#4 The Habitable Planet
Every once in a while, scientists – cosmologists in particular – claim that they have found a potentially habitable planet in some distant part of our galaxy. What do they mean? Do they mean there is a good chance that life exists on that planet? What drives them to declare such a strong hypothesis? And what kind of life are they talking about? Is it the kind of life we see on Earth – plants, animals, humans? Or are they talking about the presence of any kind of life – even if it is just microbial life?
Prerequisites for Life
Life is prevalent on our planet. We see it everywhere. It is the salient feature of Earth. To the best of our current estimates, life on our planet runs into millions of distinct species of plants and animals. In reality, life on Earth is in such an abundant variety that most of the species have still not been discovered yet. But why is the presence of life on other planets such a big deal? It would seem that if life is so pervasive and easily replicated on our planet, there is a very good chance that other planets would easily host life too. After all, nature invents a way to create and sustain life. To quote the chaos theory mathematician Ian Malcolm from Jurassic Park: “Life finds a way”.
It turns out that the conditions for harboring life on a planet are not that straightforward. There are a host of stringent conditions that must be met for the possibility of life to exist on a planet. And we’re talking about the simplest of life forms here – single-celled organisms. If we move on to more complex organisms – plants, birds, reptiles, mammals – the constraints on their existence grow rapidly. We may not realize it right away, but any planet that claims to host life must pass a bunch of strict requirements. And the list of such prerequisites grows as life forms become more complex.
Earth hosts millions of life forms. Does that mean that our planet passes all the stringent requirements for harboring life? And what are those factors necessary for life to exist on a planet? Let’s look at some of them.
Location, location, location
Our universe has a seemingly infinite real estate. We can’t even accurately estimate its ever-increasing size. Surely, this unlimited space must carry life profusely within it? We know that life – at least the one found on our planet – is primarily made up of biomolecules. Carbon, oxygen, hydrogen, nitrogen, and other metal-rich heavy elements, such as iron and silicon, are necessary for life to exist. These elements are not readily found throughout the universe. They exist simultaneously only in certain places.
Of the three types of galaxies found in the universe – elliptical, irregular, and spiral – spiral galaxies are the only ones that are conducive for life to exist. Elliptical and irregular galaxies do not contain sufficient amounts of metal-rich heavy elements. Even within spiral galaxies – the ones with long spiral arms – locations near the center are filled with dangerous radiations that kill all life within its reach. Neighborhoods further out toward the outer fringes of the galaxy are too low in heavy elements. Neither region can harbor life.
The only locations in the vast universe where life could exist are circular bands mid-way between the center and the edges of a spiral galaxy – called the Galactic Habitable Zone. Biological life cannot survive outside of this narrow band. The Milky Way galaxy – the one in which we live – is a spiral galaxy. The Solar System that we are part of is located within its Galactic Habitable Zone. Earth sits in the right location in the Milky Way galaxy to sustain life.
Earth, however, is not the only planet to lie in the Galactic Habitable Zone of a spiral galaxy. There may still be billions of planets in the entire universe that fit the requirement. Does that mean they can carry life? It turns out that it takes much more for a planet to harbor life.
The Sun
Any planet orbiting around its star must lie within the Habitable Zone of the star for it to have any chance of life on it. Unsurprisingly, Earth lies comfortably within the Sun’s Habitable Zone. If Earth were closer to the Sun – more than 5% closer than it is now – its oceans would boil over. If it were too far – more than 15% farther than it is now – the oceans would freeze up. Similar life-destroying results would occur on Earth if the Sun were slightly more or less massive or brighter than it is now. It would alter the temperature on Earth beyond the endurance level of biological life.
A star’s luminosity typically changes with time. A large variation will cause sharp temperature changes on a planet, resulting in catastrophic damage to its ecosystem. The Sun, in this regard, is fairly stable, steadily increasing 30% in brightness over the last 4 billion years of Earth’s existence. As a result, its Habitable Zone. has moved further away with time. Remarkably, our planet has remained safely within the thin and sliding Habitable Zone of the Sun throughout this time. The Sun’s location, size, brightness, variability, and distance from Earth all line up precisely to ensure a hospitable environment on Earth.
