The Universe’s Habitable Zone

Humans have a severe case of societal loneliness.  We send signals out into the void in the hopes that someone might answer, we launch spacecraft into the interstellar medium with a record of our civilization, we push the edges of our technology to seek evidence of long-extinct microbial and unicellular life on the Moon, Mars, asteroids, and other bodies in our solar system.  On a less evidential level, we seek clues, stories, and anecdotes that could enable us to believe that our species is not alone in the universe: points of light in the sky, a circle painted on a cave wall ten thousand years ago, unexplained happenings all over the world.

When an oddly shaped asteroid moving in a peculiar orbit that took it nearly perpendicular to the ecliptic plane was first detected, many scientists were prepared to seriously consider the possibility that it might be some kind of space probe from an intelligent species, sent from who knows how many lightyears away and traveling through space for who knows how many millions of years.  We eventually determined it was only an ordinary asteroid, and as our detection capabilities improved, we discovered more bodies in similar orbits.  When radar returns from a Mars orbiter suggested the existence of subsurface lakes of liquid water near the southern pole, the immediate conclusion was that, as on Earth, that meant life could exist, but a closer analysis and further experiments suggest no lakes at all, but instead just Martian clay.

As our search for exoplanets continues, and our ability to detect them improves, the focus becomes ever more upon the “habitable zone,” a range of distances from a given star in which liquid water can exist.  The whole premise of this emphasis is that liquid water is the unifying molecule that underpins all life on Earth, and therefore our search for extraterrestrial life elsewhere in the universe ought to focus on planets that exhibit conditions similar to our own blue marble.  There is logic to this, since the only life we know is carbon-based and requires liquid water, and we have yet to discover any life that does not conform to this mold.  Yet this is limiting.  It constrains our imaginations, and our openness to the infinite possibilities of our universe.  Limiting our search for life to life we would recognize on Earth creates a risk that we will find utterly alien life elsewhere in the universe, and not even recognize it as a lifeform.

The biological definition of life has five claims: the capacity to grow, metabolize, respond to stimuli, adapt, and reproduce.  According to this definition, stars are lifeforms.  They can grow, building from hydrogen and other base elements into stars of sizes as diverse as red dwarfs and blue giants.  They can metabolize, churning through their hydrogen fuel to power their nuclear fusion and producing helium and heavier elements as waste.  They respond to stimuli, generating sunspots, twisting their magnetic field lines, blowing off solar flares in response to the forces exerted upon them as they hurtle through the interstellar medium.  They adapt, evolving over their lifecycles to utilize different fuel and endure different conditions, like the changing balance between their own gravity and pressures.  And they can reproduce, their gravity causing distortions in the interstellar medium that eventually give rise to clumping of raw materials into nebulae, and the eventual collapse of nebulae into new stars, themselves perhaps formed from the remnants of their elders.

This is not an argument that stars are lifeforms, though it does sound like one.  I recognize that some of the claims, especially those relating to stimulus response and adaptation, are a bit of stretch in a classical sense of the definition.  No, this is an argument that our biological definition of life is inadequate for the task of identifying and perhaps interacting with truly alien lifeforms, beings so far removed from human experience that we very will might not recognize them as life at all.  Imagine loose collections of gas and plasma held together by electromagnetic forces in the atmosphere of a gas giant, drifting along through the hydrogen clouds life Earth-sized jellyfish, “metabolizing” the electrical energy of the superstorms.  Imagine, as in Rocheworld, a silicon-based, amorphous lifeform that can alter its own density to assume the consistency of an oil slick as easily as a rock.  Imagine, as in Dragon’s Egg (which is what inspired this essay), a species of amorphous beings .005 mm tall, living on the surface of a neutron star, living a million times faster than humans, and capable of generating energy through a heat engine powered by the temperature difference between the crust and the atmosphere.

There are thought to be at least a billion stars in our Milky Way galaxy, and as our ability to detect exoplanets has improved, we’ve found that perhaps most of them have at least one planet.  That’s a minimum of a billion planets in the galaxy, and if life could exist on neutron stars, or on brown dwarf stars, or in other, equally “inhospitable” environments, even more possibilities are available.  Life could exist even in the interstellar medium, drifting along life living ram scoops, harvesting raw materials, or perhaps living off of the variations in the cosmic microwave background.  For now, though, constrain it just to planets.  There are hundreds of billions of galaxies in the observable universe.  If each has a similar number of stars to our Milky Way, there are perhaps two hundred pentillion planets in the universe.  If just .000001% of them hosted life, that would still be two hundred trillion planets with life.

Not just an exercise in the almost unfathomable scales of the universe in which we cling to our tiny, wet dust speck hurtling through a near vacuum of less than six hydrogen atoms per cubic meter, this shows just how unlikely it is that we are alone in the universe.  Yet we might never assuage our societal loneliness if we continue to confine ourselves to looking only for life that looks like us.  There are all kinds of arguments that we’ve developed to justify looking for life that looks like what we know on Earth, from the practical (that’s all we know how to look for), to the biological (if it doesn’t look at least a little like life on Earth, then it’s probably not alive), to the improbable (there are good reasons that life on Earth looks the way it does, and so it’s highly unlikely that significantly different combinations would be viable).  Silicon is said to be too uncommon, too brittle, and have the wrong electron configuration to take the place of carbon.  Methane is said to be too poor a solvent to replace water.  Conditions on superhot Jupiters are said to be too extreme.  I say all of these are excuses to cover up a lack of imagination.  In a universe of more than two hundred pentillion planets, there is room for all of the possible variations, no matter how unlikely.  Perhaps we ourselves are the unlikely ones, and no intelligent aliens have contacted us because they think our form of life could never possibly develop and therefore is not worth searching for in the cosmos.

It is an enormous universe out there, and from our improbable collection of atoms swirled into existence from the celestial soup by a mid-class star we peer out into the cosmos and have the hubris to think we can extrapolate what might be waiting.  It is easy, comfortable, and even logical to take an anthropocentric view, but it is also limiting.  Infinite variety in infinite combinations.  Only when we open our minds to the weirdest, most unimaginable possibilities, the ones that seem to contradict everything we think we understand about life, will we be ready to meet what might be out there.