophiles (cold lovers), halophiles (salt lovers), barophiles (living under high pressures), acidophiles (living in media of the lower scale of pH). At the other end of the pH scale they are called alkaliphiles (namely, microbes that live at the higher range of the pH scale). Thermo-acidophilic microbes thrive in elevated thermo-environments with acidic levels that exist ubiquitously in hot acidic springs.Cyanidium caldarium, is a classical example of an acido-thermophilic red alga that thrives in places such as hot-springs (<570 and in the range 0.2-4 pH). This algal group shows a higher growth rate (expressed as number of cells and higher oxygen production when cultured with a stream of pure CO2, rather than when bubbled with a stream of air (Seckbach, 2010). It has been reported that Cyanidium cells resisted being submerged in sulfuric acid (1N H2SO4). This is a practical method for purifying cultures in the laboratory and eliminating other microbial contamination (Allen, 1959). The psychrophiles thrive in cold environments, such as within the territories found in the Siberian permafrost, around the North Pole in Arctic soils, and they may also grow in Antarctica.

Microbes Thriving Below Antarctic Ice

 

 

Recently, the segmented microscopic animals tardigrades, (0.1 – 1.5 mm) have been under investigations (Goldstein and Blaxter, 2002; Horikawa, 2008). These “water bears” are polyextremophilic, and are able to tolerate a temperature range from about 00C up to + 1510C (much more that other known microbial prokaryotic extremophiles, Bertolani et al., 2004). But even low Earth orbit extreme temperatures are possible: tardigrades can survive being heated for a few minutes to 151°C, or being chilled for days at -200°C, or for a few minutes at -272°C, 1° warmer than absolute zero (Jönsson et al., 2008). These extraordinary temperatures were discovered by an ESA project of research into the fundamental physiology of the tardigrade, named TARDIS. Tardigrades are also known to resist high radiation, vacuum, and anhydrous condition for a decade in a dehydrated stage and can tolerate a pressure of up to 6,000 atmospheres. These aquatic creatures are ideal candidates for extraterrestrial life and for withstanding long periods in space. They have already been used in space and have survived such stress. That’s why I find it indulging to speculate 

Hope there is life!

[Source: Astrobiology Magazine]

[Ref: Gliese 581g as a scaled-up version of Earth: atmospheric circulation simulations by Kevin Heng and Steven S. Vogt]

About bruceleeeowe
An engineering student and independent researcher. I'm researching and studying quantum physics(field theories). Also searching for alien life.

3 Responses to On The Habitability of Gliese 581g: Review

  1. Pingback: Earth-like Planets are Common in Universe!! « WeirdSciences

  2. Wow! Maybe aliens evolved from these Water Bears could survive and prosper in such an extremephile planet as this! But, perhaps there is some unknown circumstance of extremephile organisms like that, which restrict them to being VERY small? It’s odd that something like the water Bear, the incredible hulk of the microverse like that, never grew any bigger.

  3. bruceleeeowe says:

    Hi,
    nice to meet you after a long interval. May be, but there is another discovery that is exchange of arsenic and phosphorus in DNA utilized by a microbe, adds another footstep to our current view of alien life.

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