When scientists declare a distant planet to be potentially habitable, the planet’s location within its star’s habitable zone is one of the primary criteria they consider. If the planet does not pass this test, there is no chance for life to exist there. But while this condition is necessary to sustain life, is it sufficient? Let’s explore further in the context of our planet.
The Outer Planets
It may come as a surprise to you, but large outer planets of the Solar System, especially Jupiter, has a direct impact on life on Earth. Our Solar System is filled with cosmic debris flying through it. If unchecked, some of the large cosmic debris would fall on Earth and wipe out life on it. The intense gravitational pull of the massive planet, Jupiter, plucks large cosmic objects out of Earth’s trajectory diverting them to outer space. As a result, our planet sees much less cosmic debris than it otherwise would without Jupiter, thereby minimizing the chances of a cosmic hit that could wipe out life on it. Earth would be bombarded much more frequently with life-threatening large cosmic debris had it not been for Jupiter.
The Moon
Earth’s moon – about one-third the size of Earth – is unusually large for a warm inner planet. It turns out that this ratio is precisely necessary for sustaining life on Earth. Our planet rotates on its axis at a tilt between two large celestial bodies – the Sun and Jupiter. If there were no moon, or if Earth’s moon were smaller, its gravitational force on Earth would not be strong. The gravitational pull of the Sun and Jupiter would take over and fight with each other causing Earth to wobble on its axis as it rotated at an angle. Such a tug-of-war would cause Earth’s axis to tilt sharply between near horizontal to near vertical resulting in intense seasonal changes. Earth would experience abrupt and extreme temperature variations that would run beyond the endurance levels of biological life.
If the moon were larger than it is now, its strong gravitational force would lock Earth’s spin, halting its seasonal changes and causing permanent extreme temperatures on either side of the planet. Either scenario would produce an existential threat to biological life.
Apart from stabilizing Earth’s rotation, the moon is also responsible for nurturing marine life. The moon’s strong gravitational force drives the strong ocean tides on Earth that, in turn, trigger the flow of nutrients, heat, and ocean currents – all necessary for a healthy marine life. Biological life wouldn’t survive on Earth without the moon.
Besides factors external to Earth, our planet has several inherent characteristics that ensure the presence and sustainability of biological life on it. Let’s look at a handful of them.
Orbit
Earth revolves around the Sun in a slightly elliptical (oval-shaped) orbit. Such a small variation in its elliptical revolution around the Sun ensures that our planet always stays within the circular Habitable Zone of the Sun. If Earth’s orbit were sufficiently more elliptical than it is now, it would revolve around the Sun in a trajectory that would take it in and out of the Sun’s Habitable Zone. Temperatures on Earth would oscillate between too hot and too cold for survival. Biological life would quickly succumb to these extreme temperatures and die.
Rotation
Earth rotates on its imaginary axis every 24 hours as it circles the Sun in its orbit. This uninterrupted rotation produces the cyclical day/night pattern, causes temperature variations, and drives wind patterns on the planet. If Earth did not rotate, one side of it would face the Sun for months and be scorched while the other face would remain frigid cold in darkness for months. If the Earth rotated much faster than it does now, days and nights would pass by very quickly – in a few hours – severely disturbing weather and daily life patterns. Neither scenario is conducive for sustaining biological life.
Tilt
While orbiting the Sun, Earth rotates at a slight tilt – a pivotal feature that produces favorable seasonal variations, wind patterns, and regularity of rain on most parts of the planet, all of which are necessary for life to survive. If Earth had a large tilt, it would have extreme temperatures locked on either side of its two hemispheres. Its polar icecaps would melt inundating land mass. If Earth had a negligible tilt, extreme temperatures would exist between its poles and the equator. Distribution of rain would be severely inhibited resulting in much arid land and reduced vegetation. Life would be severely challenged either way.
Mass
Earth’s mass is perfectly suited for life to survive on it. A smaller Earth would not have sufficient gravitational pull to keep its atmosphere intact. Without the atmosphere, life would not exist. A larger Earth would have a weaker magnetic field around it. Relentless cosmic debris would easily fall on Earth and destroy life. The stronger gravitational force of a larger Earth would have many other calamitous effects detrimental to life, including a dense atmosphere and high atmospheric pressure. We are lucky to have the right size of Earth.
