Earth is quite a lovely little rock in space. While there is no doubt that at least most of our planet supports ideal conditions for human life, and Earth is the most “habitable” world we know of, this doesn’t necessarily mean that Earth is a member of an exclusive a club. It’s not that we magically dropped down out of the sky onto a planet that happened to be perfect. The reality is that our line of organisms has been shaped by billions of years of evolution on this planet. Earth seems so amazingly habitable to us not by happenstance, but precisely because we evolved to thrive in its environment.
Lots of types of worlds may support many types of life, but not necessarily life as we know it. There are of course certain bounds and limits. So far as we know, there must be a temperature range capable of supporting chemical reactions of stable organic molecules, and (we think) some sort of liquid. Water is ideal, but may not be the only liquid capable of serving this purpose. For example, the surface of Saturn’s moon Titan is covered in liquid hydrocarbons at a chilly -180°C (colder than liquid nitrogen). We can’t rule out the possibility of microorganisms or even sizable animals living in this environment, albeit with very un-Earthlike chemistry, relying on a methane cycle not so different from our planet’s water cycle. Perhaps small crystalline “sea snakes” glide through the freezing waters of Titan.
Life on Earth thrives across an extremely wide array of conditions, and this would be no different from other worlds. Bacteria on Earth live essentially everywhere from the upper atmosphere to the depths of Earth’s crust. They survive extremes of radiation exposure, high and low pressures and temperatures, abundance or lack of light, and utter deprivation of water and nutrients. The live in the thermal pools of Yellowstone National Park at temperatures up to 80°C, dining on a rich array of chemicals leaking from volcanic vents. Microorganisms have been found deep underground in oil wells, and suspended in lakes of liquid water trapped miles under the Antarctic ice sheet. In fact, there’s more life underneath our planet than on top of it. Bacteria live miles underground, never seeing light and consuming nothing but chemicals stored in rocks. There might be as many as a hundred trillion tons of bacteria living beneath our feet. Pile up all the underground bacteria, and it would cover our planet’s surface to a depth of over five feet.
Titan’s hazy atmosphere – the most Earthlike in our solar system
Based on recent estimates from the Kepler Space Telescope, there are billions of Earthlike planets in our galaxy alone – around one per star, on average. With billions of galaxies in the Universe, we now think that there are more Earthlike planets than grains of sand on all the beaches of Earth. That’s a lot of potential life as we know it, but if we include life in more exotic environments like icy moons, the conditions for life are ten time more common than that. Even in our own solar system, there are a dozen worlds that support liquid water and could, by that definition, be considered habitable.
Of these, Jupiter’s moon Europa is perhaps the best prospect for life, with a liquid water ocean heated by regular tidal flexing in mighty Jupiter’s gravitational field. Almost entirely isolated from the outside world (there is evidence that liquid water occasionally rises and bursts through the icy surface), Europa’s ocean floor is in direct contact with the bedrock beneath, where there should be thermal vents spewing out energy and nutrients. On Earth, these ocean floor thermal vents are cradles of life, and similar to the primitive ecosystems that nurtured the origin of life itself.
Europa has a lot more water than even our blue planet
Thus arises a question: since the conditions for life are ubiquitous, is life common in our Universe, or are there challenges to the origin that make life a relative rarity? Although intelligent beings may exist elsewhere in our galaxy, they obviously aren’t exceedingly common or else we would have extraterrestrials roaming around our solar system or perhaps a nearby star. Yet, this tells us nothing about simpler life which may indeed be common, possibly even to be found on another world orbiting our Sun. Beyond, there could be billions of worlds covered in microorganisms or even simple plants and animals, just waiting to be found. Either we’re alone or we aren’t: both prospects are equally terrifying. Our curiosity drives us to search for answers, as living beings connected to the Universe.