The Living Worlds
From its lonely vantage point, nestled between the Orion and Sagittarius arms of the Milky Way galaxy, Earth's blue-green surface seems unique, a singular island of complexity in a harsh and frigid universe. Seen from a wider angle, nothing could be further from the truth. When races expand from their home solar systems, they discover that the cosmos is awash in living worlds. These planets fall into a limited number of classes, as defined by their morphologies and their suns.
Not all suns are suitable hosts for biology. Stars much larger than Earth's sun are too short-lived to allow the formation of complex life: the worlds that circle them remain at a bacterial stage until the rising luminosity of the star boils the ocean and cooks the carbon dioxide out of the rocks, sterilizing the planet under a superheated shell of gas. Biologies are always exclusively bacterial for the first aeons of their planet's history, owing to the lack of oxygen in the atmosphere. The origin of life from inanimate matter-spontaneous and remarkably rapid on all watery worlds-occurs at deep sea vents where extremes of temperature and pressure accelerate the random walk toward chemical complexity. Without shallow seas, oxygen, and terrestrial ecosystems, however, life remains permanently at a very simple stage.
On ocean-bearing worlds, oxygen accumulates as a byproduct of photosynthetic bacteria. Despite high production, oxygen concentrations remain low for several billion years because of its reactivity. Metals rust and silicates oxidize, forming a vast array of minerals from the raw material of asteroidal rubble. Only once the process of rock-oxidation is complete can diatomic oxygen accumulate, providing the energy needed for multicellular life to thrive. On this world, the process took over three and a half billion years, ending around 850 million years ago in what Earth's geologists call the Great Oxygenation Event.
Earth's sun is near the upper size limit of life-bearing stars. It has shone for almost five billion years and will continue to fuse hydrogen for again as long. Unfortunately, the sun's luminosity increases by around ten percent every billion years. The Earth's temperature has remained relatively stable in spite of this growing brightness because the insulating nitrogen in its atmosphere has been lost to space and fixed into biological matter by bacteria. Without sustained and massive technological intervention, the sun's heat will outpace the thinning of the atmosphere in around a billion years, giving the solar system a second Venus with a moon. This timeline means that complex life on Earth has a lifespan only half as long as that of its anaerobic bacterial ancestors.
Most of the stars in the life-bearing range occupy the lower ranges of Earth's sun's G-class and the K-class below it. These suns are similar to the Earth's in behavior, though their smaller size means that they slowly sip their fusion fuel and provide hospitable environments for much longer. M-class stars-commonly known as red dwarves-are not friendly to biology, despite lifespans that can extend for hundreds of billions of years. The danger of red dwarves derives from their fully convective interiors, which create extraordinarily strong magnetic fields. The fields in turn generate powerful solar flare cycles that can roast planets in a matter of hours. Due to the very tight orbits that planets are obliged to take around these dim suns, they are also liable to tidal locking-one hot face forever turned to the star, the other frozen in permanent darkness.
The living worlds that circle the clement K- and G-class suns vary between 0.05 and 1 Earth mass. It is difficult for Earth natives to imagine that the average life-bearing world would be smaller than their own moon, but this is the truth. Other species have been known to accuse captains of starships from Earth with centrifugal gravity of torturing their crews with the high gravity (Nota bene: there are no Earth-originating starships currently. This anecdote refers to the other worldlines of Earth's history, separate from your own.) The world of humanity occupies the extreme upper size range of habitability. Any larger, and its gravity well would have attracted more comets during the formation of the solar system and the oceans would be much deeper, precluding the formation of land. Without land, the sea is deprived of the nutrients blown from continental topsoil and cannot host the diversity of life to which humans are accustomed.
The most common type of world with life is the gas-giant moon. Typically, they are part of a large family of other satellites, each tidally locked like the Earth's moon with one face toward the Jovian planet and the other facing outward. One day on such a world lasts as long as its orbital period around the central body. Moons of gas giants have many advantages over the other classes of worlds. They can be much smaller than a singleton planet while retaining the active volcanism required to release carbon into the biosphere because their cores are constantly heated by gravitational perturbations from the other moons. Several of Jupiter's moons boast liquid water oceans underneath their frozen surfaces for exactly this reason.