Plate Tectonics
Plate tectonics on Earth is a very complex process unique to it that recycles Earth’s land mass. Erosion of mountains and land mass due to rain and subduction in ocean floors causes our planet to lose its land mass. Plate tectonics ensures that new land is injected from Earth’s interior back to its surface through volcanoes and oceanic ruptures. If there were no plate tectonics on Earth, mountains would eventually flatten out causing oceans to cover the entire planet. Not only terrestrial life, but marine life would eventually die out too.
Magnetic Field
Due to its inner core of iron and nickel, Earth has a strong magnetic field around it. This magnetic shield protects the planet from the harmful solar winds and cosmic rays that constantly bombard it from space. If Earth did not have a strong magnetic field, charged particles from space would splutter away its atmospheric layer exposing life below it to lethal radiation. Biological life would have no chance of surviving such a deadly onslaught.
Atmosphere
Earth has an atmospheric layer around it that is absolutely necessary for life. The composition of gases in the atmosphere – 78% nitrogen, 21% oxygen, 1% greenhouse and trace gases – serve essential roles for the survivability of life. Nitrogen is used for recycling life’s chemistry, oxygen is indispensable for life, while the greenhouse gases regulate temperature on Earth. A small but sufficient disturbance in this delicate proportion of gases would destroy the equilibrium necessary to maintain life on our planet. The global warming that we see today – with its catastrophic effects – is due to one such man-made perturbation in nature’s equilibrium.
Earth’s atmosphere also serves as a protective canopy for its inhabitants, shielding them from the cosmic debris that burns itself while passing through the atmosphere. Without the atmosphere, Earth would be relentlessly bombarded with such cosmic debris from outer space, making it unsustainable for life to flourish.
Temperature
With an average global temperature of about 15oC, Earth has a comfortable temperature range on its surface. Its greenhouse gases trap the planet’s heat to regulate its temperature to be within the comfortable range for biological life. Without the greenhouse gases, Earth would lose its heat causing its average temperature to drop 30-40oC and making it impossible for life to exist.
Terrestrial Composition
The surface of Earth is 71% water and 29% land – a proportion well suited for a stable surface temperature, adequate water supply, and ample vegetation on land. A higher land-to-water ratio would result in more heat and less rain on land, causing it to dry up. A much lower land-to-water ratio than we have now would reduce available land resources, depriving biological life of the means to thrive.
Dynamic Cycles
Earth has a number of equilibrial mechanisms to recycle and regulate precious resources. Its water cycle ensures rain falls on most parts of the planet to sustain life. Its carbon cycle maintains carbon – the essential ingredient of all life – through plants, animals, and Earth to support the continuation of life. Other dynamic cycles – the nitrogen cycle, oxygen cycle, methane cycle, etc. – all recycle critical resources of the planet that make life possible on it. Disturbing any of Earth’s dynamic cycles would perturb the equilibrium on the planet and cause irreparable harm to, or extinction of, its biological life.
We’ve gone through a long list of factors necessary for life to exist on Earth. This list is by no means exhaustive. There are so many more elements on Earth and external to it that contribute to the survivability of its biological life. We do not need to go into their details. The factors detailed above provide a quick glimpse – and sufficient insight – into what is needed on a planet and outside of it to support life.
We certainly take things for granted on Earth. The air we breathe that keeps us alive. The protective guards above us that shield us from harmful radiations. The sunlight we are immersed in as we go about our daily routine. The Sun’s warmth we bask in without which we would freeze to death. The moon without which our lives would become almost unbearable. The tolerable temperatures that most of us dwell in. The regular rainfalls that keep us alive. The day/night cycle that lets us balance between work and rest. The cyclical seasons that rejuvenate life. When was the last time we contemplated over such complimentary wonders of our world?
Having life on any planet is not a straightforward feat. While it is certainly possible for life to exist elsewhere, a large number of interconnected factors have to become favorable concurrently for life to exist. Discovering life elsewhere will be challenging and possibly a long shot. The fact that all critical factors that contribute to the survivability of life line up so comfortably and precisely on our planet should give us some pause to think. If this is a coincidence, we must be uncannily lucky. If it is providence, we must be contemplatively curious. Our exploration continues …
Share your thoughts on this subject. What other critical factors contribute to the habitability of life on Earth? Do you think the long list of factors necessary for life exists on other planets? Do you anticipate other life forms to exist elsewhere in the universe?