The Goldilocks Conditions
In the popular fairytale, Goldilocks and The Three Bears, Goldilocks stumbles upon a bear family's home while they are away. She tried out each bed, chair, and bowl of porridge to find the one that she was most comfortable with. Trying each of the three bowls of porridge laid out, one was too hot for her, one too cold, and one she ate entirely - the one that was just right. One bed was too hard, one was too soft, but one was just right - the one she slept on. Further specifying that it had to be "just right" for her.
Like Goldilocks, we all have our preferences for something to be just right. This, of course, is not just regarding chair sizes, bed hardness, or porridge temperatures, but includes far more complex subjects. Humanity as a whole needs certain things to be exact for it to thrive. Though it's easy to overlook, given that everything went right, if we ask how come we haven't found any human life on other planets yet, our luck becomes evident. This luck dates all the way back to the creation of Earth, where four things had to go right, for you to be able to read this article today.
Don’t take Earth for granted – there are reasons other planets, like Mars and Venus, just aren’t right.
Artists rendition of a young planet forming in an early solar system.
The Creation of the Solar System
Our solar system is special in its planetary variety and number. Their placements seem simple; with an asteroid belt dividing the two types of planets neatly. But don’t be fooled! It took a lot of chaos to acquire this planetary harmony.
Nearly 4.6 billion years ago, the early solar system was way too hot and messy for life to start, yet it was just right in its own way. The Sun was formed as hot gas clumped together under gravity to fuse hydrogen atoms and ignite our host star. This ignition created an extremely massive ball of scorching gas, with an incredible gravitational pull. According to Isaac Newton's law of gravitation, the bigger a mass, the stronger its pull gravity. In turn, the Sun was able to pull the surrounding gas and dust particles into orbit around the star. This mess of gas and dust orbiting chaotically around the star meant hundreds of thousands of collisions. Through the process of accretion, any particles that found themselves in the same orbit collided and clumped together by the heat of an impact, slowly growing into the planets with their own gravitational pull. By this method, a young Earth was born alongside its 7 neighbors.
The elements in the human body all originate from accreted remnants of stars that died before the creation of the Sun and Earth. So, if the young Earth hadn’t collected calcium, potassium, iron, or any of the other metals that we find in our bodies, you wouldn’t be here reading this article today. For that, we have the mayhem of the early solar system to thank, and all the proto-planetary collisions that had to happen so you could take those magnesium supplements every morning!
2. The Habitable Zone
After gathering the necessary elements for life, the conditions they were in also had to be just right. These depend on where the young Earth settled in its orbital distance from the sun.
Due to the extreme heat of the sun's radiation, Mercury's close orbit proves to be uninhabitable for life as we know it. However, going too far out away from the sun is also detrimental to habitability; as the heat from the sun diminishes, and the frigid temperatures of empty space take over. If Earth had settled anywhere beyond the asteroid belt, its surface would be a frozen desert.
Liquid water covers a significant amount of present-day Earth's surface. But how did this water come to be on a young Earth? Sure, the accretion process works for dust and gas particles, as they are able to become molten and thus remain on the surface during such hot collisions. However, at that point in the young Earth's life, any water would have evaporated and have had a hard time settling into liquid form, without the necessary atmospheric pressure conditions. Scientists have devised a multitude of theories on how liquid water settled on our planet.
Planetary cooling is the simplest one of the bunch; explaining that young Earth simply had enough time to cool and form an atmosphere, allowing for any minerals that had the building blocks of water molecules (from the accretion period) to seep out and allow water to form. As the planet cooled further, there would have been oceans on the young Earth's surface from very early on in its life.
A weaker contender is exoplanetary sources. By 'exoplanetary', scientists refer to space rocks not classified as planets in our solar system - like comets, that may have struck Earth with their icy bodies and thus deposited masses of water onto our planet's surface. However, scientists discovered traces of water on the moon, which is thought to be made from the young Earth's body, after a collision with a Mars-sized planet. Therefore, this means that water molecules had to have been on the planet prior to the impact, fuelling the planetary cooling theory.
Earth celestial buddy plush toy
Wherever it comes, water is found on multiple planetary bodies in our solar system in its solid, icy form. One thing that differentiates Earth from these icy bodies, is that it harbors H2O in every state - gaseous, liquid, and solid. Most importantly, Liquid.
According to the theory of abiogenesis, life as we know it requires liquid water to form. Since water is only liquid at temperatures from 0-100 degrees Celsius, our frigid universe does not often make such a condition easy to meet.
The heat a planet feels from its host star depends on two things: the planet's orbital distance, and the host star's stellar type. Through observations, astronomers have identified 7 types of stars in the universe. These are classified as spectral types O,B,A,F,G,K, and M in order. An O-type star is the hottest-known star type, and M is the coolest. As astronomers discover new stars, they can determine their surface temperatures, and place them on this stellar type scale. Should they find any planet orbiting said star, they take note of its orbital distance and, looking at its host star's stellar type, can determine whether the planet is in the habitable zone. Hotter stars mean that a planet needs to be further away to sustain liquid water: too close and it'll boil. Cooler stars can mean possible habitable planets at orbits much closer to their star than Earth is, for instance.
