The Goldilocks zone was a term given to describe a band of space around a star in which temperatures would be neither too hot, nor too cold, but just right for liquid water - and thus life as we know it. Such as the one Earth inhabits, for example. First only a couple planets were discovered in this zone. The first, in 1996, 70 Virginis b, was even removed when it was discovered to have an eccentric orbit. Then came Gliese 581 d and Gliese 581 g, two planets that we have sent a little Hello! Of course, it's a 20 light year trip, and 20 light years back, so we'll be listening for a while. If there is life here, maybe they're not sentient. Maybe they've already risen and fallen, their ruins stretching through alien landscapes like so many fallen jenga cubes. We've only been capable of receiving transmissions for the past couple hundred years - and the universe is very old. Massive species have walked this earth and been annihilated. Perhaps they beamed us a message, found no reply, and faded into the millenia. But now I'm just daydreaming.
Anyway, most recently, NASA released 54 more planets out of 1235 around 156000 stars that fall into the habitable zone. And this is just in our galaxy.
Simple extrapolation.. 156 thousand stars.. 54 planets in the habitable zone around those stars... .00034%. Not a big number. Lets say there are 10^24 stars in the universe. That means....
(10^24) x .00034 = 3.4 x 10^20.
I don't even know how do describe that number, but it is vast. Did I do maths wrong? Doesnt matter, the point is that with so many stars, so many planets, the odds are good for something else to have plodded, waggled, swaggered, slithered, or lurched across some distant landscape - or seascape.
Back to the goldilocks zone itself. This term is interesting to me. Mostly because it deals with extraterrestrial life, but also because it makes an assumption. An assumption which is so incredibly human in its hubris that it doesn't surprise me - that life on another planet would require the same conditions we have here.
Lets assume for a moment that a planet does need liquid water to act as a solvent for life, and that no other compound will do. Where might we find water? The moon has water. Mars has water. It's a little scarce, though, and also a little frozen. I'll just pause right here to mention Ice worms.
Concerning our own solar system, Mars isn't where I would look. Instead I'd go much, much farther outside our own Goldilocks zone. I'd go to Jupiter, and to Saturn. Not those planets themselves of course, but their moons.
This is Enceladus. Of the 62 moons of Saturn, it is the sixth largest. What you see in this picture taken by the Cassini probe is water vapor, spewing forth like a massive geyser. Other moons are long dead, but not this one. It's still geologically active, which means that beneath its surface of ice it's highly likely that there is a liquid ocean. Liquid water, and lots of it. But this isn't just any liquid water either - It's salt water, the same as that which covers most of our planet. Cassini also found organic compounds in the plume such as nitrogen, carbon, hydrogen, oxygen and various hydrocarbons.
Jupiter's moon Europa is similar. Covered in ice, it is thought that below this is a vast liquid ocean, twice the volume of our own. According to Richard Greenberg of the University of Arizona, cosmic rays impacting the surface of Europa could release oxygen which is then absorbed by the ocean below - resulting in an ocean rich enough in oxygen to support fish.
Ah, but isn't the Sun also essential to life? Well, yes, but life has proven to be incredibly resilient on this planet. Forms of life that are most relevant in this case are in our deep oceans. Thermal vents spew forth a highly toxic (to us) mix of chemicals, and no sunlight reaches their communities. And yet, life exists there. Microbes use chemosynthesis instead of photosynthesis, and larger lifeforms such as tubeworms and clams feed on these microbes. There are even some crabs.
So here, far outside the Goldilocks zone, is perhaps liquid water and maybe a candidate for life. Even so, I've fallen into the familiar trap of comparing life on earth to what to look for out there. But as a starting point, I think this is all we have to go on, and as we find more and more extreme examples closer to home - such as this bacteria which lives on arsenic, deadly poisonous to other forms of life on earth, our concept of just what to look for will continue to expand.
In the short time that I have been alive, I have heard echoes of the same question repeated throughout the human ages.
"Are we alone?"
If I may be so bold, how about a little rework of that question?
"Where are you?"