“Next to reasoning, the greatest handicap to the optimum development of Man lies in the fact that this planet is just barely habitable. Its minimum temperatures are too low, and its maximum temperatures too high. Its day is not long enough, and its night is too long… These factors encourage depression, fear, war, and lack of vitality. They describe a planet, which is by no means perfectly devised for the nurturing or for the perpetuation of a higher intelligence.” –James Thurber
It’s been just about three years, now, since the first announced discovery of a planet in another star system found within its parent star’s habitable zone.
That star happened to be Gliese 581, a red dwarf star — an M-class star — located about 20 light years away. There are a whole slew of relatively nearby stars named “Gliese” followed by a number; that’s because in the second half of the 20th Century, the German astronomer Wilhelm Gliese started doing a massive survey of stars located within 20 parsecs (or 65 light-years) of Earth. It’s been extended since then, and we now have a very extensive catalogue of thousands of stars located in our local neighborhood of 82 light-years or so: the Gliese Catalogue of Nearby Stars.
The vast majority of these stars are M-class stars, the lowest-mass, reddest, and longest-lived type. It turns out — as we’ve learned from this and other surveys — that these red dwarfs, the M-class stars, are by far the most common type of star in the Universe: some 3-out-of-every-4 stars are like them! The Sun, a G-class star, is a relative rarity, as it’s brighter and more massive than 95% of the other stars out there.
So it’s no surprise that the first star that they discovered a planet potentially capable of harboring life-as-we-know-it (i.e., a planet with Earth-like conditions) was around one of these nearby M-class stars. Initially found to have four planets labelled a-through-d, where “c” was just a little too hot and “d” a little too cold, subsequent data and analysis showed evidence for a planet in between those two: Gliese 581 g.
It was determined to fall dead-center in its star’s habitable zone, making it a prime candidate for life as we know it, and the first discovered planet outside of our Solar System that’s potentially habitable, as far as we understand it.
But just because something’s potentially habitable doesn’t mean it’s actually inhabited! Still, you’d never know it from the 2010 nterview with Steven Vogt, the lead author on the paper announcing its discovery:
I’m not a biologist, nor do I want to play one on TV. Personally, given the ubiquity and propensity of life to flourish wherever it can, I would say that… the chances of life on this planet are 100%. I have almost no doubt about it.
There are some things that are really worth thinking hard about before we make the leap from habitable to inhabited. Here are some of them.
Here in our Solar System, Earth is the largest of all the rocky bodies present. The next largest body, after Earth, is Uranus, which outweighs us by a factor of around 15, and is a little more than double our planet’s diameter.
No one seriously considers that gas giant planets are good candidates for Earth-like life to develop. That isn’t to say that life couldn’t develop on a gas giant, just that it would likely be very, very different from the life we find on our world. And yet, what we typically call super-Earths might actually be more like mini-Uranuses; Sara Seager calls them gas dwarfs. With a mass of around four times our Earth, it’s far from clear that a rocky planet like Earth is the right analogy for this world.
There’s also the important issue of heat transfer. On the surface of Earth, we have relatively warm days and relatively cool nights, but the short duration of our day means that the night side of the planet doesn’t freeze solid, and the day side doesn’t boil away. But the story would be very different if Earth were tidally locked to the Sun, the same way the Moon is locked to us. If the Earth had a permanent day-side and a permanent night-side, it might make habitability a pretty interesting question.
Well, guess what? Gliese 581 g is tidally locked to its star! In a worst-case scenario, that means there’s only a thin ring, on the border between night-and-day, that has a shot at life, but in a better-case scenario, a thicker atmosphere does an excellent job at transferring heat from the day side to the night side, keeping it in relative thermal equilibrium, like Venus. (Although hopefully it’s not some 700 Kelvin like Venus, with its awful, poisonous atmosphere is!)
My point is, there are some unknowns here, and they’re some big ones.
Some other things worth considering?
- This star is older than the Sun: at least seven billion years old and possibly as much as 11 billion years old. If there ever was life on this world, could it have sustained itself this long without catastrophically poisoning its environment, and rendering it uninhabitable?
- This star contains just 46% of the heavy elements that the Sun contains, an indication that there are fewer dense, heavy elements in the planets in this Solar System. Does that make it impossibly hard for life to develop?
- The average temperature on this planet, based on its distance from the Sun, is cold, as in well-below-freezing cold. Is there a sufficient greenhouse gas effect to have liquid water on its surface, what we typically think of as the “gold standard” for habitability?
And finally, does this planet even exist? It’s actually very controversial, because the follow-up studies we typically do to confirm an exoplanet’s existence have turned up empty-handed on this world. It may still exist, but it remains unconfirmed.
That isn’t to say that it’s likely that there are no inhabited planets out there; there almost definitely are.
It’s just that we’re a long way from being able to state anything with 100% certainty as respects the most ideal planetary candidate for life, and Gliese 581 g is far from ideal. I’d still bet on life if you offered me even odds, but 100%? There’s a big difference between potentially habitable and actually inhabited.