Cav's Place

Little Gliese 581 is an entirely unremarkable star. Like the vast majority of known stars in our galaxy, it’s an M dwarf. More exactly, its spectral class is M3V, which is about a third of a class away from the dimmest possible main sequence stars, the lower limit for them being estimated around M6.5. Compared to the Sun, its luminosity is a mere 1.3%, its mass 31% and its diameter 29%. Though it’s in our galactic neighborhood, it’s only the 117th closest star to us and, due to its very low luminosity, there’s no way you could ever see it while looking up at the sky. In addition, its low metallicity, with estimated values ranging between -0.02 and -0.33, could have made it uninteresting for exoplanet hunters, since it is generally assumed that planets are more likely to form around stars with higher metallicity.
However, since 2007 it has been one of the most interesting stars because of the planets discovered around it. First it was Gliese 581 c, at the time thought to potentially be in the habitable zone. However, that assumption was quickly dropped as further research showed that the planet is a little too close to the star and likely suffers from a runaway greenhouse effect similar to that seen on Venus. But the enthusiasm never really subsided, because Gliese 581 d, discovered at the same time, was just outside the star’s habitable zone and massive enough to hold on to a very thick atmosphere, which could create a strong enough greenhouse effect to bring its surface temperature above freezing.

Until yesterday, Gliese 581 d was considered the known exoplanet most likely to have conditions that could potentially support life. It was a little too far from its star to avoid being completely frozen under normal circumstances and right at the upper mass limit for potentially rocky planets, therefore somewhat likely to in fact be something of a small gas giant, but it was the best we had. Until yesterday…
But now, two more planets have been discovered around good old Gliese 581, and one of them, Gliese 581 g, certainly seems to be the first discovered exoplanet that truly matches our definition of habitable. At 0.146 AU from such a dim star, it’s very clearly inside the habitable zone, and a mass estimated at 3.1 to 4.3 times that of Earth means it’s certainly not a gas giant. Its radius should be larger than Earth’s, but no more than double, and in fact most likely no more than 50% larger. Its surface gravity should therefore be between 10% and 70% stronger than Earth’s, which means that a human would be able to walk on it, though with some difficulty. As for the temperature, which is the other crucial factor, alongside mass, in determining whether a planet fits our definition of habitable, the global average is estimated to be between 236 and 261°K (-37 to -12°C, -35 to 10°F). However, that estimate is based on an Earth-like greenhouse effect, but Gliese 581 g’s stronger gravity makes it likely to have a thicker atmosphere, or at least one that is thicker at low altitudes, which could enhance the greenhouse effect and therefore increase the temperature.
A potential problem is that, being so close to its parent star, Gliese 581 g is very likely tidally locked, which means one of its sides constantly faces the star and the other constantly faces away from it. That can create some really interesting weather patterns, but certainly doesn’t have to prevent the existence of life. In fact, seeing as the temperature varies dramatically between the light side and the dark side, with the estimated averages likely only found in the “land of constant twilight” that separates the two, this could even help life, as it means that any organisms that exist on Gliese 581 g can move slightly towards one side or the other to find just the right temperature for their survival. Since conditions on the light and dark sides of the planet are likely to be exteme, it’s unlikely that any reasonably advanced life exists outside this “land of constant twilight”, yet simple organisms could certainly be found all over the planet, seeing as certain organisms thrive in extreme habitats even here on Earth.

Interestingly, the two new discoveries also mean that the Gliese 581 system is now the system with the highest number of confirmed planets, namely six, because HD 10180 only has five confirmed planets, the other two still being considered only planet candidates among scientists, though the team that discovered the system is pretty confident that the signals have been correctly identified. This also means that the 55 Cancri system finally drops down the order, after being the one with the most confirmed planets ever since 2004, though matched by HD 10180 recently.
So good old Gliese 581 really is the little star that could and perhaps will remain so no matter what we’ll keep discovering from now on, because it was the first around which potentially habitable planets have been discovered. Also, seeing how utterly unremarkable its characteristics make it out to be, and especially seeing as its low metallicity wouldn’t appear to make it a suitable candidate for even having planets in the first place, these discoveries should make us see our galaxy in a new light because, if they could be found around such a star, then such planets must be really common indeed. Which is, of course, no surprise, but many people need to be reminded of it…

Now let’s see what’s next, because exciting times are certainly ahead for exoplanet hunters. I for one am still waiting for the flood of Kepler candidate signals to be verified and the confirmed planets published, though it doesn’t appear likely that any significant number of discoveries will be published soon and, if they stick to hoarding the data and not letting anyone help for fear of stealing their glory, I’m even starting to doubt I’ll see the full results of the mission within my lifetime. But only time shall tell…
Until then, I’m curious to see who’ll discover the 500th exoplanet and what its characteristics will be, seeing as The Extrasolar Planets Encyclopaedia currently lists 492.