Earlier today, a team of researchers lead by noted exoplanet hunter Michel Mayor announced a pair of blockbuster discoveries - the lowest mass planet yet discovered orbiting another star, and a new analysis suggesting that another, previously discovered planet is orbiting that same star within the theoretical "habitable zone" where liquid water can exist. Both of these discoveries are huge, but for different reasons.
The small planet, now known as Gliese 581 e, may weigh in at only about 1.9 Earth masses. That makes it the smallest exoplanet yet discovered. Gliese 581 is a red dwarf star, with a mass about a third of the suns. Gliese 581 e is orbiting the star at about 0.03 AU (1 AU = distance from here to the sun), and completes an orbit in just over three days.
As cool as it would be to live someplace where I'd already be far older than Methuselah, Gliese 581 e probably isn't a very hospitable place to live. It's way too close to the star - it's probably an oversize, overcooked Mercury. Even if it's not a vacation spot, it's still a really cool find - it shows that scientists are getting much better at finding smaller exoplanets.
Cool as that is, it pales compared to Mayor's team's second announcement. They were able to refine the estimates for the orbit of Gliese 581 d - one of three other planets that had already been discovered orbiting Gliese 581. It appears that this planet is well within the star's habitable zone, which means that liquid water - and possibly life - can exist there.
The question is, does this mean that we should dust off the welcome mat?
Probably not. Gliese 581 is only about 20 light years away, and if there was an advanced civilization there right now, I suspect that the SETI folks would probably have seen some indications by now (if they use radio, anyway). But maybe they don't, for whatever reason. Could there be life there, and might they reach space?
Obviously, we can't do any more than speculate about what kind of life might evolve on a planet like Gliese 581 d. Our speculations are, at best, going to be almost completely uninformed, but that's never stopped people from speculating before, so I certainly see no reason to let it stop us now.
Let's start by looking at what we know about the planet. That's something that's really not going to take us very long. We know, according to the press release, that it's at least 7.7 Earth masses, and it orbits the star about every 66 days.
I told you that wouldn't take long. Now, let's speculate.
One of the many physical factors that's important to life is gravity - particularly when we're going to think about the potential for flight. We know that Gliese 581 d is a lot more massive than Earth, but we don't actually know what the planet's surface gravity is. In order to know what the surface gravity of the planet is, we need to know its radius. At the moment, we've got no idea what Gliese 581 d's radius is, because we also don't know what its density is.
Not all planets are equally dense. The planet we're on is about 5.5 times as dense as water. Venus and Mercury are a little less dense than we are, and Mars is a lot less dense - it comes in at about 3.9. If 581 d is a rocky planet, it's density is probably somewhere in that range. The research team that discovered the planet doesn't think that it's a rocky one, though. They say that, given how massive it is, it's more likely to be an icy-type planet. In our solar system, Pluto and Saturn's moon Titan fall into this category of object. Their densities are about 2.
That gives us quite a range of densities to look at, but we can use the possible densities to work out the possible size of the planet. If we make some simplifying assumptions (like pretending that the planet is perfectly spherical), it's fairly simple to figure out. We know the mass of the planet, and we know that volume equals mass divided by density, so we can punch in a range of densities and get a range of volumes.
If we dust off our 10th-grade geometry, we can figure out what the radii for spheres with each of those volumes. Once we've done that, we can figure out the surface gravity that goes with each of the possibilities. That's also fairly simple (g = m/r^2, where mass and radius are expressed relative to Earth's). I went ahead and looked a a range of densities that ran from 0.55 (0.1 Earth-density) to 11 (2 Earth-density).
As you can see, that's quite a range. Here's the bottom line, though: the Earth, as I've mentioned, is the densest of the rocky planets in our solar system. If 581 d is as dense as the Earth, it's got just about twice our surface gravity. However, if the scientists who discovered the planet are right, and it has a density similar to that of Pluto or Titan, (~0.3 of Earth's density), it probably has a surface gravity very similar to ours.
In that case, though, there's a decent chance that it's a water world - a planet that's one giant ocean. Not only do these planets have no continents, islands, or even rocks breaking the surface, they don't even really have an ocean floor as we understand the concept. The water simply gets deeper and deeper until the pressure increases to the point where the water becomes ice. There's no solid evidence to suggest that Gliese 581 d is an ice planet, but its size is right in the predicted range for an ocean world. Given that the estimated age of the Gliese 581 system is in the neighborhood of seven billion years, it's certainly plausible that 581 d started out as an outer solar system ice world, and has since migrated into the habitable zone.
It's fun to speculate about what sort of intelligent life might evolve on a water world. What would their myths and legends involve, and how would their environment shape their view of the universe? Would they have the same desire to reach for the stars that we do?
Maybe they do, but haven't made it yet. Or maybe they did, but they're long gone.
If you're at all familiar with anything that has to do with the search for extraterrestrial intelligent life, you're probably familiar with the Drake Equation, which is sometimes used to estimate the number of intelligent species out there in the galaxy (and is more appropriately used to remind us of just how much more we need to learn to be able to make anything like a reasonable guess about that number). One of the unknowns in the Drake Equation is L: the length of time that an advanced civilization lasts, on average. Even if the value for L is high - on the order of 1,000,000 years - it's a small window when we're talking about planets that have been around for billions.
Gliese 581 isn't very far from us - at least in galactic terms. It's only about 20 light years away. We're clearly not going to go there in the immediate future, but it would not surprise me at all if we do someday, some time in the next few generations.
I wonder if we can even imagine what might be there when we arrive.