Starship Asterisk*

http://www.hellofromearth.net/ wrote:
This site is collecting messages that will be transmitted to Gliese 581d, a planet outside our Solar System which may support life. Comments are moderated: inappropriate messages will be rejected, and messages must be in English (so we can evaluate them).

Register here to send your message before the deadline: 5pm Monday 24 August 2009 Sydney time (07:00 GMT Monday 24 August 2009)

http://en.wikipedia.org/wiki/Gliese_581_d wrote:
An artist's impression of Gliese 581 d and speculative moons.


<<Gliese 581 d (pronounced /ˈɡliːzə/) or Gl 581 d is an extrasolar planet approximately 20 light-years away in the constellation of Libra. Because of its mass, nearly 8 times that of Earth, the planet is classified as a super-Earth. In late April 2009 new observations by the original discovery team concluded that the planet is within the habitable zone where liquid water, and therefore, life, could exist.

The planet was discovered by the team of Stéphane Udry of the Geneva Observatory in Switzerland using the HARPS instrument on the European Southern Observatory 3.6 meter telescope in La Silla, Chile on 24 April 2007. Udry's team employed the radial velocity technique, in which the size and mass of a planet are determined based on the small perturbations it induces in its parent star’s orbit via gravity.

The team is confident that the planet exists but recognizes that unlikely events could mimic its existence. They believe the issue will be settled by upcoming studies. Dynamical simulations of the Gliese 581 system assuming that the orbits of the three planets are coplanar show that the system becomes unstable if the masses of the planets exceed 1.6 – 2 times the minimum values. The upper mass limit for Gliese 581 d is 13.8 Earth masses.

It was originally thought that Gliese 581 d orbits outside the theoretical habitable zone of its star. Further studies released in April of 2009 confirmed that the planet is within the habitable zone where liquid water could exist. According to Stephan Udry, "d could be covered by a 'large and deep ocean'; it is the first serious Ocean planet candidate." Gliese 581 d is probably too massive to be made only of rocky material, but we can speculate that it is an icy planet that has migrated closer to the star." Scientists originally believed that Gliese 581 d would be too cold for liquid water to exist, and therefore could not support life in forms as existing on Earth. However, since Earth's temperature would be about -18°C without any greenhouse gases, and due to a theorized greenhouse effect of Gliese 581 d, research now suggests that atmospheric conditions on the planet could create temperatures at which liquid water can exist, and therefore the planet may be capable of supporting life.

On 21 April 2009, ESO announced, together with the discovery of Gliese 581 e, that they had refined the models of Gliese 581 d's orbit, discovering it lay well within the habitable zone. Calculations by Barnes et al. suggest, however, that tidal heating is too low to keep plate tectonics active on the planet, unless radiogenic heating is somewhat higher than expected.>>

http://antwrp.gsfc.nasa.gov/apod/ap070502.html wrote:
Explanation: How might a sunrise appear on Gliese 581c? One artistic guess is shown above. Gliese 581c is the most Earth-like planet yet discovered and lies a mere 20 light-years distant. The central red dwarf is small and redder than our Sun but one of the orbiting planets has recently been discovered to be in the habitable zone where liquid water could exist on its surface. Although this planet is much different from Earth, orbiting much closer than Mercury and containing five times the mass of Earth, it is now a candidate to hold not only oceans but life enabled by the oceans. Were future observations to confirm liquid water, Gliese 581c might become a worthy destination or way station for future interstellar travelers from Earth. Drawn above in the hypothetical, the red dwarf star Gliese 581 rises through clouds above a calm ocean of its planet Gliese 581c.

http://antwrp.gsfc.nasa.gov/apod/ap070426.html wrote:
Explanation: The unremarkable star centered in this skyview is Gliese 581, a mere 20 light-years away toward the constellation Libra. But astronomers are now reporting the discovery of a remarkable system of three planets orbiting Gliese 581, including the most earth-like planet found beyond our solar system. Gliese 581 itself is not a sun-like star, though. Classified as a red dwarf, the star is much smaller and colder than the Sun. Still, the smallest planet known to orbit the star is estimated to be five times as massive as Earth with about 1.5 times Earth's diameter. That super-earth orbits once every 13 days, about 14 times closer to its parent star than the Earth-Sun distance. The close-in orbit around the cool star implies a mean surface temperature of between 0 and 40 degrees C - a range over which water would be liquid - and places the planet in the red dwarf's habitable zone.

• lanl.arXiv.org > astro-ph > arXiv:1005.5098 > 27 May 2010

The recently discovered exoplanet Gl581d is extremely close to the outer edge of its system's habitable zone, which has led to much speculation on its possible climate. We have performed a range of simulations to assess whether, given simple combinations of chemically stable greenhouse gases, the planet could sustain liquid water on its surface. For best estimates of the surface gravity, surface albedo and cloud coverage, we find that less than 10 bars of CO2 is sufficient to maintain a global mean temperature above the melting point of water. Furthermore, even with the most conservative choices of these parameters, we calculate temperatures above the water melting point for CO2 partial pressures greater than about 30 bar. However, we note that as Gl581d is probably in a tidally resonant orbit, further simulations in 3D are required to test whether such atmospheric conditions are stable against the collapse of CO2 on the surface.