Gas-giant moons also tend to have thicker atmospheres that keep temperatures constant even at the poles, as the host planet provides a stronger electromagnetic shield against the solar wind than the Earth does. Finally, the extra tectonic activity means that continents form more rapidly and often occupy the majority of the planet's area, allowing more terrestrial living space. Under low gravity, mountains can be much taller than on Earth, but their summits remain fairly warm because the atmosphere is not so tightly compressed to the surface. Sentients originating on such moons often place great significance on the sub-Jovian point, where the gas giant hangs in the sky directly overhead, storms swirling in brilliant colors across its face. Often, multiple moons in a system contain complex life, a great treat for the first explorers of any race.
Singleton worlds are the next most common class. The absence of gravitational resonance with another body makes these planets much less tectonically active, meaning fewer continents arise from the sea and less carbon is available for living structures to incorporate. There is rarely time in the world's lifespan to fully oxygenate the atmosphere, leaving the biosphere dominated by bacteria until the end. Only ten percent or less of the surface consists of land, surrounded by a vast but unproductive ocean. Other disadvantages also arise from singletons' lonely status. They wobble severely about their axes, resulting in ages where most of the world's surface experiences half-year periods of polar darkness alternating with endless day. This can eliminate the eternal spring of the tropical regions, in which the great majority of biodiversity develops. Furthermore, their magnetic fields tend to be so weak that the atmosphere is at risk of being entirely stripped away. Their frozen and dry fate contrasts with the Venusian death spiral suffered by the other living planets.
The Earth is a textbook example of the rarest form of world playing host to life: the double planet. Early in the planet's history-as was the case with Earth-two large bodies collide, the rubble reforming as a second world in orbit around the first. These two-planet systems have many of the same advantages as gas-giant moons: stable axes, active tectonics, and magnetic fields. Sadly, Earth's moon is too distant to be protected by its host's field, leaving Luna with a bone-dry surface exposed to the vacuum of space. The relative weakness of the Earth's magnetic field has actually been a boon for the biosphere. Its Sun's brightening is so rapid that the Earth would have already experienced runaway global warming terminating in a Venus-like state if its atmosphere were as protected as that of a gas-giant moon.
Earth occupies a special place in the family of living worlds: short-lived on account of its star, much larger than the average, and orbited by a very uncommon kind of satellite. If humans should choose to explore beyond the bounds of this solar system, they will find many excellent and interesting worlds, but none nearby will remind them of home. Moreover, none of the other worlds will contain biology that is inter-compatible with Earth's. If humans care at all for their future, they must preserve their ecosystem, the only truly irreplaceable thing that they possess.
Not all suns are suitable hosts for biology. Stars much larger than Earth's sun are too short-lived to allow the formation of complex life: the worlds that circle them remain at a bacterial stage until the rising luminosity of the star boils the ocean and cooks the carbon dioxide out of the rocks, sterilizing the planet under a superheated shell of gas. Biologies are always exclusively bacterial for the first aeons of their planet's history, owing to the lack of oxygen in the atmosphere. The origin of life from inanimate matter-spontaneous and remarkably rapid on all watery worlds-occurs at deep sea vents where extremes of temperature and pressure accelerate the random walk toward chemical complexity. Without shallow seas, oxygen, and terrestrial ecosystems, however, life remains permanently at a very simple stage.
On ocean-bearing worlds, oxygen accumulates as a byproduct of photosynthetic bacteria. Despite high production, oxygen concentrations remain low for several billion years because of its reactivity. Metals rust and silicates oxidize, forming a vast array of minerals from the raw material of asteroidal rubble. Only once the process of rock-oxidation is complete can diatomic oxygen accumulate, providing the energy needed for multicellular life to thrive. On this world, the process took over three and a half billion years, ending around 850 million years ago in what Earth's geologists call the Great Oxygenation Event.
Earth's sun is near the upper size limit of life-bearing stars. It has shone for almost five billion years and will continue to fuse hydrogen for again as long. Unfortunately, the sun's luminosity increases by around ten percent every billion years. The Earth's temperature has remained relatively stable in spite of this growing brightness because the insulating nitrogen in its atmosphere has been lost to space and fixed into biological matter by bacteria. Without sustained and massive technological intervention, the sun's heat will outpace the thinning of the atmosphere in around a billion years, giving the solar system a second Venus with a moon. This timeline means that complex life on Earth has a lifespan only half as long as that of its anaerobic bacterial ancestors.