If we were to take the radiation from a sun-like star and a fair atmospheric pressure into consideration, the perfect distance for liquid water to be found is between 0.8-1.5 AU (1AU = 150 million km). Thankfully, Earth’s address lies at 1AU, making way for the right conditions for the development of simple life. This orbital distance range is known amongst astronomers as the habitable zone. Another term commonly used is 'The Goldilocks Zone', as it is not too hot or too cold for liquid water, but just right.
Illustration of jupiter and the neighboring asteroids that it influences with its gravitational pull, preventing them from hitting Earth.
3. Planetary Bodyguards
After a young Earth has gathered heavy elements and obtained the perfect orbital distance for water to begin hosting life, it must now sustain it. The hard part begins. Over 4 billion years are yet to pass before human life can be introduced, and the Earth is still not safe enough. Millions of rocks left over from the formation of the solar system are still in unpredictable orbits, moving at high speeds. Because of this, Earth becomes a sitting duck, waiting to be bombarded by these asteroids that could wipe out any life on the planet. In addition, the new planets formed in this early solar system, develop a gravitational pull, as a result of their heavier masses. This influences the motion of these rocky bodies further - it can catapult them at higher speeds towards Earth. On the plus side, gravity can also act as a protector and pull the asteroids away from their course toward our planet.
Earth has planetary bodyguards in the two largest gas giant planets – Jupiter and Saturn. Jupiter is the largest planet in our solar system, with a mass 318 times that of Earth. In second place is Saturn, weighing in at 95 times the mass of the Earth. Being the second and third most massive bodies in the entire solar system, these planets hold these places in the gravitational strength leaderboard too. On account of their great masses, their gravitational influence on the other planets and asteroids in the solar system is incredibly significant. Referring back to Newton's law of gravitation, a gravitational field is not only stronger the more massive a planet is, but it also is able to span a much larger distance. This explains how asteroids that are far away from Jupiter or Saturn are still able to feel their gravitational pull. The influence of Jupiter and Saturn and the importance of their own orbital positions can best be realized when we simulate a solar system without them. In such a solar system, the Earth becomes an easy target for any of the fast-moving, dangerously sized Asteroid belt, Kuiper belt, or Oort cloud objects. Thankfully, Jupiter has a strong hold on the asteroids, and Saturn keeps the objects coming from farther out in the solar system at bay.
Working together, the two planets' joint gravitational hold on these icy, rocky objects, can influence their orbital trajectory to our benefit. In other words, we have these two giants to thank for making Earth’s orbit safe enough for life to form and evolve into us.
Artists rendition of the Earth's magnetic field.
4. The Magnetic Field
Despite having two gas giants as bodyguards at a distance, the Earth was left to its own devices when asked to sustain life for 4 billion years. Earth faces a danger from its very own heat and light source – the sun. This danger comes in the form of the solar wind’s extreme radiation, neutrinos, and charged particles that can erode the atmosphere and create an unliveable environment. Life is threatened by high-energy solar radiation, like gamma and X-rays, which ionize particles and break bonds in living organisms. For humans and animals like us, this causes mutations, cancerous growths, and developmental dangers. Notwithstanding its intense heating effects, which kills microorganisms, plants and is detrimental to marine life, from whence humans are theorized to have evolved. Being in such a danger means that Earth needs to be protected from this harmful radiation.
The Earth’s iron and nickel core rotates in a molten mess at the heart of the planet. This rotation of charged particles induces a magnetic field that surrounds the whole planet. Though this might seem like sorcery, physics explains it all! A magnetic field is an area of space where there is a change in energy. When particles that carry charge, like protons and electrons, move, they create a change in energy and thus a magnetic field. In metals, electrons are free to move around more than in other elements. Meaning that they are able to create stronger mangetic fields, especially when they are molten as this adds fluidity and hence more degrees of motion. In the case of the Earth's core; because it is such a large mass of molten metal, the magnetic field becomes large enough to cover the whole planet.
This field shields our planet from the oncoming harmful solar winds as it redirects protons and electrons, which are caught in it before they bombard Earth. The redirection mostly moves them towards the north and south poles of the planet, which also act as the poles of the magnet- the strongest points in the field. As the charged particles hit our atmosphere, they ionize not the particles in our bodies, thankfully, but the particles in our atmosphere. This causes the atmosphere to glow in greens, pinks, and purples as we see the effects of solar radiation hitting the nitrogen, carbon, and oxygen in the air. Reminding us of our invisible shield that is just right.
Photo of the Aurora Borealis from the International space station.
Conclusion
Human life is picky. Like Goldilocks, it likes things to be just so on its home planet, for it to form, evolve, and flourish. The planet needs to gather exact, necessary heavy elements, including water. It must then ensure to sustain this water, in its liquid form, at just the right orbital distance from its host star. Should the planet be in danger of being hit by space rocks, it must have neighboring planets with strong, protecting gravities. And finally, it must protect itself by producing and maintaining a magnetic field, so that any life on its surface is shielded from its host star's radiation.
While I may have lost some hope we'd ever find alien life, my research taught me to be far more grateful for the planet we call home.