Garden World

Garden Worlds are a special kind of silicate planet: one that is capable of harbouring life as we know it. The only known example in the universe is Earth. While technically referring to any kind of life-bearing world, garden worlds are typically planets that can support plant life (or its equivalent) and a complex ecosphere. Below are listed the requirements for a planet to be considered habitable and thus qualify as a garden world.

Requirements for Habitable Planets
Note that since the sample size of garden worlds is limited to Earth, all of the below is just speculation at best. It also does not take into account alternate biochemistries.

Mass
Garden Worlds require a relatively high mass for a number of reasons: However, just as too little mass inhibits the development of life, so too does too much mass: Overall, the optimum mass is around 2 M⊕or less, with a radius under 1.6 R⊕ and a density similar to that of Earth (this corresponds to the Fe/MgSiO3band on the mass-radius graph).
 * Low mass planets have low gravity, making atmospheric retention difficult. Planets without thick atmospheres lack the 608 Pa atmospheric pressure necessary for liquid water to exist. They also lack sufficient heat transfer, insulation, protection from external hazards (meteorites and harmful radiation) and the matter itself necessary for primal biochemistry. A denser atmosphere may also contribute towards the regularisation of relief and the decreasing of ocean basins, allowing for greater shallow-water diversity.
 * Lower mass planets generally have a smaller volume-surface area ratio, meaning energy from the planet’s formation is quickly lost and the planet becomes geologically dead: unable to host tectonic activity and volcanism, which are necessary for recycling essential minerals and supplying the planet with temperature moderators, like CO2(there are exceptions to these rules: Jupiter’s moon Io, for example, is volcanically active due to gravitational stresses caused by its orbit). Tectonic activity peaks in bodies of masses 1 M⊕ to 5 M⊕, with the optimum at 2 M⊕.
 * Lower mass planets are also less likely to have a large enough molten iron core, which is necessary for the planet to possess a magnetosphere and thus protect itself from harmful cosmic and solar radiation (there are exceptions, see Coreless Planet).
 * Larger terrestrial planets may have overly thick atmospheres which, if too close to the star, would result in a runaway greenhouse effect. Thus larger planets would need to be further from their star to remain habitable.

Orbit and Rotation
There are a number of orbital and rotational factors that affect a planet's habitability. Garden Worlds require an axial tilt that is neither too low nor too high: Garden worlds require a relatively quick rotational period:
 * A Garden World must have liquid water in order to sustain life as we know it. Therefore the planet must orbit within the habitable zone (where water is in its liquid state), or else must exist in some other form (such as subsurface aquifers or subsurface oceans).
 * In order to remain in the habitable zone, the planet must have a low eccentricity (Earth’s is a very circular 0.02). Too high an eccentricity would result in high seasonal variation which could cause water to change from its liquid state, making the development of life complicated, to say the least.
 * Low axial tilt means very little seasonal variation as well as a planet dominated by cold weather (as warm weather cannot move poleward if heat radiation is consistently centered within a few degrees of the equator).
 * Very high axial tilt would create extreme seasonal variation, which would complicate the life-harbouring process by increasing temperature difference across seasons.
 * Tidally-locked planets, or planets with very long day-night cycles, would have an immense temperature difference between day and night sides. A tidally-locked planet may have life-friendly temperatures at the terminator (the dividing line between day and night) however.
 * A planet should rotate fast enough to produce a dynamo effect at its iron core, so as to produce a magnetosphere (there are exceptions, see Coreless Planet).

Worldbuilding in Practice
"Triun is the idyllic homeworld of the Florian race. A 'superhabitable' garden world orbiting the quiet K star Buranda, it sports a diverse and flourishing ecology thanks to its uncanny habitability. With a mass of about 1.97 M⊕and a radius of about 1.24 R⊕, giving it an average density of roughly 5.7 g/cm3and a comfortable surface gravity of about 1.28 Gs. At a close 0.67 AU from Buranda and with a near-perfect 0.013 eccentricity, it orbits almost exactly in the middle of the star's habitable zone. Combined with a heavy, oxygen-rich atmosphere, a day cycle of about 18 Earth hours and an axial tilt of 21°, it is often regarded as the 'perfect world'." "The same cannot be said for Gathka, a fringe Naxarl colony. With a mass 3.04 M⊕and a radius of about 1.21 R⊕, it has an uncomfortable 2.1 Gs of gravity. With a high eccentricity of 0.1 and an axial tilt of 39°, its seasons are violent and its storms brutal. The planet's ecology relies heavily on Gathka's high levels of arsenic, making the world toxic to just about every race other than the Naxarl, who also make use of arsenic in their biochemistry. While a few large fauna and flora exist, most of Gathka's lifeforms are microbial, and the world is famous for its deadly diseases. Its inhabitants eke out a miserable existence working in mines and factories, lorded over by greedy corporate dictators. As a result, Gathka is widely regarded as 'barely habitable' and one of the worst planets in the galaxy. It receives almost no tourism, despite its aggressive self-promotion, which has spawned the saying, 'a Gathka honeymoon' to describe a marriage that is doomed to fail."