Most of the stars in the life-bearing range occupy the lower ranges of Earth's sun's G-class and the K-class below it. These suns are similar to the Earth's in behavior, though their smaller size means that they slowly sip their fusion fuel and provide hospitable environments for much longer. M-class stars-commonly known as red dwarves-are not friendly to biology, despite lifespans that can extend for hundreds of billions of years. The danger of red dwarves derives from their fully convective interiors, which create extraordinarily strong magnetic fields. The fields in turn generate powerful solar flare cycles that can roast planets in a matter of hours. Due to the very tight orbits that planets are obliged to take around these dim suns, they are also liable to tidal locking-one hot face forever turned to the star, the other frozen in permanent darkness.
The living worlds that circle the clement K- and G-class suns vary between 0.05 and 1 Earth mass. It is difficult for Earth natives to imagine that the average life-bearing world would be smaller than their own moon, but this is the truth. Other species have been known to accuse captains of starships from Earth with centrifugal gravity of torturing their crews with the high gravity (Nota bene: there are no Earth-originating starships currently. This anecdote refers to the other worldlines of Earth's history, separate from your own.) The world of humanity occupies the extreme upper size range of habitability. Any larger, and its gravity well would have attracted more comets during the formation of the solar system and the oceans would be much deeper, precluding the formation of land. Without land, the sea is deprived of the nutrients blown from continental topsoil and cannot host the diversity of life to which humans are accustomed.
The most common type of world with life is the gas-giant moon. Typically, they are part of a large family of other satellites, each tidally locked like the Earth's moon with one face toward the Jovian planet and the other facing outward. One day on such a world lasts as long as its orbital period around the central body. Moons of gas giants have many advantages over the other classes of worlds. They can be much smaller than a singleton planet while retaining the active volcanism required to release carbon into the biosphere because their cores are constantly heated by gravitational perturbations from the other moons. Several of Jupiter's moons boast liquid water oceans underneath their frozen surfaces for exactly this reason.
Gas-giant moons also tend to have thicker atmospheres that keep temperatures constant even at the poles, as the host planet provides a stronger electromagnetic shield against the solar wind than the Earth does. Finally, the extra tectonic activity means that continents form more rapidly and often occupy the majority of the planet's area, allowing more terrestrial living space. Under low gravity, mountains can be much taller than on Earth, but their summits remain fairly warm because the atmosphere is not so tightly compressed to the surface. Sentients originating on such moons often place great significance on the sub-Jovian point, where the gas giant hangs in the sky directly overhead, storms swirling in brilliant colors across its face. Often, multiple moons in a system contain complex life, a great treat for the first explorers of any race.
Singleton worlds are the next most common class. The absence of gravitational resonance with another body makes these planets much less tectonically active, meaning fewer continents arise from the sea and less carbon is available for living structures to incorporate. There is rarely time in the world's lifespan to fully oxygenate the atmosphere, leaving the biosphere dominated by bacteria until the end. Only ten percent or less of the surface consists of land, surrounded by a vast but unproductive ocean. Other disadvantages also arise from singletons' lonely status. They wobble severely about their axes, resulting in ages where most of the world's surface experiences half-year periods of polar darkness alternating with endless day. This can eliminate the eternal spring of the tropical regions, in which the great majority of biodiversity develops. Furthermore, their magnetic fields tend to be so weak that the atmosphere is at risk of being entirely stripped away. Their frozen and dry fate contrasts with the Venusian death spiral suffered by the other living planets.
The Earth is a textbook example of the rarest form of world playing host to life: the double planet. Early in the planet's history-as was the case with Earth-two large bodies collide, the rubble reforming as a second world in orbit around the first. These two-planet systems have many of the same advantages as gas-giant moons: stable axes, active tectonics, and magnetic fields. Sadly, Earth's moon is too distant to be protected by its host's field, leaving Luna with a bone-dry surface exposed to the vacuum of space. The relative weakness of the Earth's magnetic field has actually been a boon for the biosphere. Its Sun's brightening is so rapid that the Earth would have already experienced runaway global warming terminating in a Venus-like state if its atmosphere were as protected as that of a gas-giant moon.
Earth occupies a special place in the family of living worlds: short-lived on account of its star, much larger than the average, and orbited by a very uncommon kind of satellite. If humans should choose to explore beyond the bounds of this solar system, they will find many excellent and interesting worlds, but none nearby will remind them of home. Moreover, none of the other worlds will contain biology that is inter-compatible with Earth's. If humans care at all for their future, they must preserve their ecosystem, the only truly irreplaceable thing that they possess.
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