Liquid water is stable on the surface of Gliese 581d according to new global circulation models, and combined with observations from Canadian space telescope MOST that show the host star has a low level of activity, the finding bodes well for the habitability of the "super-Earth".

GJ 581d, which is at least 5.6 times the mass of Earth, and thus nicknamed a super-Earth, is one of four confirmed planets in the Gliese 581 system, with the existence of two additional planets – one of which supposedly lies in the habitable zone – much debated. GJ 581d also lies in the habitable zone, the "goldilocks" not-too-hot, not-too-cold region around a star where liquid water is stable. In the search for life as we know it, any location where water is stable provides an obvious starting point for locating life elsewhere in the Universe, and at just 20 light years away, this system is right on our cosmic doorstep.

Although GJ581d receives 35 percent less solar energy than Mars does in our own Solar System, initial models of the planet's atmosphere suggested that a strong greenhouse effect from a dense carbon dioxide-rich atmosphere could still place Gliese 581d within the habitable zone. But the planet is likely locked in tidal resonance with the star, meaning that cold regions on the night side would quickly deplete the atmosphere of the carbon dioxide necessary to keep the planet warm.

In a new global circulation model, presented in a paper lead by Robin Wordsworth of the Institut Pierre Simon Laplace in Paris, two types of planet are considered: one composed of rock and one with a liquid water ocean, both with predominantly carbon dioxide atmospheres that can form clouds, ice or precipitation. By varying the thickness of the atmosphere and the orbital configuration of the planet around the star, surface temperatures on the planet are derived to determine its stability.

The models revealed that planets with carbon dioxide atmospheres with pressures of under approximately 10 bars (Earth's atmospheric pressure at sea level is one bar) were unstable, with the planet becoming too cold to support liquid water. For atmospheres above 10 bars however, the global warming effect of carbon dioxide combined with the efficiency of heat transport around the planet allowed liquid water to be stable on its surface.

Direct spectroscopic observations of the planet's atmosphere by future instruments would be able to determine the composition of the atmosphere, and since the Gliese 581 system lies just 20 light years from Earth, perhaps we won't have too long to wait to test the integrity of the models.

Meanwhile, an international team of astronomers have been looking at Gliese 581's innermost planet GJ 581e using Canada's Microvariability & Oscillations of STars space telescope MOST. GJ 581e has a minimum mass of 1.7 Earth masses, but lies just 0.03 astronomical units from the star and therefore is likely bathed in intense radiation from the star. But the MOST team were on the hunt for transits of the planet, given that all of the star's planets were detected using the Doppler method, whereby the wobble induced in the star by its orbiting companions can be measured from the wavelength shift in the star's spectral lines. For the transit method of detection, if the orbital inclination of the planetary system is just right then a planet may be observed to cross the disc of the star, temporarily dimming its brightness, allowing astronomers to constrain the size of the planet.

If Gliese 581e was found to be transiting its star then its radius must be smaller than 1.4 times the Earth's radius, since a larger planet would be detected by MOST. But even though MOST did not see any transits, the null result still provides certain constraints about the planet.

"The key here is that we've ruled out transits of larger than 1.4 times the Earth radius planets," explains Diana Dragomir, a PhD student at the University of British Columbia who led the MOST study. "That means that either the planet transits but is smaller than that, or does not transit at all. The second possibility is more likely, but it's also important to rule out transits, not just find them. It's good to know confidently as early as possible which observing methods (like transit, radial velocity, direct imaging, etc.) can be used for a specific exoplanetary system and which cannot."

The study also provided important information about the planets' host star, an M-class red dwarf, finding that it has a low level of activity, which bodes well for the potential habitability of planets such as GJ581d.

"The longer we observe the star, the better the precision of the data," adds Dragomir. "And the more precise the data, the smaller a transit we can exclude for Gliese 581e."

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