Planet Gliese 581 g has just been listed as the most likely to be habitable out of the list of known planets around other stars – a list that currently contains over 750 confirmed and over 2300 unconfirmed candidates.
The fact that we can detect planets at interstellar distances is an amazing feat all in itself. But that got me thinking about what kind of life might be possible on Gliese 581 g (G581g from now on!).
Gliese 581 (G581) itself is a red dwarf star with a mass around one-third of our sun. This can be both good and bad from the point of view of life on surrounding planets. With a mass that low, G581 fuses its hydrogen at a much lower rate than that of the sun. This means the star is more stable and longer-lived than the sun and so stays in the “main sequence” longer. Longer stable conditions mean that any orbiting planets should have more stable environments and subsequently a greater chance to develop life.
The down-side to this is that G581 is much cooler than the sun and delivers relatively less energy to orbiting planets and energy is the key building block of life. So if life does develop, it’s likely going to take longer than it did here on Earth. However, G581g is within the star’s “Goldilocks zone” – the area where it’s possible for a planet to maintain liquid water, which is considered another essential for life.
The surface temperature of G581g is thought to be in the range of -64 to -45 degrees C (-84 to -49 F), which doesn’t look promising initially; though if the planet maintains an atmosphere of a similar level as the Earth (which is possible) this could raise these figures to a more life-friendly -37 to -12 degrees C (-35 to 10 F).
As G581g is so close to its parent star (around fifteen percent of the distance between the earth and sun) the chances are high that it is “tidally locked”;in other words it turns in lockstep with its orbit so that it presents only one face to its star, like the Moon does to the Earth.
Being locked means that one hemisphere of G581g receives all the energy from its star while the other gets virtually none. Similar objects in our own solar system characteristically display extremes of temperature from several hundred degrees above zero C, on the side facing the star, to hundreds below. An atmosphere might soften this a little comma but we can guess that G581g has high surface temperature variations.
One possibility is that the planet may be a “ribbon world”, a name suggested by Isaac Asimov. In this scenario, in between the constant hot and cold hemispheres, a zone exists where the temperatures balance. This forms a lush strip favorable to lifeforms in a “continuous garden, swimming in the eternal morning of an eternal June”.
So what kind of life might live there?
We can surmise that the pace of life and evolution on such a planet would be necessarily slow. With so much less energy to “play” with, nature would likely be constrained. Balancing that would be the longer-term stability, which would give rise to a great variety of life,though possibly as “variants” on a theme.
The most successful life forms on Earth have been algae, bacteria and viruses, which have been resident for billions of years, so we could reasonably expect to see similar life on G581g. The sheer amount of variations possible in DNA mean that such life would not necessarily appear in similar forms as we see here on Earth.
Going beyond the microscopic level, the next type of life to develop on Earth were arthropods, invertebrate animals with a “shell” or external skeleton. They later developed into developed later into arachnids (spiders), insects and crustaceans (shellfish). This type of life was incredibly successful in the early parts of Earth’s history, during the “Cambrian explosion” (around 600 million years ago) when such life exploded and diversified into every possible evolutionary niche.
We could imagine a huge variety of trilobyte-like creatures in great variety dominating the life-bearing shores around coastlines and coral-like structures in the ribbon region, while on land crab and spider-like creatures would hunt in packs, preying on their smaller and weaker cousins.
The surface gravity of G581g is thought to be between ten and seventy percent higher than here on Earth, which would tend to lead to the development of creatures larger and more solidly built than we see here, with a lower center of gravity. Imagine a cross between a crab, a centipede and a wild boar, hard scales protecting the segmented body and legs, with large multi-faceted eyes to make the most of the dim light coming from G581. They may move at a slow rate compared to earthly contemporaries, but so would you in their gravity!
The Gravity would mean that there would be far less chance of tall creatures evolving, anything with the delicate height of a Giraffe for example, would be unlikely. Also there would likely be a dearth of creatures that fly. Insect like creatures would probably have the best chance if their body size was small, but it’s unlikely that anything larger would be able to lift off on such a planet.
Could intelligence develop? Could we find ourselves sitting down to a negotiation table facing smart “bugs” from another world in the distant future? It’s certainly possible, though probably unlikely. Let’s hope that if we do, they’re friendly!